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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2017	Feb. 23, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	CYTK
Entity Registrant Name	CYTOKINETICS INC
Entity Central Index Key	0001061983
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		54,008,113
Entity Public Float			$ 630.8

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 125,206	$ 66,874
Short-term investments	143,685	89,375
Accounts receivable	1,112	24
Prepaid and other current assets	4,292	2,360
Total current assets	274,295	158,633
Long-term investments	16,518	7,672
Property and equipment, net	3,568	3,637
Other assets	429	200
Total assets	294,810	170,142
Current liabilities:
Accounts payable	5,253	4,236
Accrued liabilities	17,392	18,047
Deferred revenue, current	9,572	8,060
Current portion of long-term debt	0	2,500
Other current liabilities	227	415
Total current liabilities	32,444	33,258
Long-term debt	31,777	27,381
Liability related to the sale of future royalties, net	104,650	0
Deferred revenue, non-current	15,000	15,000
Other long-term liabilities	1,097	142
Total liabilities	184,968	75,781
Commitments and contingencies (Note 10)	0	0
Stockholders’ equity:
Preferred stock, $0.001 par value: Authorized: 10,000,000 shares; Issued and outstanding: none	0	0
Common stock, $0.001 par value: Authorized: 163,000,000 Issued and outstanding: 53,960,832 shares at December 31, 2017	54	41
Additional paid-in capital	755,526	612,474
Accumulated other comprehensive income	343	137
Accumulated deficit	(646,081)	(518,291)
Total stockholders’ equity	109,842	94,361
Total liabilities and stockholders’ equity	$ 294,810	$ 170,142

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	163,000,000	163,000,000
Common stock, shares issued	53,960,832	40,646,595
Common stock, shares outstanding	53,960,832	40,646,595

Consolidated Statement of Operations And Comprehensive (Loss) Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Research and development, grant and other revenues, net	$ 4,569	$ 44,236	$ 14,740
License revenues	8,799	62,171	13,918
Total revenues	13,368	106,407	28,658
Operating expenses:
Research and development	90,296	59,897	46,398
General and administrative	36,468	27,823	19,667
Total operating expenses	126,764	87,720	66,065
Operating (loss) income	(113,396)	18,687	(37,407)
Interest expense	(3,016)	(2,698)	(268)
Non-cash interest expense on liability related to sale of future royalties	(13,980)	0	0
Interest and other income, net	2,602	464	174
Net (loss) income	$ (127,790)	$ 16,453	$ (37,501)
Net (loss) income per share — basic	$ (2.59)	$ 0.41	$ (0.97)
Net (loss) income per share — diluted	$ (2.59)	$ 0.39	$ (0.97)
Weighted-average number of shares used in computing net (loss) income per share — basic	49,404	39,943	38,814
Weighted-average number of shares used in computing net (loss) income per share — diluted	49,404	42,561	38,814
Other comprehensive (loss) income:
Unrealized (losses) gains on available-for-sale securities, net	$ 206	$ (12)	$ 153
Comprehensive (loss) income	$ (127,584)	$ 16,441	$ (37,348)

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	CE Offering [Member]	Common Stock [Member]	Common Stock [Member] CE Offering [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] CE Offering [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2014	$ 92,064		$ 39		$ 589,272		$ (4)	$ (497,243)
Beginning Balance, shares at Dec. 31, 2014			38,659,738
Exercise of stock options, value	427				427
Exercise of stock options, shares			68,635
Issuance of common stock under Employee Stock Purchase Plan, value	69				69
Issuance of common stock under Employee Stock Purchase Plan, shares			21,167
Vesting of restricted stock units, net of taxes withheld, value	(144)				(144)
Vesting of restricted stock units, net of taxes withheld, shares			23,725
Exercise of warrants, shares			234
Issuance of common stock under CE Offering at net of commission and issuance costs		$ 8,673		$ 1		$ 8,672
Issuance of common stock under CE Offering, shares				808,193
Issuance of warrants	282				282
Stock-based compensation	4,567				4,567
Other comprehensive income (loss)	153						153
Net (loss) income	(37,501)							(37,501)
Ending Balance at Dec. 31, 2015	68,590		$ 40		603,145		149	(534,744)
Ending Balance, shares at Dec. 31, 2015			39,581,692
Exercise of stock options, value	503				503
Exercise of stock options, shares			74,556
Issuance of common stock under Employee Stock Purchase Plan, value	917				917
Issuance of common stock under Employee Stock Purchase Plan, shares			129,604
Vesting of restricted stock units, net of taxes withheld, value	(135)				(135)
Vesting of restricted stock units, net of taxes withheld, shares			25,745
Exercise of warrants, value	611		$ 1		610
Exercise of warrants, shares			834,998
Issuance of warrants	288				288
Stock-based compensation	7,146				7,146
Other comprehensive income (loss)	(12)						(12)
Net (loss) income	16,453							16,453
Ending Balance at Dec. 31, 2016	94,361		$ 41		612,474		137	(518,291)
Ending Balance, shares at Dec. 31, 2016			40,646,595
Exercise of stock options, value	1,918				1,918
Exercise of stock options, shares			264,164
Issuance of common stock under Employee Stock Purchase Plan, value	1,167				1,167
Issuance of common stock under Employee Stock Purchase Plan, shares			120,959
Vesting of restricted stock units, net of taxes withheld, value	(904)				(904)
Vesting of restricted stock units, net of taxes withheld, shares			120,959
Exercise of warrants, value	12,071		$ 3		12,068
Exercise of warrants, shares			3,450,122
Issuance of common stock under secondary offeringnet of issuance costs of $3,400	82,370		$ 6		82,364
Issuance of common stock under secondary offering, shares			6,049,000
Issuance of common stock under CE Offering at net of commission and issuance costs		$ 29,855		$ 3		$ 29,852
Issuance of common stock under CE Offering, shares				2,425,625
Issuance of common stock pursuant to Royalty Purchase Agreement	7,560		$ 1		7,559
Issuance of common stock pursuant to Royalty Purchase Agreement, shares			875,656
Stock-based compensation	9,028				9,028
Other comprehensive income (loss)	206						206
Net (loss) income	(127,790)							(127,790)
Ending Balance at Dec. 31, 2017	$ 109,842		$ 54		$ 755,526		$ 343	$ (646,081)
Ending Balance, shares at Dec. 31, 2017			53,960,832

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Issuance of common stock under secondary offering, net of issuance costs	$ 3,400
CE Offering [Member]
Issuance of common stock under CE Offering, net of commission and issuance costs	$ 992	$ 205

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net (loss) income	$ (127,790)	$ 16,453	$ (37,501)
Adjustments to reconcile net (loss) income to net cash provided by (used in) operating activities:
Depreciation and amortization of property and equipment	1,920	741	589
Net gain on disposal of equipment	(67)	(18)	(18)
Non-cash interest expense related to long-term debt	635	534	3
Non-cash interest expense on liability related to sale of future royalties	14,028	0	0
Non-cash stock-based compensation	9,028	7,146	4,567
Gain on sale of investments	0	0	(3)
Changes in operating assets and liabilities:
Accounts receivable	(1,088)	(12)	46,634
Prepaid and other assets	(2,161)	(707)	(396)
Accounts payable	1,457	1,698	755
Accrued and other liabilities	766	8,945	2,995
Deferred revenue	1,513	2,202	(12,742)
Net cash (used in) provided by operating activities	(101,759)	36,982	4,883
Cash flows from investing activities:
Purchases of investments	(240,413)	(145,158)	(115,566)
Sales and maturities of investments	177,462	94,645	132,190
Purchases of property and equipment	(2,877)	(1,596)	(562)
Sales of property and equipment	0	33	1
Net cash (used in) provided by investing activities	(65,828)	(52,076)	16,063
Cash flows from financing activities:
Proceeds from public offerings of common stock, net of issuance costs	112,224	0	8,673
Proceeds from sale of future royalties, net of issuance costs	90,621	0	0
Proceeds from issuance of common stock related to sale of future royalties, net of issuance costs	7,560	0	0
Net proceeds from long term debt, net of debt discount and issuance costs	1,261	14,996	14,890
Proceeds from stock based award activities and warrants, net	14,253	1,896	352
Net cash provided by financing activities	225,919	16,892	23,915
Net increase in cash and cash equivalents	58,332	1,798	44,861
Cash and cash equivalents, beginning of period	66,874	65,076	20,215
Cash and cash equivalents, end of period	125,206	66,874	65,076
Supplemental disclosure of cash flow information
Cash paid for interest	2,128	1,899	94
Cash paid for taxes	$ 1	$ 1	$ 1

Organization and Significant Accounting Policies	12 Months Ended
Organization and Significant Accounting Policies	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Significant Accounting Policies	Note 1 — Organization and Significant Accounting Policies Organization Cytokinetics, Incorporated (the “Company”, “we” or “our”) was incorporated under the laws of the state of Delaware on August 5, 1997. The Company is a late stage biopharmaceutical company focused on the discovery and development of novel small molecule therapeutics that modulate muscle function for the potential treatment of serious diseases and medical conditions. The Company’s financial statements contemplate the conduct of the Company’s operations in the normal course of business. The Company has incurred an accumulated deficit of $646.1 million since inception and there can be no assurance that the Company will attain profitability. The Company had a net loss of $127.8 million and net cash used in operations of $101.8 million for the year ended December 31, 2017. Cash, cash equivalents and investments increased to $285.4 million at December 31, 2017 from $163.9 million at December 31, 2016. The Company anticipates that it will have operating losses and net cash outflows in future periods. The Company is subject to risks common to late stage biopharmaceutical companies including, but not limited to, development of new drug candidates, dependence on key personnel, and the ability to obtain additional capital as needed to fund its future plans. The Company’s liquidity will be impaired if sufficient additional capital is not available on terms acceptable to the Company. To date, the Company has funded its operations primarily through sales of its common stock and convertible preferred stock, contract payments under its collaboration agreements, sale of future royalties, debt financing arrangements, government grants and interest income. Until it achieves profitable operations, the Company intends to continue to fund operations through payments from strategic collaborations, additional sales of equity securities, grants and debt financings. The Company has never generated revenues from commercial sales of its drugs and may not have drugs to market for at least several years, if ever. The Company’s success is dependent on its ability to enter into new strategic collaborations and/or raise additional capital and to successfully develop and market one or more of its drug candidates. As a result, the Company may choose to raise additional capital through equity or debt financings to continue to fund its operations in the future. The Company cannot be certain that sufficient funds will be available from such a financing or through a collaborator when required or on satisfactory terms. Additionally, there can be no assurance that the Company’s drug candidates will be accepted in the marketplace or that any future products can be developed or manufactured at an acceptable cost. These factors could have a material adverse effect on the Company’s future financial results, financial position and cash flows. Based on the current status of its research and development plans, the Company believes that its existing cash, cash equivalents and investments will be sufficient to fund its cash requirements for at least the next 12 months after the issuance of the consolidated financial statements. If, at any time, the Company’s prospects for financing its research and development programs decline, the Company may decide to reduce research and development expenses by delaying, discontinuing or reducing its funding of one or more of its research or development programs. Alternatively, the Company might raise funds through strategic collaborations, public or private financings or other arrangements. Such funding, if needed, may not be available on favorable terms, or at all. The financial statements do not include any adjustments that might result from the outcome of this uncertainty. Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Basis of Presentation The consolidated financial statements include the accounts of Cytokinetics and its wholly owned subsidiary and have been prepared in accordance with U.S. generally accepted accounting principles (“US GAAP”). Intercompany transactions and balances have been eliminated in consolidation. Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject the Company to concentrations of risk consist principally of cash and cash equivalents, investments, long term debt and accounts receivable. The Company’s cash, cash equivalents and investments are invested in deposits with three major financial institutions in the United States. Deposits in these banks may exceed the amount of insurance provided on such deposits. The Company’s exposure to credit risk associated with non-payment is limited to its strategic partners Amgen Inc., (“Amgen”) and Astellas Pharma Inc., (“Astellas”) and any material non-payment from our partners would result in a material breach of the agreements underlying the strategic partnerships. Drug candidates developed by the Company may require approvals or clearances from the U.S. Food and Drug Administration (“FDA”) or other regulatory agencies prior to commercial sales. There can be no assurance that the Company’s drug candidates will receive any of the required approvals or clearances. If the Company was to be denied approval or clearance or any such approval or clearance was to be delayed, it would have a material adverse impact on the Company. Cash and Cash Equivalents The Company considers all highly liquid investments with a maturity of three months or less at the time of purchase to be cash equivalents. Investments Available-for-sale investments. The Company’s investments consist of U.S. Treasury securities, agency bonds, and money market funds. The Company designates all investments as available-for-sale and therefore reports them at fair value, based on quoted marked prices, with unrealized gains and losses recorded in accumulated other comprehensive loss. The cost of securities sold is based on the specific-identification method. Investments with original maturities greater than three months and remaining maturities of one year or less are classified as short-term investments. Investments with remaining maturities greater than one year are classified as long-term investments. The amortized cost of debt securities in this category is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization is included in interest income. Recognized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in other income or expense. Interest and dividends on securities classified as available-for-sale are included in Interest and other, net. Other-than-temporary impairment. All of the Company’s available-for-sale investments are subject to a periodic impairment review. The Company recognizes an impairment charge when a decline in the fair value of its investments below the cost basis is judged to be other-than-temporary. Factors considered by management in assessing whether an other-than-temporary impairment has occurred include: the nature of the investment; whether the decline in fair value is attributable to specific adverse conditions affecting the investment; the financial condition of the investee; the severity and the duration of the impairment; and whether the Company has the intent and ability to hold the investment to maturity. When the Company determines that an other-than-temporary impairment has occurred, the investment is written down to its market value at the end of the period in which it is determined that an other-than-temporary decline has occurred. Property and Equipment, net Property and equipment are stated at cost less accumulated depreciation and are depreciated on a straight-line basis over the estimated useful lives of the related assets, which are generally three years for computer equipment and software, five years for laboratory equipment and office equipment, and seven years for furniture and fixtures. Amortization of leasehold improvements is computed using the straight-line method over the shorter of the remaining lease term or the estimated useful life of the related assets, typically ranging from three to seven years. Upon sale or retirement of assets, the costs and related accumulated depreciation and amortization are removed from the balance sheet and the resulting gain or loss is reflected in operations. Impairment of Long-lived Assets Long-lived assets, the Company reviews long-lived assets, including property and equipment, are reviewed for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Impairment is measured as the amount by which the carrying amount of a long-lived asset exceeds its fair value. The Company would recognize an impairment loss when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are materially less than its carrying amount. Revenue Recognition The Company recognizes revenue after the following criteria are met: (i) persuasive evidence of an arrangement exists; (ii) delivery has occurred or services have been rendered; (iii) the fee is fixed or determinable; and (iv) collectability is reasonably assured. Determination of whether persuasive evidence of an arrangement exists and whether delivery has occurred or services have been rendered are based on management’s judgments regarding the fixed nature of the fee charged for research performed and milestones met, and the collectability of those fees. Should changes in conditions cause management to determine these criteria are not met for certain future transactions, revenue recognized for any reporting period could be adversely affected. Revenue under the Company’s strategic alliances are recognized based on the performance requirements of the alliance. Revenues may include research and development revenues earned for research and development activities, non-refundable license fees, milestones and royalties. In order to account for multiple element arrangements, we identify the deliverables at the inception of the arrangement and each deliverable within a multiple deliverable revenue arrangement is accounted for as a separate unit of accounting if both of the following criteria are met: (1) the delivered item or items have value to the customer on a standalone basis and (2) for an arrangement that includes a general right of return relative to the delivered items, delivery or performance of the undelivered items is considered probable and substantially in our control. A delivered item or items that do not qualify as a separate unit of accounting within the arrangement is combined with the other applicable undelivered items within the arrangement. For a combined unit of accounting, non-refundable upfront payments are recognized in a manner consistent with the final deliverable, generally ratably over the period we provide research and development services. If we determine that multiple deliverables exist for consideration received, the consideration is allocated to one or more units of accounting based upon the best estimate of the selling price (“BESP”), third-party evidence (“TPE”), or vendors specific objective evidence (“VSOE”) of each deliverable. The selling price used for each deliverable is based on vendor-specific objective evidence, if available, third-party evidence if vendor-specific objective evidence is not available, or estimated selling price if neither vendor-specific or third-party evidence is available. Amounts received in advance of our service performance are recorded as deferred revenue and are recognized as revenue as we perform services over estimated performance period. We review the estimated periods of performance. Our estimates of our performance period may change over the course of the collaboration term. Such a change in a current period could have a material impact on the amount of revenue we recognize in current and future periods. Payments that are contingent upon achievement of a substantive event that can only be achieved based on our performance and there is substantive uncertainty about whether the event will be achieved at the inception of the arrangement, commonly referred to as a milestone, are recognized as revenue in their entirety in the period in which the milestone is achieved. Payments for a milestone must relate solely to prior performance, be reasonable relative to all of the deliverables and payment terms within the agreement and commensurate with our performance to achieve the milestone after commencement of the agreement. Payments contingent upon achievement of events that are not considered substantive milestones are allocated to the respective arrangements unit of accounting when received and recognized as revenue based on the revenue recognition policy for that unit of accounting. Other contingent event-based payments received for which payments are the result of a collaborative partner’s performance are not considered milestones and recognized when the four criteria are met. Research and development revenues and cost reimbursements are based upon negotiated rates for the Company’s full-time employee equivalents (“FTE”) and actual out-of-pocket costs. FTE rates are set based upon the Company’s costs, and which the Company believes approximate fair value. None of the revenues recognized to date are refundable if the relevant research effort is not successful. In arrangements in which both parties make payments to each other, the Company evaluates the payments for arrangements under which consideration is given to determine whether payments made by us will be recognized as a reduction of revenue or as expense. Revenue may be reduced by payments made by us to another party unless the Company receives a separate and identifiable benefit in exchange for the payments and the Company can reasonably estimate the fair value of the benefit received. In arrangements in which the Company is the primary obligor, the Company records payments from the other party as research and development revenue. If the Company is not the primary obligor, the Company records payments as a reduction of revenue. Funds received from third parties under grant arrangements may be treated as revenue if the Company is deemed to be the principal participant in the grant arrangement where the activities under the grant are part of the Company’s development program. Otherwise, the funds received are recognized as a reduction to research and development expense. Non-refundable grant funds received are recognized when the related qualified research and development costs are incurred. Funds received in advance are deferred revenue. Preclinical Studies and Clinical Trial Accruals A substantial portion of the Company’s preclinical studies and all of the Company’s clinical trials have been performed by third-party contract research organizations (“CROs”) and other vendors and our accruals for expenses for preclinical studies and clinical trials may be significant. For preclinical studies, the significant factors used in estimating accruals include the percentage of work completed to date and contract milestones achieved. For clinical trial expenses, the significant factors used in estimating accruals include the number of patients enrolled, duration of enrollment, milestones achieved and percentage of work completed to date. The Company monitors patient enrollment levels and related activities to the extent practicable through internal reviews, correspondence and status meetings with CROs, and review of contractual terms. The Company depends on the timeliness and accuracy of data provided by its CROs and other vendors to accrue expenses. If the Company receives and relies on incomplete or inaccurate data accruals and expenses may be too high or too low at a given point in time and corresponding adjustments to accruals and expenses would be made in future periods when the actual expense becomes known. Research and Development Expenditures Research and development costs are charged to operations as incurred. Research and development expenses consist primarily of clinical manufacturing costs, preclinical study expenses, consulting and other third party costs, employee compensation, supplies and materials, allocation of overhead and occupancy costs, facilities costs and depreciation of equipment. Income Taxes The Company accounts for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to affect taxable income. Valuation allowances are established when necessary to reduce deferred tax assets to the amounts expected to be realized. The Company recognizes uncertain tax positions taken or expected to be taken on a tax return. Tax positions are initially recognized when it is more likely than not that the position will be sustained upon examination by the tax authorities. Such tax positions are initially and subsequently measured as the largest amount of tax benefit that is more likely than not of being realized upon ultimate settlement with the tax authority assuming full knowledge of the position and relevant facts. The Company recognizes interest accrued related to unrecognized tax benefits and penalties as income tax expense. Stock-Based Compensation The Company calculates non-cash stock-based compensation for stock-based awards made to employees and directors, at the grant date based on the calculated fair value of the award, and recognizes expense on a straight-line basis over the requisite service period, generally the vesting period of the award. Stock compensation for non-employees is measured at the fair value of the award for each period until the award is fully vested. Compensation cost for restricted stock awards that contain performance conditions is based on the grant date fair value of the award and compensation expense is recorded over the implicit or explicit requisite service period based on management’s best estimate as to whether it is probable that the shares awarded are expected to vest. The Company reviews the valuation assumptions at each grant date and, as a result, assumptions used to value awards in one period may differ significantly from another period. The assumptions used in estimating the fair value of share-based payment awards involve inherent uncertainties and the application of management judgment and represent management’s best estimates at the time the Company estimates the expected forfeiture rate and recognizes expense only for those shares expected to vest. If the actual forfeiture rate in the future is materially different from our estimate, stock-based compensation expense could be significantly different from what has been recorded in the current period. During 2017, the Company adopted ASU No. 2016-09, Stock Compensation on a modified retrospective approach. the Company recognizes all excess tax benefits and tax deficiencies as income tax expense or benefit in the income statement and recognizes previously unrecognized excess tax benefits upon adoption as a cumulative-effect adjustment in retained earnings. As of January 1, 2017, the Company recognized excess tax benefit of $0.7 million as an increase to deferred tax assets. This increase was fully offset by a valuation allowance. Accordingly, no cumulative-effect adjustment to retained earnings was recorded as of December 31, 2017. The Company estimates forfeitures expected to occur to determine stock-based compensation expense. The adoption of this aspect of the guidance did not have a material impact on our financial statements and disclosures. Non-Cash Interest Expense on Liability Related to Sale of Future Royalties The Company treated the Liability related to sale of future royalties as a debt financing, to be amortized under the effective interest rate method over the life of the related royalty stream. The Liability related to sale of future royalties and the debt amortization are based on the Company’s current estimates of future royalties expected to be paid over the life of the arrangement. The Company will periodically assess the expected royalty payments using a combination of internal projections and forecasts from external sources. To the extent the Company’s future estimates of future royalty payments are greater or less than its previous estimates or the estimated timing of such payments is materially different than its previous estimates, the Company will adjust the Liability related to sale of future royalties and prospectively recognize related non-cash interest expense. Prior Year’s Presentations Certain amounts in the prior year’s presentations have been reclassified to conform to the current presentation. These reclassifications had no effect on previously reported net income. Recent Accounting Pronouncements In August 2016, the FASB issued ASU 2016-15, ‘Statement of cash flows (Topic 230): Classification of certain cash receipts and cash payments’. ASU 2016-15 issued guidance to clarify how certain cash receipts and payments should be presented in the statement of cash flows. ASU 2016-15 is effective for annual and interim reporting periods beginning after December 15, 2017 and early adoption is permitted. The Company does not expect the adoption of this standard to have a material effect on its financial statements or disclosures. In June 2016, the FASB issued ASU 2016-13, ‘Financial Instruments — Credit Losses — Measurement of Credit Losses on Financial Instruments. ASU 2016-13 changes the impairment model for most financial assets and certain other instruments. ASU 2016-13 is effective for annual and interim reporting periods beginning after December 15, 2019. The Company is in the process of evaluating the impact the adoption of this standard would have on its financial statements and disclosures. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). ASU 2016-02 requires management to record right-to-use asset and lease liability on the statement of financial position for operating leases. ASU 2016-02 is effective for annual and interim reporting periods beginning on or after December 15, 2018 and the modified retrospective approach is required. The Company is in the process of evaluating the impact the adoption of this standard would have on its financial statements and disclosures. In March 2016, the FASB issued ASU 2016-09, Stock compensation (Topic 718). ASU 2016-09 simplifies various aspects of accounting for share-based payments and presentation in the financial statements. During the three months ended March 31, 2017, the Company adopted ASU No. 2016-09 on a modified retrospective approach. The guidance requires us to recognize all excess tax benefits and tax deficiencies as income tax expense or benefit in the income statement and recognize previously unrecognized excess tax benefits upon adoption as a cumulative-effect adjustment in retained earnings, which eliminates the need to track unrecognized excess tax benefits for both new and existing awards. As of January 1, 2017, the Company recognized excess tax benefit of $0.7 million as an increase to deferred tax assets related to tax loss carryover. However, the entire amount was offset by a full valuation allowance. Accordingly, no cumulative-effect adjustment to retained earnings was recorded as of December 31, 2017. The Company will maintain its current forfeiture policy to estimate forfeitures expected to occur to determine stock-based compensation expense. The adoption of this aspect of the guidance did not have a material impact on our financial statements and disclosures. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). ASU 2016-02 requires management to record right-to-use asset and lease liability on the statement of financial position for operating leases. ASU 2016-02 is effective for annual and interim reporting periods beginning on or after December 15, 2018 and modified retrospective approach is required. The Company is in the process of evaluating the impact the adoption of this standard would have on its financial statements and disclosures. In January 2016, the FASB issued ASU 2016-01, Financial instruments (Subtopic 825-10). ASU 2016-01 requires management to measure equity investments at fair value with changes in fair value recognized in net income. ASU 2016-01 is effective for annual and interim reporting periods beginning on or after December 15, 2017 and early adoption is not permitted. The Company does not expect the adoption of ASU 2016-01 to have a material effect upon its financial statements or disclosures. In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The ASU will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. The new standard will become effective and the Company will adopt the standard on January 1, 2018. The standard permits the use of either the modified retrospective method or full retrospective approach for all periods presented. The Company currently anticipates adopting the standard using the modified retrospective method. The Company has performed a preliminary assessment and continues to evaluate the impact of the pending adoption of the new revenue standard on its consolidated financial statements and has determined that the collaborations with both Amgen and Astellas are within its scope. Upon completion of the Company’s analysis, the Company will determine the cumulative effect of initially applying the new standard, including areas we expect to be impacted such as license-related revenue and payments in connection with the Co-invest Option. The Company is also currently updating its accounting policy and designing and implementing the necessary changes to processes and controls to account for revenue under the new standard and anticipates completing its implementation in connection with its first quarter 2018 interim financial statements.

Net (Loss) Income Per Share

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Net (Loss) Income Per Share

Note 2 — Net (Loss) Income Per Share

Basic net (loss) income per share is computed by dividing net (loss) income by the weighted average number of vested common shares outstanding during the period. Diluted net (loss) income per share is computed by giving effect to all potentially dilutive common shares, including outstanding stock options, unvested restricted stock, warrants, convertible preferred stock and shares issuable under the Company’s Employee Stock Purchase Plan (“ESPP”), by applying the treasury stock method. The following is the calculation of basic and diluted net (loss) income per share (in thousands, except per share data):

	Years Ended December 31,
	2017			2016		2015
Net (loss) income	$	(127,790	)	$	16,453	$	(37,501	)
Weighted-average shares used in computing net (loss) income per share — basic		49,404			39,943		38,814
Effect of dilutive securities:
Warrants to purchase common stock		—			2,019		—
Options to purchase common stock		—			409		—
Restricted stock units		—			181		—
Shares issuable related to the ESPP		—			9		—
Dilutive potential common shares		—			2,618		—
Weighted-average shares used in computing net (loss) income per share — diluted		49,404			42,561		38,814
Net (loss) income per share — basic	$	(2.59	)	$	0.41	$	(0.97	)
Net (loss) income per share — diluted	$	(2.59	)	$	0.39	$	(0.97	)

The following instruments were excluded from the computation of diluted net (loss) income per share for the periods presented because their effect would have been antidilutive (in thousands):

	December 31,
	2017		2016		2015
Options to purchase common stock		5,957		3,688		4,835
Warrants to purchase common stock		100		—		5,641
Restricted and Performance stock units		457		—		757
Shares issuable related to the ESPP		20		—		16
Total shares		6,534		3,688		11,249

Cash Equivalents and Investments

12 Months Ended

Dec. 31, 2017

Investments Debt And Equity Securities [Abstract]

Cash Equivalents and Investments

Note 3 — Cash Equivalents and Investments

Cash Equivalents and Available for Sale Investments

The amortized cost and fair value of cash equivalents and available for sale investments at December 31, 2017 and 2016 were as follows (in thousands):

	December 31, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents — U. S. Treasury securities and money market funds	$	111,501	$	—	$	—		$	111,501
Short-term investments — U.S. Treasury securities and Agency bonds	$	143,895	$	—	$	(210	)	$	143,685
Long-term investments — Equity and U.S. Treasury securities	$	16,538	$	—	$	(20	)	$	16,518

	December 31, 2016
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents — U. S. Treasury securities and money market funds	$	55,658	$	—	$	—		$	55,658
Short-term investments — U.S. Treasury securities	$	89,396	$	2	$	(23	)	$	89,375
Long-term investments — Equity and U.S. Treasury securities	$	7,513	$	176	$	(17	)	$	7,672

As of December 31, 2017, the Company’s long-term investments in U.S. Treasury securities have maturity dates less than 1.5 years. As of December 31, 2017, none of the investments were other-than temporarily impaired, no investment was in a continuous unrealized loss position for more than one year, unrealized losses were not due to change in credit risk and the Company believes investments with an unrealized loss would be held until maturity.

Fair Value Measurements

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value Measurements

Note 4 — Fair Value Measurements

The Company values its financial assets and liabilities at fair value, defined as the price that would be received for assets when sold or paid to transfer a liability in an orderly transaction between market participants at the measurement date (exit price). The Company utilizes market data or assumptions that the Company believes market participants would use in pricing the asset or liability, including assumptions about risk and the risks inherent in the inputs to the valuation technique. These inputs can be readily observable, market corroborated or generally unobservable.

The Company primarily applies the market approach for recurring fair value measurements and endeavors to utilize the best information reasonably available. Accordingly, the Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible, and considers the security issuers’ and the third-party insurers’ credit risk in its assessment of fair value.

The Company classifies the determined fair value based on the observability of those inputs. Fair value accounting guidance establishes a fair value hierarchy that prioritizes the inputs used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurement) and the lowest priority to unobservable inputs (Level 3 measurement). The three defined levels of the fair value hierarchy are as follows:

Level 1 — Observable inputs, such as quoted prices in active markets for identical assets or liabilities;

Level 2 — Inputs, other than the quoted prices in active markets, that are observable either directly or through corroboration with observable market data; and

Level 3 — Unobservable inputs, for which there is little or no market data for the assets or liabilities, such as internally-developed valuation models.

Financial assets measured at fair value on a recurring basis as of December 31, 2017 and 2016 are classified in the table below in one of the three categories described above (in thousands):

	December 31, 2017
	Fair Value Measurements Using						Assets
	Level 1		Level 2		Level 3		At Fair Value
Assets:
Money market funds	$	51,001	$	—	$	—	$	51,001
U.S. Treasury securities		165,801		—		—		165,801
Agency bonds		—		54,329				54,329
Equity securities		573		—		—		573
Total	$	217,375	$	54,329	$	—	$	271,704
Amounts included in:
Cash and cash equivalents	$	111,501	$	—	$	—	$	111,501
Short-term investments		89,356		54,329		—		143,685
Long-term investments		16,518		—		—		16,518
Total	$	217,375	$	54,329	$	—	$	271,704

	December 31, 2016
	Fair Value Measurements Using						Assets
	Level 1		Level 2		Level 3		At Fair Value
Assets:
Money market funds	$	52,657	$	—	$	—	$	52,657
U.S. Treasury securities		99,872		—		—		99,872
Equity securities		176		—		—		176
Total	$	152,705	$	—	$	—	$	152,705
Amounts included in:
Cash and cash equivalents	$	55,658	$	—	$	—	$	55,658
Short-term investments		89,375		—		—		89,375
Long-term investments		7,672		—		—		7,672
Total	$	152,705	$	—	$	—	$	152,705

The carrying amount of the Company’s accounts receivable and accounts payable approximates fair value due to the short-term nature of these instruments.

Fair value of financial liabilities:

As of December 31, 2017 and December 31, 2016, the fair value of the long-term debt, payable in installments through year ended 2020, approximated its carrying value of $31.8 million and $29.9 million, respectively, because it is carried at a market observable interest rate, which are considered Level 2.

As of December 31, 2017, the fair value of liability related to the sale of future royalties is based on the Company’s current estimates of future royalties expected to be paid to RPI Finance Trust (“RPI”), an entity related to Royalty Pharma, over the life of the arrangement, which are considered Level 3 (See Note 9 – “Liability Related to Sale of Future Royalties”).

There were no transfers between Level 1, Level 2, and Level 3 during the periods presented.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Balance Sheet Components

12 Months Ended

Dec. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Balance Sheet Components

Note 5 — Balance Sheet Components

Property and equipment balances were as follows (in thousands):

	December 31,
	2017			2016
Property and equipment, net:
Laboratory equipment	$	17,100		$	16,742
Computer equipment and software		2,890			2,699
Office equipment, furniture and fixtures		1,137			856
Leasehold improvements		5,067			4,458
Total property and equipment		26,194			24,755
Less: Accumulated depreciation and amortization		(22,626	)		(21,118	)
Total property and equipment, net	$	3,568		$	3,637

Depreciation expense was $1.9 million, $0.7 million and $0.6 million for the years ended December 31, 2017, 2016, and 2015 respectively.

Accrued liabilities were as follows (in thousands):

	December 31,
	2017		2016
Accrued liabilities:
Clinical and preclinical costs	$	8,370	$	10,092
Bonus		4,054		3,800
Other payroll related		2,207		1,888
Other accrued expenses		1,426		897
Consulting and professional fees		1,335		698
Leasehold improvements		—		672
Total accrued liabilities	$	17,392	$	18,047

The Company sponsors a 401(k) defined contribution plan covering all employees. In 2017, 2016 and 2015, employer contributions to the 401(k) plan were $0.5 million, $0.5 million and $0.4 million, respectively.

Research and Development Arrangements

12 Months Ended

Dec. 31, 2017

Research And Development [Abstract]

Research and Development Arrangements

Note 6 — Research and Development Arrangements

Amgen Inc. (“Amgen”)

The Company and Amgen collaborate on a worldwide basis to discover, develop and commercialize novel small molecule therapeutics, including omecamtiv mecarbil, that activate cardiac muscle contractility for potential applications in the treatment of heart failure under the Collaboration and Option Agreement dated December 29, 2006, as amended (the “Amgen Agreement”). Amgen is responsible for the development and commercialization of omecamtiv mecarbil and related compounds at its expense worldwide, subject to the Company’s development and commercialization participation rights. The Company recognizes research and development revenue from Amgen for reimbursement of internal costs of certain full-time employee equivalents, supporting a collaborative research program directed to the discovery of next-generation cardiac sarcomere activator compounds and the development program for omecamtiv mecarbil, and other costs related to the research and development program.

In 2016, Amgen and Les Laboratories Servier and Institut de Researches Servier (“Servier”) announced Servier’s decision to exercise its option to commercialize omecamtiv mecarbil in Europe as well as the Commonwealth of Independent States (“CIS”), including Russia. The option and related commercialization sublicense to Servier is subject to the terms and conditions of the Amgen Agreement. Amgen remains responsible for the performance of its obligations under the Amgen Agreement, including the payment of milestones and royalties relating to the development and commercialization of omecamtiv mecarbil in Europe and the CIS.

Under the Amgen Agreement, the Company is eligible to receive development milestone payments which are based on various clinical milestones, including the initiation of certain clinical studies, the submission of an application for marketing authorization for a drug candidate to certain regulatory authorities and the receipt of such approvals. Additionally, the Company is eligible to receive commercial milestone payments provided certain sales targets are met. Due to the nature of drug development, including the inherent risk of development and approval of drug candidates by regulatory authorities, it is not possible to estimate if and when these milestone payments could be achieved or become due. The achievement of each of these milestones is dependent upon the results of Amgen’s development and commercialization activities.

The Company provided notice of its exercise of its option to co-invest $40.0 million in the Phase 3 development program of omecamtiv mecarbil in exchange for a total incremental royalty from Amgen of up to 4% on increasing worldwide sales of omecamtiv mecarbil outside Japan (the “Co-invest Option”). Through December 31, 2017, the Company paid $21.3 million of the $40.0 million co-investment and the remaining $18.8 million is scheduled to be paid quarterly through the third calendar quarter of 2018. Because these Co-Investment Option payments are contingent on Amgen continuing the development program of omecamtiv mecarbil and the benefit to be received in exchange for these payments are not sufficiently separable from the Amgen Agreement, the payments made are recorded as contra-revenue to research and development revenues.

Revenue from Amgen

Revenue from Amgen for the years ended December 31, 2017, 2016, and 2015 was as follows (in thousands):

	Years Ended December 31,
	2017			2016			2015
Research and development revenues
Reimbursement of internal costs	$	1,279		$	2,466		$	2,460
Research and development milestone fees		11,000			26,666			—
Co-invest option payment		(20,000	)		(1,250	)		—
Allocated consideration		—			—			21
Total net revenues from Amgen	$	(7,721	)	$	27,882		$	2,481

Accounts receivable due from Amgen was $1.0 million at December 31, 2017 and zero at December 31, 2016.

During the year ended December 31, 2017, the Company recognized $11.0 million in milestone fees, consisting of $10.0 million related to the start in Japan of GALACTIC-HF, the Phase 3 cardiovascular outcomes clinical trial of omecamtiv mecarbil and $1.0 million related to a next-generation cardiac muscle activator that was nominated as a development candidate by the Joint Research Committee. During the year ended December 31, 2016, the Company recognized $26.7 million in development milestone fees related to the start of GALACTIC-HF as the Company has no remaining deliverables under the Amgen Agreement.

Prior to April 1, 2017, the Company considered Amgen to be a related party, due in part to Amgen’s equity ownership percentage, and reported revenue under the Amgen Agreement as revenues from a related party. Effective April 1, 2017, in part due to a decrease in Amgen’s equity ownership percentage, the Company no longer considers Amgen to be a related party.

Astellas Pharma Inc. (“Astellas”)

In 2013, the Company and Astellas entered into a license and collaboration agreement under which the Company granted Astellas an exclusive license to co-develop and jointly commercialize reldesemtiv, a fast skeletal muscle troponin activator (“FSTA”), for potential application in non-neuromuscular indications worldwide (the “Original Astellas Agreement”). In 2014, the Company and Astellas amended and restated the license and collaboration agreement and expanded the objective of the collaboration to include spinal muscular atrophy (“SMA”) and potentially other neuromuscular indications for reldesemtiv and other FSTAs, in addition to the non-neuromuscular indications provided for in the Original Astellas Agreement (the “2014 Astellas Agreement”). In 2016, Cytokinetics and Astellas further amended the collaboration agreement to expand the collaboration to include the development of reldesemtiv for the potential treatment of ALS, as well as the possible development in ALS of other FSTAs previously licensed by us to Astellas (“2016 Astellas Amendment”). Together, these agreements and amendments are referred to as the Astellas Agreement.

In collaboration with Astellas, the Company is conducting two Phase 2 clinical trials of reldesemtiv, one in patients with spinal muscular atrophy (“SMA”) and one in patients with amyotrophic lateral sclerosis (“ALS”), called FORTITUDE-ALS (Functional Outcomes in a Randomized Trial of Investigational Treatment with CK-2127107 to Understand Decline in Endpoints – in ALS).

The Company and Astellas share equally the costs of developing reldesemtiv in ALS for potential registration and marketing authorization in the U.S. and Europe, provided that (i) Astellas has agreed to solely fund Phase 2 development costs of reldesemtiv in ALS subject to a right to recoup the Company’s share of such costs plus a 100% premium by reducing future milestone and royalty payments to the Company and (ii) the Company may defer (but not eliminate) a portion of its co-funding obligation for development activities after Phase 2 for up to 18 months, subject to certain conditions.

In the years ended December 31, 2017, 2016 and 2015, the Company has recognized research and development revenue from Astellas for reimbursements of internal costs of certain full-time employee equivalents supporting collaborative research and development programs, and of other costs related to those programs.

In 2015, in connection with the 2014 Astellas Agreement, Astellas paid the Company a $30 million non-refundable upfront license fee and a $15.0 million milestone payment relating to Astellas’ decision to advance reldesemtiv into Phase 2 clinical development. The Company determined that the license and did not have stand-alone value and that the research and development services relating to the 2014 Astellas Agreement are a single unit of accounting. Accordingly, the Company recognizes this license fee as revenue over the specified term of the deliverables using the proportional performance model.

In 2016, in connection with the 2016 Astellas Amendment, Astellas paid the Company $50.0 million, consisting of a $35.0 million non-refundable upfront amendment fee (the “ALS License”) and an accelerated $15.0 million milestone payment for the initiation of the first Phase 2 clinical trial of reldesemtiv in ALS and committed consideration for additional research services (“Additional Research Services”) of $5.1 million and consideration for development services in ALS through Phase 2 activities (“ALS Development Services”) of $39.1 million, for total arrangement consideration of $94.2 million (the “Arrangement Consideration”).

In 2016, the Company considered the 2016 Astellas Amendment to be a modification of the 2014 Astellas Agreement. At that time, the remaining deliverables under the 2014 Astellas Agreement were: (1) the SMA license; (2) Research Services in connection with the Research Plan; and (3) SMA Development Services in connection with the Development Plan. The Company evaluated the components and consideration of the 2016 Astellas Amendment and determined that the 2016 deliverables had standalone value and are delivered at fair value. Therefore, no reallocation of consideration to the 2014 deliverables was performed.

The Company determined that the deliverables under the 2016 Astellas Amendment included the ALS License, the ALS Development Services and the Additional Research Services and that these three deliverables were two units of accounting with stand-alone value: (1) the ALS License and (2) the Additional Research Services and ALS Development Services (“Research and ALS Development Services”). The ALS License had stand-alone value because (i) Astellas received a worldwide license for ALS to perform further research in the field of ALS, to develop and use reldesemtiv and to make, have make, sell or otherwise commercialize reldesemtiv in ALS; (ii) Astellas received the right to sublicense the rights to reldesemtiv in ALS to a third party; and (iii) Astellas had the technical capabilities to advance further development on reldesemtiv in ALS without the continued involvement of the Company. The Company determined that the ALS Development Services and the Additional Research Services did not have standalone value and combined these two deliverables into one unit of accounting.

The Company allocated the Arrangement Consideration among the two units of accounting on a relative fair value basis using the best estimated selling price (“BESP”), as follows (in millions):

	Allocated Consideration		Upfront Revenue Recognition		Revenue Recognition over Performance Period
Units of Accounting:
ALS License	$	74.9	$	50.0	$	24.9
Research and ALS Development Services		19.3		—		19.3
Total consideration	$	94.2	$	50.0	$	44.2

The BESP of the ALS License was determined using a discounted cash flow, risk adjusted for probability of success. The BESP of the Research and ALS Development Services was determined using estimated research and development costs included in a research and development program plan approved by the Company and Astellas.

Since the $50 million upfront consideration was less than the $74.9 million consideration allocated to the ALS License, the Company recognized $50.0 million of upfront consideration as license revenue in 2016 and records license revenue for the remaining $24.9 million as an allocation from research and development services and $19.3 million as research and development revenues, as those research and development services are performed, using the proportional performance model over the development term, through the completion of the ALS Development Services.

Astellas’ Option on Tirasemtiv

In 2016, Astellas paid the Company a $15.0 million non-refundable option fee for an option for a global collaboration for the development and commercialization of tirasemtiv (the “Option on Tirasemtiv”).

While Astellas holds the Option on Tirasemtiv, the Company is responsible for the development of tirasemtiv at its own expense and retains the final decision making authority on the development of tirasemtiv. Therefore, the Company concluded that it had no obligation to Astellas related to any development services pursuant to the Option on Tirasemtiv.

If Astellas exercises the Option on Tirasemtiv:

•

the Company will grant Astellas an exclusive license to develop and commercialize tirasemtiv outside the Company’s own commercialization territory of North America, Europe and other select countries under a license and collaboration agreement for tirasemtiv (the “License on Tirasemtiv”). Each party would be primarily responsible for the further development of tirasemtiv in its territory and have the exclusive right to commercialize tirasemtiv in its territory.

•

the Company will receive an option exercise payment ranging from $25.0 million (if exercise occurs following receipt of data from VITALITY-ALS) to $80.0 million (if exercise occurs following receipt of FDA approval) and a milestone payment of $30.0 million from Astellas associated with the Company’s initiation of the open-label extension trial for tirasemtiv (VIGOR-ALS). If Astellas exercises the option after the defined review period following receipt of data from VITALITY-ALS, Astellas will at the time of option exercise reimburse the Company for a share of any additional costs incurred after such review period.

•

the parties will share the future development costs of tirasemtiv in North America, Europe and certain other countries (with Cytokinetics bearing 75% of such shared costs and Astellas bearing 25% of such costs), and Astellas will be solely responsible for the development costs of tirasemtiv specific to its commercialization territory.

Contingent upon the successful development of tirasemtiv, the Company may receive from Astellas milestone payments up to $100.0 million for the initial indication and up to $50.0 million for each subsequent indication. If tirasemtiv is commercialized, Astellas will pay the Company royalties (at rates ranging from the mid-teens to twenty percent) on sales of tirasemtiv in Astellas’ territory, and the Company will pay Astellas royalties (at rates up to the mid-teens) on sales of tirasemtiv in the Company’s territory, in each case subject to various possible adjustments.

In 2016, the Company concluded that the Option on Tirasemtiv was a substantive option, and is therefore not considered a deliverable at the execution of the 2016 Astellas Amendment. The Company determined that the License on Tirasemtiv is contingent upon the exercise of the Option on Tirasemtiv, and is therefore not effective during the periods presented, since the option has not been exercised as of the latest balance sheet date. In addition, the Company did evaluate the consideration set to be received for the License on Tirasemtiv in relation to the fair value of the License on Tirasemtiv, and determined that it was not being provided at a significant incremental discount.

The Company further determined that the option fee of $15.0 million was deemed to be a prepayment towards the License on Tirasemtiv, and therefore deferred revenue recognition of the option fee either until the Option on Tirasemtiv is exercised or expires unexercised. Unless exercised, the Option on Tirasemtiv expires following the receipt of the approval letter for tirasemtiv from the FDA. If the Option on Tirasemtiv expires unexercised, the $15.0 million received would be added to the 2016 Astellas Amendment consideration, to be allocated to the units of accounting.

Revenue and deferred revenue from Astellas

Research and development revenue from Astellas in the years ended December 31, 2017, 2016 and 2015 was as follows (in thousands):

	Year Ended
	December 31, 2017		December 31, 2016		December 31, 2015
License revenues	$	8,799	$	62,171	$	13,918
Research and development revenues		11,934		15,110		12,184
Total Revenue from Astellas	$	20,733	$	77,281	$	26,102

Deferred Revenue reflecting the unrecognized portion of the license revenue, option fee and payment of expenses from the Astellas Agreement was as follows (in thousands):

December 31,

2017

December 31,

2016

Deferred revenue, current

9,572

8,060

Deferred revenue, non-current

15,000

Under the Astellas Agreement, additional research and early and late state development milestone payments which are based on various research and clinical milestones, including the initiation of certain clinical studies, the submission of an application for marketing authorization for a drug candidate to certain regulatory authorities and the commercial launch of collaboration products could total over $600.0 million, including up to $95.0 million relating to reldesemtiv in non-neuromuscular indications, and over $100.0 million related to reldesemtiv in each of SMA, ALS and other neuromuscular indications. Additionally, $200.0 million in commercial milestones could be received under the Astellas Agreement provided certain sales targets are met. The achievement of each of the late stage development milestones and the commercialization milestones were determined to be dependent solely upon the results of Astellas’ development activities and therefore these potential milestone payments were not deemed to be substantive. The Company is eligible to receive up to $2.0 million in research milestone payments under the collaboration for each future potential drug candidate. The Company believes that each of the milestones related to research under the Astellas Agreement is substantive and can only be achieved with the Company’s past and current performance. Due to the nature of drug development, including the inherent risk of development and approval of drug candidates by regulatory authorities, it is not possible to estimate if and when these milestone payments could be achieved or become due.

Other Research and Development Revenue Arrangements Grants	12 Months Ended
	Dec. 31, 2017
Research And Development [Abstract]
Other Research and Development Revenue Arrangements Grants	Note 7 — Other Research and Development Revenue Arrangements Grants In July 2015, The ALS Association (the “ALSA Grant”) awarded to the Company a $1.5 million grant to support the conduct of VITALITY-ALS as well as the collection of clinical data and plasma samples from patients in VITALITY-ALS in order to help advance the discovery of potentially useful biomarkers in ALS. On August 28, 2015, the Company achieved its first milestone under the ALSA Grant which triggered a payment of $0.5 million in accordance with the ALSA Grant. The Company recorded $0.3 million, $1.1 million, and $0.1 million, as grant revenue as qualified expenses were incurred, for years ended December 31, 2017, 2016 and 2015, respectively. At December 31, 2017, the Company had no deferred revenue under the ALSA Grant, reflecting the unrecognized portion of the grant revenue.

Long-Term Debt

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Long-Term Debt

Note 8 — Long-Term Debt

Long-term debt and unamortized debt discount balances are as follows (in thousands):

	December 31,
	2017			2016
Notes payable, gross	$	32,000		$	30,000
Less: Unamortized debt discount		(325	)		(472	)
Accretion of final exit fee		102			353
Carrying value of notes payable		31,777			29,881
Less: Current portion of long-term debt		—			(2,500	)
Long-term debt	$	31,777		$	27,381

In October 2017, the Company entered into a Second Amendment to Loan and Security Agreement (the “Amended Loan Agreement”) with Oxford Finance LLC and Silicon Valley Bank to amend the Loan Agreement entered into in October 2015. Per the terms of the Amended Loan Agreement, upon closing, the Company immediately drew $32.0 million and retired the Company’s existing debt outstanding of $30.0 million under the existing Loan Agreement, and approximately $0.5 million related to the accrued portion of the final payment fee under the Loan Agreement. Payments on the new outstanding loan balance of $32.0 million will be interest only through May 2019, followed by 41 months of equal monthly payments of interest and principal. The Company will be required to make a final payment upon loan maturity of 6.5% of the amounts advanced. The interest rate under the Amended Loan Agreement is the greater of (a) 8.05% or (b) the sum of 6.81% plus the 30-day U.S. LIBOR rate.

In October 2015, the Company issued warrants to purchase 65,189 shares of the Company’s common stock at an exercise price of $6.90 and in February 2016, the Company issued warrants to purchase 68,285 shares of the Company’s common stock at an exercise price of $6.59 per share. In January 2017, the Company issued 16,126 shares of common stock related to cashless exercises of some of these warrants. The loan carries prepayment penalties of 3% and 2% for prepayment within one and two years, respectively, of the loan origination and 1% thereafter. The warrants issued in the Loan Agreement became exercisable upon issuance and will remain exercisable for five years from issuance or the closing of a merger consolidation transaction in which the Company is not the surviving entity.

The Loan Agreement contains customary representations and warranties and customary affirmative and negative covenants applicable to the Company and its subsidiaries, including, among other things, restrictions on dispositions, changes in business, management, ownership or business locations, mergers or acquisitions, indebtedness, encumbrances, distributions, investments, transactions with affiliates and subordinated debt. The Agreement also includes customary events of default, including but not limited to the nonpayment of principal or interest, violations of covenants, material adverse changes, attachment, levy, restraint on business, cross-defaults on material indebtedness, bankruptcy, material judgments, misrepresentations, subordinated debt, governmental approvals, lien priority and delisting. Upon an event of default, the Lenders may, among other things, accelerate the loans and foreclose on the collateral. The Company’s obligations under the Agreement are secured by substantially all of the Company’s current and future assets, other than its intellectual property.

Future minimum payments under the Loan, as of December 31, 2017 are as follows (in thousands):

2018	$	2,615
2019		7,963
2020		11,187
2021		10,417
2022		10,176
Total minimum payments		42,358
Less: Interest and final payment		(10,358	)
Notes payable, gross	$	32,000

Liability Related to Sale of Future Royalties

12 Months Ended

Dec. 31, 2017

Deferred Revenue Disclosure [Abstract]

Note 9 - Liability Related to Sale of Future Royalties

In February 2017, the Company entered into a Royalty Purchase Agreement (the “Royalty Agreement”) with RPI, an entity related to Royalty Pharma. Under the Royalty Agreement, the Company sold a portion of the Company’s right to receive royalties on potential net sales of omecamtiv mecarbil (and potentially other compounds with the same mechanism of action) under the Amgen Agreement to RPI for a payment of $90.0 million (the “Royalty Monetization”). The Royalty Monetization is non-refundable, even if omecamtiv mecarbil is never commercialized. The Company accounts for the Royalty Monetization as a liability reported as Liability related to sale of future royalties, primarily because the Company has significant continuing involvement in generating the royalty stream under the Amgen Agreement, including the Company’s option to co-invest in the Phase 3 development program of omecamtiv mecarbil. The Liability related to sale of future royalties is accreted to the expected future cash flows using the interest method at an effective pre-tax annual interest rate of approximately 17%.

Also in February 2017, pursuant to a concurrently-executed Common Stock Purchase Agreement with RPI, the Company issued 875,656 shares of its common stock to RPI for $10.0 million (the “RPI Common Stock”). The Company determined the fair value of the RPI Common Stock at March 31, 2017 to be $8.1 million, based on the closing stock price at the transaction date and adjusted for the trading restrictions.

The Company concluded that there are two units of accounting for the Royalty Monetization and the RPI Common Stock: (1) the liability related to sale of future royalties and (2) the RPI Common Stock. The Company allocated the $90 million from the Royalty Monetization and the $10 million from the RPI Common Stock among the two units of accounting on a relative fair value basis. The Company determined the fair value for the liability related to sale of future royalties at the time of the Royalty Monetization to be $96.7 million, including the then statutory tax rate of 35%. As of December 31, 2017, the Company determined the fair value should be increased to $131.6 million due to the new statutory effective tax rate of 21%.

At the time of the Royalty Monetization, the Company allocated the transaction consideration on a relative fair value basis to the liability and the common stock, as follows (in millions):

	Allocated Consideration
Units of Accounting:
Liability related to sale of future royalties	$	92.3
Common stock		7.7
Total consideration	$	100.0

The Company allocated $1.8 million of transaction costs incurred in connection with the Royalty Monetization and the RPI Common Stock to the liability and common stock in proportion to the allocation of proceeds to those components. The transaction costs allocated to the liability will be amortized to non-cash interest expense over the estimated term of the Royalty Agreement.

The following table shows the activity within liability related to sale of future royalties during the year ended December 31, 2017 (in thousands):

Liability related to sale of future royalties at February 1, 2017	$	92,300
Non-cash interest expense recognized		13,980
Liability related to sale of future royalties at December 31, 2017		106,280
Less: Unamortized transaction costs		(1,630	)
Carrying value of liability related to sale of future royalties at December 31, 2017		104,650

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

Note 10 — Commitments and Contingencies

Commitments

Operating Lease

The Company leases office space under a non-cancelable operating lease that expires in 2021. The lease terms provide for rental payments on a graduated scale and the Company’s payment of certain operating expenses. The Company recognizes rent expense on a straight-line basis over the lease period.

Rent expense was as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Rent expense	$	3,627	$	3,448	$	3,297

As of December 31, 2017, future minimum lease payments under noncancelable operating leases were as follows (in thousands):

2018	$	3,789
2019		4,682
2020		4,846
2021		2,465
Total	$	15,782

Co-investment option

The Company provided notice to Amgen of its exercise of its option under the Amgen Agreement to fully co-invest $40.0 million in the Phase 3 development program of omecamtiv mecarbil in exchange for a total incremental royalty from Amgen of up to 4% on increasing worldwide sales of omecamtiv mecarbil outside Japan and the right to co-promote omecamtiv mecarbil in institutional care settings in North America, with reimbursement by Amgen for certain sales force activities. Quarterly co-investment payments are contingent on Amgen continuing the Phase 3 development program of omecamtiv mecarbil. As of December 31, 2017, a total of $18.8 million is scheduled to be paid through the third calendar quarter of 2018.

Contingencies

In the ordinary course of business, the Company may provide indemnifications of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters, including, but not limited to, losses arising out of the Company’s breach of such agreements, services to be provided by or on behalf of the Company, or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with its directors and certain of its officers and employees that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors, officers or employees. The Company maintains director and officer insurance, which may cover certain liabilities arising from its obligation to indemnify its directors and certain of its officers and employees, and former officers and directors in certain circumstances. The Company maintains product liability insurance and comprehensive general liability insurance, which may cover certain liabilities arising from its indemnification obligations. It is not possible to determine the maximum potential amount of exposure under these indemnification obligations due to the limited history of prior indemnification claims and the unique facts and circumstances involved in each particular indemnification obligation. Such indemnification obligations may not be subject to maximum loss clauses. Management is not currently aware of any matters that could have a material adverse effect on the financial position, results of operations or cash flows of the Company.

In December 2014, the Company filed a lawsuit alleging fraudulent inducement, breach of contract and negligence on the part of a contract research organization for BENEFIT-ALS. In 2016, the Company received $4.5 million related to the settlement with that contract research organization and classified the payment as a reduction of R&D expense.

Stockholders' Equity

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Stockholders' Equity

Note 11 — Stockholders’ Equity

Committed Equity Offering

In September 2015, the Company entered into a Committed Equity Offering (an “CE Offering”) that is an at-the-market issuance sales agreement (the “Cantor Fitzgerald Agreement”) with Cantor Fitzgerald & Co. During 2015, the Company issued 808,193 shares under the CE Offering for total net proceeds of $8.9 million. During 2017, the Company issued 2,425,625 shares of common stock under the Cantor Fitzgerald Agreement for net proceeds totaling $29.9 million and completed the CE offering.

Warrants

Pursuant to the Loan Agreement described in Note 8 “Long Term Debt,” the Company issued warrants to purchase 65,189 shares of the Company’s common stock at an exercise price of $6.90 per share and additional warrants to purchase 68,285 shares of the Company’s common stock at an exercise price of $6.59 per share. In January 2017, the Company issued 16,126 shares of common stock related to cashless exercises of some of these warrants. At December 31, 2017, 100,106 warrants with a weighted average exercise price of $6.74 per share were outstanding.

In June 2012, the Company issued warrants with expiration in June 2017 pursuant to public offerings of our securities in 2012. In 2017 and 2016, the Company issued 3,450,122 and 834,998 shares of common stock for exercises of these warrants, respectively.

Equity Incentive Plan

The Company’s amended and restated 2004 Equity Incentive Plan (the “2004 Plan”) provides for the granting of incentive stock options, nonstatutory stock options, restricted stock, stock appreciation rights, restricted stock units, performance shares and performance units to employees, directors and consultants. Options may be granted at prices not lower than 100% of the fair market value of the common stock on the date of grant for nonstatutory stock options and incentive stock options and may be granted for terms of up to ten years from the date of grant. Options granted to new employees generally vest 25% after one year and monthly thereafter over a period of four years. Options granted to existing employees generally vest monthly over a period of four years. At the May 2017 Annual Meeting of Stockholders, the number of shares of common stock authorized for issuance under the 2004 Plan was increased by 3.9 million. As of December 31, 2017, there were 3.8 million shares of common stock reserved and available for issuance under the 2004 Plan.

Stock Options

Stock Option Activity under the Equity Incentive Plan was as follows:

	Stock Options Outstanding			Weighted Average Exercise Price per Share - Stock Options		Weighted Average Remaining Contractual Life		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2016		5,192,813		$	9.27
Options granted		1,310,674			11.74
Options exercised		(264,164	)		7.26
Options forfeited/expired		(281,865	)		24.45
Balance at December 31, 2017		5,957,458		$	9.19		6.61	$	4,287
Exercisable at December 31, 2017		4,019,836		$	9.01		5.65	$	3,300
Vested and expected to vest as of December 31, 2017		5,866,326		$	9.18		6.57	$	4,250

Total intrinsic value of stock options exercised, calculated as the difference between the market value at the date of exercise and the exercise price of the shares, was $1.8 million, $0.2 million, and $0.1 million during the years ended December 31, 2017, 2016, and 2015, respectively. The market value as of December 31, 2017 was $8.15 per share as reported by NASDAQ. The weighted average grant date fair value of stock options granted was $7.95, $4.77 and $5.35 per share during the years ended December 31, 2017, 2016, and 2015, respectively.

The grant date fair value of option shares vested was $6.5 million, $4.9 million and $3.6 million in 2017, 2016 and 2015, respectively.

Restricted Stock Units

Restricted stock unit activity in 2017 was as follows:

	Number of Shares			Weighted Average Award Date Fair Value per Share
Unvested restricted stock units outstanding at December 31, 2016		64,502			7.19
Restricted stock units granted		269,000			10.60
Restricted stock units released		(43,500	)		6.67
Restricted stock units forfeited		(4,500	)		9.73
Unvested restricted stock units outstanding at December 31, 2017		285,502			10.44

Restricted stock units generally vest monthly over 48 months. For 2017, the fair value of restricted stock units vested, calculated based on the units vested multiplied by the closing price of the Common Stock on the date of vesting, was $1.5 million.

Share-based Awards that Contain Performance Conditions (“Performance Units”)

Performance unit activity in 2017 was as follows:

	Number of Shares			Weighted Average Award Date Fair Value per Share
Performance units outstanding at December 31, 2016		685,000		$	7.00
Performance units granted		—			—
Performance units released		(171,250	)		7.00
Performance units forfeited		(342,500	)		7.00
Performance units outstanding at December 31, 2017		171,250		$	7.00

During 2015, the Company granted 685,000 Performance units with a grant date fair value of $7.00 per share. In 2017, performance criteria for the 342,500 Performance units were met, 171,250 of those units vested, and while remaining 171,250 are expected to vest in March 2018. In 2017, 342,500 Performance units were forfeited when the Company determined that the performance criteria for those units would not be met.

Valuation Assumptions

The Company uses the Black-Scholes option pricing model to determine the fair value of stock option grants to employees and directors and employee stock purchase plan shares. The key input assumptions used to estimate fair value of these awards include the exercise price of the award, the expected option term, the expected volatility of the Company’s stock over the option’s expected term, the risk-free interest rate over the option’s expected term, and the Company’s expected dividend yield, if any.

The fair value of share-based payments was estimated on the date of grant using the Black-Scholes option pricing model based on the following weighted average assumptions:

	Year Ended December 31, 2017						Year Ended December 31, 2016						Year Ended December 31, 2015
	Employee Stock Options			ESPP			Employee Stock Options			ESPP			Employee Stock Options			ESPP
Risk-free interest rate		2.2	%		1.3	%		1.9	%		0.5	%		1.7	%		0.3	%
Volatility		74.0	%		74.0	%		74.0	%		74.0	%		79.4	%		75.3	%
Expected term in years		6.52			0.50			6.44			0.50			6.38			0.56
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%		0.0	%		0.0	%

The risk-free interest rate that the Company uses in the option pricing model is based on the U.S. Treasury zero-coupon issues with remaining terms similar to the expected terms of the options. The Company does not anticipate paying dividends in the foreseeable future and therefore uses an expected dividend yield of zero in the option pricing model. The Company is required to estimate forfeitures at the time of grant and revise those estimates in subsequent periods if actual forfeitures differ from those estimates. Historical data is used to estimate pre-vesting option forfeitures and record stock-based compensation expense only on those awards that are expected to vest.

The Company uses its own historical exercise activity and extrapolates the life cycle of options outstanding to arrive at its estimated expected term for new option grants. The Company uses its own volatility history based on its stock’s trading history for the expected term. The Company measures compensation expense for awards of restricted stock and restricted stock units at fair value on the date of grant and recognizes the expense over the expected vesting period. The fair value for restricted stock and restricted stock unit awards is based on the closing price of the Company’s common stock on the date of grant.

As of December 31, 2017, there was $11.1 million of unrecognized compensation cost related to unvested stock options, which is expected to be recognized over a weighted-average period of 2.4 years, and there was $2.1 million of unrecognized compensation cost related to unvested restricted stock and performance stock units, which is expected to be recognized over a weighted-average period of 1.3 years. The fair value for restricted stock units is based on the closing price of the Company’s common stock on the grant date.

Employee Stock Purchase Plans

Under the Company’s terminated 2004 Employee Stock Purchase Plan, employees purchased common stock of the Company up to a specified maximum amount at a price equal to 85% of the fair market value at certain plan-defined dates.

Under the Company’s 2015 Employee Stock Purchase Plan (the “2015 ESPP”) employees may purchase common stock of the Company up to a specified maximum amount at a price equal to 85% of the fair market value at certain plan-defined dates.

The Company issued 120,959, 129,604 and 21,167 shares of common stock during 2017, 2016 and 2015, respectively, pursuant to these plans at an average price of $9.65, $7.08 and $3.24 per share, in 2017, 2016 and 2015, respectively.

At December 31, 2017 the Company had 398,439 shares of common stock reserved for issuance under the 2015 ESPP.

Non-cash Stock-Based Compensation

The Company recognizes non-cash stock-based compensation expense for share-based payment awards made to employees, non-employees and directors, including employee stock options, and employee stock purchases. Under this guidance, stock-based compensation cost is measured at the grant date based on the calculated fair value of the award, and is recognized as an expense on a straight-line basis over the employee’s requisite service period, generally the vesting period of the award.

The following table summarizes non-cash stock-based compensation related to stock options, restricted stock unit, and restricted stock units that contain performance criteria, as well as activity under the 2004 (in thousands):

	Years Ended December 31,
	2017		2016		2015
Research and development	$	5,656	$	4,252	$	1,828
General and administrative		3,372		2,894		2,739
Stock-based compensation included in operating expenses	$	9,028	$	7,146	$	4,567

In connection with services rendered by non-employees, the Company recorded stock-based compensation expense of $532,000, $147,000, and $27,000 in 2017, 2016 and 2015, respectively.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Taxes

Note 12 — Income Taxes

The Company did not record an income tax provision in the years ended December 31, 2017, 2016, and 2015 because the Company either had net taxable losses or was able to utilize tax attributes to offset taxable income.

The following is a reconciliation of the statutory federal income tax rate to the Company’s effective tax rate:

	Years Ended December 31,
	2017			2016			2015
Tax at federal statutory tax rate		(34	)%		34	%		(34	)%
State income tax, net of federal tax benefit		—			2	%		0	%
State Apportionment		—			(7	)%		0	%
Tax credits (net)		(8	)%		(32	)%		(7	)%
Federal statutory rate reduction		51	%		—			—
Deferred tax assets (utilized) not benefited		(10	)%		(15	)%		37	%
Stock-based compensation		—			7	%		2	%
NOL Expiration		—			9	%		2	%
Other		1	%		2	%		0	%
Total		0	%		0	%		0	%

	Years Ended December 31,
	2017			2016		2015
United States	$	(127,235	)	$	16,453	$	(37,501	)
Foreign		(555	)		—		—
Total	$	(127,790	)	$	16,453	$	(37,501	)

Deferred tax assets reflect the net tax effect of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. The significant components of the Company’s deferred tax assets and liabilities were as follows (in thousands):

	As of December 31,
	2017			2016			2015
Deferred tax assets:
Net operating loss ("NOL") carryforwards	$	98,630		$	146,961		$	153,251
Tax credits		64,185			46,998			38,742
Liability related to sale of future royalties		24,593			—			—
Reserves and accruals		10,524			10,258			12,899
Capitalized R&D		6,432			11,675			13,150
Depreciation and amortization		546			766			769
Total deferred tax assets		204,910			216,658			218,811
Less: Valuation allowance		(204,910	)		(216,658	)		(218,811	)
Net deferred tax assets	$	—		$	—		$	—

At December 31, 2017, federal NOL carryforwards were $382.8 million and apportioned state NOL carryforwards before federal benefits were $244.8 million. If not utilized, the federal and state operating loss carryforwards will begin to expire in various amounts beginning 2022 and 2028, respectively.

At December 31, 2017, tax credits were $61.1 million and $14.7 million for federal and state income tax purposes, respectively and consisted of Research and Development Credits and Orphan Drug Credits. If not utilized, the federal carryforwards will expire in various amounts beginning in 2021. California based credit carryforwards do not expire.

Realization of deferred tax assets is dependent upon future earnings, if any, the timing and amount of which are uncertain. Based upon the weight of available evidence, which includes the Company’s historical operating performance, reported cumulative net losses since inception, expected future losses, and difficulty in accurately forecasting the Company’s future results and an assessment of both positive and negative evidence when determining whether it is more likely than not that deferred tax assets are recoverable, the Company maintained a full valuation allowance on the net deferred tax assets as of December 31, 2017, 2016, and 2015. The valuation allowance decreased by $11.7 million in 2017, decreased by $2.1 million in 2016, and increased by $13.9 million in 2015.

In general, under Section 382 of the Internal Revenue Code (“Section 382”), a corporation that undergoes an ‘ownership change’ is subject to limitations on its ability to utilize its pre-change net operating losses and tax credits to offset future taxable income. The Company does not believe it has experienced an ownership change since 2006. The Company expects a portion of its NOLs and tax credits from prior to 2007 will be subject to limitations under Section 382.

Activity related to the Company’s gross unrecognized tax benefits were (in thousands):

	Years Ended December 31,
	2017		2016
Balance at the beginning of the year	$	7,565	$	6,715
Decrease related to prior year tax positions		-		5
Increase related to current year tax positions		1,800		845
Balance at the end of the year	$	9,365	$	7,565

The significant jurisdictions in which the Company files income tax returns are the United States and California. The Company is subject to income tax examination for all fiscal years since inception. Included in the balance of unrecognized tax benefits as of December 31, 2017, 2016, and 2015 are $8.1 million, $6.3 million and $5.5 million of tax benefits, respectively, that, if recognized, would result in adjustments to other tax accounts, primarily deferred taxes.

Tax Reform

On December 22, 2017, the Tax Cuts and Jobs Act of 2017 (the “Tax Act”) was signed into law making significant changes to the Internal Revenue Code. Changes include, but are not limited to, a corporate tax rate decrease from 34% to 21% (the “Rate Reduction”) effective for tax years beginning after December 31, 2017. The Company reduced deferred tax assets at December 31, 2017 for the effect of the Rate Reduction. The Rate Reduction did not impact the Company's provision for income taxes for 2017 due to the full valuation allowance on deferred tax assets.

Staff Accounting Bulletin No. 118 ("SAB 118") was issued to address the application of US GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Act. The Company determined that $68.3 million of the reduction in deferred tax assets resulting from Rate Reduction was both provisional and a reasonable estimate at December 31, 2017. Additionally, the Company is still in the process of analyzing certain provisions of the Act including the application of new executive compensation limitation provisions under Internal Revenue Section 162(m). These items are subject to revisions from further analysis of the Tax Act and interpretation of any additional guidance issued by the U.S. Treasury Department, IRS, FASB, and other standard-setting and regulatory bodies.

Interest and Other Income, Net	12 Months Ended
Interest and Other Income, Net	Dec. 31, 2017
Other Income And Expenses [Abstract]
Interest and Other Income, Net	Note 13 — Interest and Other Income, Net Interest and other income, net for the years ended December 31, 2017, 2016, and 2015, primarily consisted of interest income generated from the Company’s cash, cash equivalents and investments.

Quarterly Financial Data

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Data

Note 14 — Quarterly Financial Data (Unaudited)

Quarterly results were as follows (in thousands, except per share data):

	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
2017
Total revenues, net	$	4,153		$	3,053		$	6,180		$	(18	)
Net loss		(25,867	)		(29,081	)		(32,357	)		(40,484	)
Net loss per share — basic and diluted	$	(0.62	)	$	(0.60	)	$	(0.60	)	$	(0.75	)
2016
Total revenues, net	$	8,421		$	5,802		$	59,047		$	33,138
Net income (loss)		(12,455	)		(11,611	)		33,362			7,157
Net income (loss) per share — basic	$	(0.31	)	$	(0.29	)	$	0.84		$	0.18
Net income (loss) per share —diluted	$	(0.31	)	$	(0.29	)	$	0.77		$	0.16

Organization and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	Organization Cytokinetics, Incorporated (the “Company”, “we” or “our”) was incorporated under the laws of the state of Delaware on August 5, 1997. The Company is a late stage biopharmaceutical company focused on the discovery and development of novel small molecule therapeutics that modulate muscle function for the potential treatment of serious diseases and medical conditions. The Company’s financial statements contemplate the conduct of the Company’s operations in the normal course of business. The Company has incurred an accumulated deficit of $646.1 million since inception and there can be no assurance that the Company will attain profitability. The Company had a net loss of $127.8 million and net cash used in operations of $101.8 million for the year ended December 31, 2017. Cash, cash equivalents and investments increased to $285.4 million at December 31, 2017 from $163.9 million at December 31, 2016. The Company anticipates that it will have operating losses and net cash outflows in future periods. The Company is subject to risks common to late stage biopharmaceutical companies including, but not limited to, development of new drug candidates, dependence on key personnel, and the ability to obtain additional capital as needed to fund its future plans. The Company’s liquidity will be impaired if sufficient additional capital is not available on terms acceptable to the Company. To date, the Company has funded its operations primarily through sales of its common stock and convertible preferred stock, contract payments under its collaboration agreements, sale of future royalties, debt financing arrangements, government grants and interest income. Until it achieves profitable operations, the Company intends to continue to fund operations through payments from strategic collaborations, additional sales of equity securities, grants and debt financings. The Company has never generated revenues from commercial sales of its drugs and may not have drugs to market for at least several years, if ever. The Company’s success is dependent on its ability to enter into new strategic collaborations and/or raise additional capital and to successfully develop and market one or more of its drug candidates. As a result, the Company may choose to raise additional capital through equity or debt financings to continue to fund its operations in the future. The Company cannot be certain that sufficient funds will be available from such a financing or through a collaborator when required or on satisfactory terms. Additionally, there can be no assurance that the Company’s drug candidates will be accepted in the marketplace or that any future products can be developed or manufactured at an acceptable cost. These factors could have a material adverse effect on the Company’s future financial results, financial position and cash flows. Based on the current status of its research and development plans, the Company believes that its existing cash, cash equivalents and investments will be sufficient to fund its cash requirements for at least the next 12 months after the issuance of the consolidated financial statements. If, at any time, the Company’s prospects for financing its research and development programs decline, the Company may decide to reduce research and development expenses by delaying, discontinuing or reducing its funding of one or more of its research or development programs. Alternatively, the Company might raise funds through strategic collaborations, public or private financings or other arrangements. Such funding, if needed, may not be available on favorable terms, or at all. The financial statements do not include any adjustments that might result from the outcome of this uncertainty.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of Cytokinetics and its wholly owned subsidiary and have been prepared in accordance with U.S. generally accepted accounting principles (“US GAAP”). Intercompany transactions and balances have been eliminated in consolidation.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject the Company to concentrations of risk consist principally of cash and cash equivalents, investments, long term debt and accounts receivable. The Company’s cash, cash equivalents and investments are invested in deposits with three major financial institutions in the United States. Deposits in these banks may exceed the amount of insurance provided on such deposits. The Company’s exposure to credit risk associated with non-payment is limited to its strategic partners Amgen Inc., (“Amgen”) and Astellas Pharma Inc., (“Astellas”) and any material non-payment from our partners would result in a material breach of the agreements underlying the strategic partnerships. Drug candidates developed by the Company may require approvals or clearances from the U.S. Food and Drug Administration (“FDA”) or other regulatory agencies prior to commercial sales. There can be no assurance that the Company’s drug candidates will receive any of the required approvals or clearances. If the Company was to be denied approval or clearance or any such approval or clearance was to be delayed, it would have a material adverse impact on the Company.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with a maturity of three months or less at the time of purchase to be cash equivalents.
Investments	Investments Available-for-sale investments. The Company’s investments consist of U.S. Treasury securities, agency bonds, and money market funds. The Company designates all investments as available-for-sale and therefore reports them at fair value, based on quoted marked prices, with unrealized gains and losses recorded in accumulated other comprehensive loss. The cost of securities sold is based on the specific-identification method. Investments with original maturities greater than three months and remaining maturities of one year or less are classified as short-term investments. Investments with remaining maturities greater than one year are classified as long-term investments. The amortized cost of debt securities in this category is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization is included in interest income. Recognized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in other income or expense. Interest and dividends on securities classified as available-for-sale are included in Interest and other, net. Other-than-temporary impairment. All of the Company’s available-for-sale investments are subject to a periodic impairment review. The Company recognizes an impairment charge when a decline in the fair value of its investments below the cost basis is judged to be other-than-temporary. Factors considered by management in assessing whether an other-than-temporary impairment has occurred include: the nature of the investment; whether the decline in fair value is attributable to specific adverse conditions affecting the investment; the financial condition of the investee; the severity and the duration of the impairment; and whether the Company has the intent and ability to hold the investment to maturity. When the Company determines that an other-than-temporary impairment has occurred, the investment is written down to its market value at the end of the period in which it is determined that an other-than-temporary decline has occurred.
Property and Equipment, net	Property and Equipment, net Property and equipment are stated at cost less accumulated depreciation and are depreciated on a straight-line basis over the estimated useful lives of the related assets, which are generally three years for computer equipment and software, five years for laboratory equipment and office equipment, and seven years for furniture and fixtures. Amortization of leasehold improvements is computed using the straight-line method over the shorter of the remaining lease term or the estimated useful life of the related assets, typically ranging from three to seven years. Upon sale or retirement of assets, the costs and related accumulated depreciation and amortization are removed from the balance sheet and the resulting gain or loss is reflected in operations.
Impairment of Long-lived Assets	Impairment of Long-lived Assets Long-lived assets, the Company reviews long-lived assets, including property and equipment, are reviewed for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Impairment is measured as the amount by which the carrying amount of a long-lived asset exceeds its fair value. The Company would recognize an impairment loss when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are materially less than its carrying amount.
Revenue Recognition	Revenue Recognition The Company recognizes revenue after the following criteria are met: (i) persuasive evidence of an arrangement exists; (ii) delivery has occurred or services have been rendered; (iii) the fee is fixed or determinable; and (iv) collectability is reasonably assured. Determination of whether persuasive evidence of an arrangement exists and whether delivery has occurred or services have been rendered are based on management’s judgments regarding the fixed nature of the fee charged for research performed and milestones met, and the collectability of those fees. Should changes in conditions cause management to determine these criteria are not met for certain future transactions, revenue recognized for any reporting period could be adversely affected. Revenue under the Company’s strategic alliances are recognized based on the performance requirements of the alliance. Revenues may include research and development revenues earned for research and development activities, non-refundable license fees, milestones and royalties. In order to account for multiple element arrangements, we identify the deliverables at the inception of the arrangement and each deliverable within a multiple deliverable revenue arrangement is accounted for as a separate unit of accounting if both of the following criteria are met: (1) the delivered item or items have value to the customer on a standalone basis and (2) for an arrangement that includes a general right of return relative to the delivered items, delivery or performance of the undelivered items is considered probable and substantially in our control. A delivered item or items that do not qualify as a separate unit of accounting within the arrangement is combined with the other applicable undelivered items within the arrangement. For a combined unit of accounting, non-refundable upfront payments are recognized in a manner consistent with the final deliverable, generally ratably over the period we provide research and development services. If we determine that multiple deliverables exist for consideration received, the consideration is allocated to one or more units of accounting based upon the best estimate of the selling price (“BESP”), third-party evidence (“TPE”), or vendors specific objective evidence (“VSOE”) of each deliverable. The selling price used for each deliverable is based on vendor-specific objective evidence, if available, third-party evidence if vendor-specific objective evidence is not available, or estimated selling price if neither vendor-specific or third-party evidence is available. Amounts received in advance of our service performance are recorded as deferred revenue and are recognized as revenue as we perform services over estimated performance period. We review the estimated periods of performance. Our estimates of our performance period may change over the course of the collaboration term. Such a change in a current period could have a material impact on the amount of revenue we recognize in current and future periods. Payments that are contingent upon achievement of a substantive event that can only be achieved based on our performance and there is substantive uncertainty about whether the event will be achieved at the inception of the arrangement, commonly referred to as a milestone, are recognized as revenue in their entirety in the period in which the milestone is achieved. Payments for a milestone must relate solely to prior performance, be reasonable relative to all of the deliverables and payment terms within the agreement and commensurate with our performance to achieve the milestone after commencement of the agreement. Payments contingent upon achievement of events that are not considered substantive milestones are allocated to the respective arrangements unit of accounting when received and recognized as revenue based on the revenue recognition policy for that unit of accounting. Other contingent event-based payments received for which payments are the result of a collaborative partner’s performance are not considered milestones and recognized when the four criteria are met. Research and development revenues and cost reimbursements are based upon negotiated rates for the Company’s full-time employee equivalents (“FTE”) and actual out-of-pocket costs. FTE rates are set based upon the Company’s costs, and which the Company believes approximate fair value. None of the revenues recognized to date are refundable if the relevant research effort is not successful. In arrangements in which both parties make payments to each other, the Company evaluates the payments for arrangements under which consideration is given to determine whether payments made by us will be recognized as a reduction of revenue or as expense. Revenue may be reduced by payments made by us to another party unless the Company receives a separate and identifiable benefit in exchange for the payments and the Company can reasonably estimate the fair value of the benefit received. In arrangements in which the Company is the primary obligor, the Company records payments from the other party as research and development revenue. If the Company is not the primary obligor, the Company records payments as a reduction of revenue. Funds received from third parties under grant arrangements may be treated as revenue if the Company is deemed to be the principal participant in the grant arrangement where the activities under the grant are part of the Company’s development program. Otherwise, the funds received are recognized as a reduction to research and development expense. Non-refundable grant funds received are recognized when the related qualified research and development costs are incurred. Funds received in advance are deferred revenue.
Preclinical Studies and Clinical Trial Accruals	Preclinical Studies and Clinical Trial Accruals A substantial portion of the Company’s preclinical studies and all of the Company’s clinical trials have been performed by third-party contract research organizations (“CROs”) and other vendors and our accruals for expenses for preclinical studies and clinical trials may be significant. For preclinical studies, the significant factors used in estimating accruals include the percentage of work completed to date and contract milestones achieved. For clinical trial expenses, the significant factors used in estimating accruals include the number of patients enrolled, duration of enrollment, milestones achieved and percentage of work completed to date. The Company monitors patient enrollment levels and related activities to the extent practicable through internal reviews, correspondence and status meetings with CROs, and review of contractual terms. The Company depends on the timeliness and accuracy of data provided by its CROs and other vendors to accrue expenses. If the Company receives and relies on incomplete or inaccurate data accruals and expenses may be too high or too low at a given point in time and corresponding adjustments to accruals and expenses would be made in future periods when the actual expense becomes known.
Research and Development Expenditures	Research and Development Expenditures Research and development costs are charged to operations as incurred. Research and development expenses consist primarily of clinical manufacturing costs, preclinical study expenses, consulting and other third party costs, employee compensation, supplies and materials, allocation of overhead and occupancy costs, facilities costs and depreciation of equipment.
Income Taxes	Income Taxes The Company accounts for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to affect taxable income. Valuation allowances are established when necessary to reduce deferred tax assets to the amounts expected to be realized. The Company recognizes uncertain tax positions taken or expected to be taken on a tax return. Tax positions are initially recognized when it is more likely than not that the position will be sustained upon examination by the tax authorities. Such tax positions are initially and subsequently measured as the largest amount of tax benefit that is more likely than not of being realized upon ultimate settlement with the tax authority assuming full knowledge of the position and relevant facts. The Company recognizes interest accrued related to unrecognized tax benefits and penalties as income tax expense.
Stock-Based Compensation	Stock-Based Compensation The Company calculates non-cash stock-based compensation for stock-based awards made to employees and directors, at the grant date based on the calculated fair value of the award, and recognizes expense on a straight-line basis over the requisite service period, generally the vesting period of the award. Stock compensation for non-employees is measured at the fair value of the award for each period until the award is fully vested. Compensation cost for restricted stock awards that contain performance conditions is based on the grant date fair value of the award and compensation expense is recorded over the implicit or explicit requisite service period based on management’s best estimate as to whether it is probable that the shares awarded are expected to vest. The Company reviews the valuation assumptions at each grant date and, as a result, assumptions used to value awards in one period may differ significantly from another period. The assumptions used in estimating the fair value of share-based payment awards involve inherent uncertainties and the application of management judgment and represent management’s best estimates at the time the Company estimates the expected forfeiture rate and recognizes expense only for those shares expected to vest. If the actual forfeiture rate in the future is materially different from our estimate, stock-based compensation expense could be significantly different from what has been recorded in the current period. During 2017, the Company adopted ASU No. 2016-09, Stock Compensation on a modified retrospective approach. the Company recognizes all excess tax benefits and tax deficiencies as income tax expense or benefit in the income statement and recognizes previously unrecognized excess tax benefits upon adoption as a cumulative-effect adjustment in retained earnings. As of January 1, 2017, the Company recognized excess tax benefit of $0.7 million as an increase to deferred tax assets. This increase was fully offset by a valuation allowance. Accordingly, no cumulative-effect adjustment to retained earnings was recorded as of December 31, 2017. The Company estimates forfeitures expected to occur to determine stock-based compensation expense. The adoption of this aspect of the guidance did not have a material impact on our financial statements and disclosures.
Non-Cash Interest Expense on Liabilities Related to Sale of Future Royalties	Non-Cash Interest Expense on Liability Related to Sale of Future Royalties The Company treated the Liability related to sale of future royalties as a debt financing, to be amortized under the effective interest rate method over the life of the related royalty stream. The Liability related to sale of future royalties and the debt amortization are based on the Company’s current estimates of future royalties expected to be paid over the life of the arrangement. The Company will periodically assess the expected royalty payments using a combination of internal projections and forecasts from external sources. To the extent the Company’s future estimates of future royalty payments are greater or less than its previous estimates or the estimated timing of such payments is materially different than its previous estimates, the Company will adjust the Liability related to sale of future royalties and prospectively recognize related non-cash interest expense.
Prior Year’s Presentations	Prior Year’s Presentations Certain amounts in the prior year’s presentations have been reclassified to conform to the current presentation. These reclassifications had no effect on previously reported net income.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In August 2016, the FASB issued ASU 2016-15, ‘Statement of cash flows (Topic 230): Classification of certain cash receipts and cash payments’. ASU 2016-15 issued guidance to clarify how certain cash receipts and payments should be presented in the statement of cash flows. ASU 2016-15 is effective for annual and interim reporting periods beginning after December 15, 2017 and early adoption is permitted. The Company does not expect the adoption of this standard to have a material effect on its financial statements or disclosures. In June 2016, the FASB issued ASU 2016-13, ‘Financial Instruments — Credit Losses — Measurement of Credit Losses on Financial Instruments. ASU 2016-13 changes the impairment model for most financial assets and certain other instruments. ASU 2016-13 is effective for annual and interim reporting periods beginning after December 15, 2019. The Company is in the process of evaluating the impact the adoption of this standard would have on its financial statements and disclosures. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). ASU 2016-02 requires management to record right-to-use asset and lease liability on the statement of financial position for operating leases. ASU 2016-02 is effective for annual and interim reporting periods beginning on or after December 15, 2018 and the modified retrospective approach is required. The Company is in the process of evaluating the impact the adoption of this standard would have on its financial statements and disclosures. In March 2016, the FASB issued ASU 2016-09, Stock compensation (Topic 718). ASU 2016-09 simplifies various aspects of accounting for share-based payments and presentation in the financial statements. During the three months ended March 31, 2017, the Company adopted ASU No. 2016-09 on a modified retrospective approach. The guidance requires us to recognize all excess tax benefits and tax deficiencies as income tax expense or benefit in the income statement and recognize previously unrecognized excess tax benefits upon adoption as a cumulative-effect adjustment in retained earnings, which eliminates the need to track unrecognized excess tax benefits for both new and existing awards. As of January 1, 2017, the Company recognized excess tax benefit of $0.7 million as an increase to deferred tax assets related to tax loss carryover. However, the entire amount was offset by a full valuation allowance. Accordingly, no cumulative-effect adjustment to retained earnings was recorded as of December 31, 2017. The Company will maintain its current forfeiture policy to estimate forfeitures expected to occur to determine stock-based compensation expense. The adoption of this aspect of the guidance did not have a material impact on our financial statements and disclosures. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). ASU 2016-02 requires management to record right-to-use asset and lease liability on the statement of financial position for operating leases. ASU 2016-02 is effective for annual and interim reporting periods beginning on or after December 15, 2018 and modified retrospective approach is required. The Company is in the process of evaluating the impact the adoption of this standard would have on its financial statements and disclosures. In January 2016, the FASB issued ASU 2016-01, Financial instruments (Subtopic 825-10). ASU 2016-01 requires management to measure equity investments at fair value with changes in fair value recognized in net income. ASU 2016-01 is effective for annual and interim reporting periods beginning on or after December 15, 2017 and early adoption is not permitted. The Company does not expect the adoption of ASU 2016-01 to have a material effect upon its financial statements or disclosures. In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The ASU will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. The new standard will become effective and the Company will adopt the standard on January 1, 2018. The standard permits the use of either the modified retrospective method or full retrospective approach for all periods presented. The Company currently anticipates adopting the standard using the modified retrospective method. The Company has performed a preliminary assessment and continues to evaluate the impact of the pending adoption of the new revenue standard on its consolidated financial statements and has determined that the collaborations with both Amgen and Astellas are within its scope. Upon completion of the Company’s analysis, the Company will determine the cumulative effect of initially applying the new standard, including areas we expect to be impacted such as license-related revenue and payments in connection with the Co-invest Option. The Company is also currently updating its accounting policy and designing and implementing the necessary changes to processes and controls to account for revenue under the new standard and anticipates completing its implementation in connection with its first quarter 2018 interim financial statements.

Net (Loss) Income Per Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Calculation of Basic and Diluted Net (Loss) Income Per Share

The following is the calculation of basic and diluted net (loss) income per share (in thousands, except per share data):

	Years Ended December 31,
	2017			2016		2015
Net (loss) income	$	(127,790	)	$	16,453	$	(37,501	)
Weighted-average shares used in computing net (loss) income per share — basic		49,404			39,943		38,814
Effect of dilutive securities:
Warrants to purchase common stock		—			2,019		—
Options to purchase common stock		—			409		—
Restricted stock units		—			181		—
Shares issuable related to the ESPP		—			9		—
Dilutive potential common shares		—			2,618		—
Weighted-average shares used in computing net (loss) income per share — diluted		49,404			42,561		38,814
Net (loss) income per share — basic	$	(2.59	)	$	0.41	$	(0.97	)
Net (loss) income per share — diluted	$	(2.59	)	$	0.39	$	(0.97	)

Instruments Excluded from the Computation of Diluted Net (Loss) Income Per Share

The following instruments were excluded from the computation of diluted net (loss) income per share for the periods presented because their effect would have been antidilutive (in thousands):

	December 31,
	2017		2016		2015
Options to purchase common stock		5,957		3,688		4,835
Warrants to purchase common stock		100		—		5,641
Restricted and Performance stock units		457		—		757
Shares issuable related to the ESPP		20		—		16
Total shares		6,534		3,688		11,249

Cash Equivalents and Investments (Tables)

12 Months Ended

Dec. 31, 2017

Investments Debt And Equity Securities [Abstract]

Amortized Cost and Fair Value of Cash Equivalents and Available for Sale Investments

The amortized cost and fair value of cash equivalents and available for sale investments at December 31, 2017 and 2016 were as follows (in thousands):

	December 31, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents — U. S. Treasury securities and money market funds	$	111,501	$	—	$	—		$	111,501
Short-term investments — U.S. Treasury securities and Agency bonds	$	143,895	$	—	$	(210	)	$	143,685
Long-term investments — Equity and U.S. Treasury securities	$	16,538	$	—	$	(20	)	$	16,518

	December 31, 2016
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents — U. S. Treasury securities and money market funds	$	55,658	$	—	$	—		$	55,658
Short-term investments — U.S. Treasury securities	$	89,396	$	2	$	(23	)	$	89,375
Long-term investments — Equity and U.S. Treasury securities	$	7,513	$	176	$	(17	)	$	7,672

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on Recurring Basis

Financial assets measured at fair value on a recurring basis as of December 31, 2017 and 2016 are classified in the table below in one of the three categories described above (in thousands):

	December 31, 2017
	Fair Value Measurements Using						Assets
	Level 1		Level 2		Level 3		At Fair Value
Assets:
Money market funds	$	51,001	$	—	$	—	$	51,001
U.S. Treasury securities		165,801		—		—		165,801
Agency bonds		—		54,329				54,329
Equity securities		573		—		—		573
Total	$	217,375	$	54,329	$	—	$	271,704
Amounts included in:
Cash and cash equivalents	$	111,501	$	—	$	—	$	111,501
Short-term investments		89,356		54,329		—		143,685
Long-term investments		16,518		—		—		16,518
Total	$	217,375	$	54,329	$	—	$	271,704

	December 31, 2016
	Fair Value Measurements Using						Assets
	Level 1		Level 2		Level 3		At Fair Value
Assets:
Money market funds	$	52,657	$	—	$	—	$	52,657
U.S. Treasury securities		99,872		—		—		99,872
Equity securities		176		—		—		176
Total	$	152,705	$	—	$	—	$	152,705
Amounts included in:
Cash and cash equivalents	$	55,658	$	—	$	—	$	55,658
Short-term investments		89,375		—		—		89,375
Long-term investments		7,672		—		—		7,672
Total	$	152,705	$	—	$	—	$	152,705

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Summary of Property and Equipment Balances

Property and equipment balances were as follows (in thousands):

	December 31,
	2017			2016
Property and equipment, net:
Laboratory equipment	$	17,100		$	16,742
Computer equipment and software		2,890			2,699
Office equipment, furniture and fixtures		1,137			856
Leasehold improvements		5,067			4,458
Total property and equipment		26,194			24,755
Less: Accumulated depreciation and amortization		(22,626	)		(21,118	)
Total property and equipment, net	$	3,568		$	3,637

Summary of Accrued Liabilities

Accrued liabilities were as follows (in thousands):

	December 31,
	2017		2016
Accrued liabilities:
Clinical and preclinical costs	$	8,370	$	10,092
Bonus		4,054		3,800
Other payroll related		2,207		1,888
Other accrued expenses		1,426		897
Consulting and professional fees		1,335		698
Leasehold improvements		—		672
Total accrued liabilities	$	17,392	$	18,047

Research and Development Arrangements (Tables)

12 Months Ended

Dec. 31, 2017

Amgen [Member]

Summary of Research and Development Revenue

Revenue from Amgen for the years ended December 31, 2017, 2016, and 2015 was as follows (in thousands):

	Years Ended December 31,
	2017			2016			2015
Research and development revenues
Reimbursement of internal costs	$	1,279		$	2,466		$	2,460
Research and development milestone fees		11,000			26,666			—
Co-invest option payment		(20,000	)		(1,250	)		—
Allocated consideration		—			—			21
Total net revenues from Amgen	$	(7,721	)	$	27,882		$	2,481

Astellas [Member]

Summary of Allocation of Arrangement Consideration

The Company allocated the Arrangement Consideration among the two units of accounting on a relative fair value basis using the best estimated selling price (“BESP”), as follows (in millions):

	Allocated Consideration		Upfront Revenue Recognition		Revenue Recognition over Performance Period
Units of Accounting:
ALS License	$	74.9	$	50.0	$	24.9
Research and ALS Development Services		19.3		—		19.3
Total consideration	$	94.2	$	50.0	$	44.2

Summary of Research and Development Revenue

Research and development revenue from Astellas in the years ended December 31, 2017, 2016 and 2015 was as follows (in thousands):

	Year Ended
	December 31, 2017		December 31, 2016		December 31, 2015
License revenues	$	8,799	$	62,171	$	13,918
Research and development revenues		11,934		15,110		12,184
Total Revenue from Astellas	$	20,733	$	77,281	$	26,102

Summary of Deferred Revenue

Deferred Revenue reflecting the unrecognized portion of the license revenue, option fee and payment of expenses from the Astellas Agreement was as follows (in thousands):

December 31,

2017

December 31,

2016

Deferred revenue, current

9,572

8,060

Deferred revenue, non-current

15,000

Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Schedule of Long Term Debt and Unamortized Debt Discount

Long-term debt and unamortized debt discount balances are as follows (in thousands):

	December 31,
	2017			2016
Notes payable, gross	$	32,000		$	30,000
Less: Unamortized debt discount		(325	)		(472	)
Accretion of final exit fee		102			353
Carrying value of notes payable		31,777			29,881
Less: Current portion of long-term debt		—			(2,500	)
Long-term debt	$	31,777		$	27,381

Schedule of Future Minimum Payments under Loan

Future minimum payments under the Loan, as of December 31, 2017 are as follows (in thousands):

2018	$	2,615
2019		7,963
2020		11,187
2021		10,417
2022		10,176
Total minimum payments		42,358
Less: Interest and final payment		(10,358	)
Notes payable, gross	$	32,000

Liability Related to Sale of Future Royalties (Tables)

12 Months Ended

Dec. 31, 2017

Deferred Revenue Disclosure [Abstract]

Schedule Represents Allocation of Transaction Consideration on a Relative Fair Value Basis to the Liability and the Common Stock

At the time of the Royalty Monetization, the Company allocated the transaction consideration on a relative fair value basis to the liability and the common stock, as follows (in millions):

	Allocated Consideration
Units of Accounting:
Liability related to sale of future royalties	$	92.3
Common stock		7.7
Total consideration	$	100.0

Schedule Represents Activity Within Liability Related to Sale of Future Royalties

The following table shows the activity within liability related to sale of future royalties during the year ended December 31, 2017 (in thousands):

Liability related to sale of future royalties at February 1, 2017	$	92,300
Non-cash interest expense recognized		13,980
Liability related to sale of future royalties at December 31, 2017		106,280
Less: Unamortized transaction costs		(1,630	)
Carrying value of liability related to sale of future royalties at December 31, 2017		104,650

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Rent Expense

Rent expense was as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Rent expense	$	3,627	$	3,448	$	3,297

Future Minimum Lease Payments under Noncancelable Operating Leases

As of December 31, 2017, future minimum lease payments under noncancelable operating leases were as follows (in thousands):

2018	$	3,789
2019		4,682
2020		4,846
2021		2,465
Total	$	15,782

Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Summary of Stock Option Activity

Stock Option Activity under the Equity Incentive Plan was as follows:

	Stock Options Outstanding			Weighted Average Exercise Price per Share - Stock Options		Weighted Average Remaining Contractual Life		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2016		5,192,813		$	9.27
Options granted		1,310,674			11.74
Options exercised		(264,164	)		7.26
Options forfeited/expired		(281,865	)		24.45
Balance at December 31, 2017		5,957,458		$	9.19		6.61	$	4,287
Exercisable at December 31, 2017		4,019,836		$	9.01		5.65	$	3,300
Vested and expected to vest as of December 31, 2017		5,866,326		$	9.18		6.57	$	4,250

Fair Value of Share-Based Payments was Estimated on Date of Grant Using Black-Scholes Option Pricing Model Based on Weighted Average Assumptions

The fair value of share-based payments was estimated on the date of grant using the Black-Scholes option pricing model based on the following weighted average assumptions:

	Year Ended December 31, 2017						Year Ended December 31, 2016						Year Ended December 31, 2015
	Employee Stock Options			ESPP			Employee Stock Options			ESPP			Employee Stock Options			ESPP
Risk-free interest rate		2.2	%		1.3	%		1.9	%		0.5	%		1.7	%		0.3	%
Volatility		74.0	%		74.0	%		74.0	%		74.0	%		79.4	%		75.3	%
Expected term in years		6.52			0.50			6.44			0.50			6.38			0.56
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%		0.0	%		0.0	%

Non-cash Stock-Based Compensation Related to Stock Options, Restricted Stock Awards, Restricted Stock Unit, and Employee Stock Purchases

	Years Ended December 31,
	2017		2016		2015
Research and development	$	5,656	$	4,252	$	1,828
General and administrative		3,372		2,894		2,739
Stock-based compensation included in operating expenses	$	9,028	$	7,146	$	4,567

Restricted Stock Units [Member]

Summary of Stock Unit Activity

Restricted stock unit activity in 2017 was as follows:

	Number of Shares			Weighted Average Award Date Fair Value per Share
Unvested restricted stock units outstanding at December 31, 2016		64,502			7.19
Restricted stock units granted		269,000			10.60
Restricted stock units released		(43,500	)		6.67
Restricted stock units forfeited		(4,500	)		9.73
Unvested restricted stock units outstanding at December 31, 2017		285,502			10.44

Performance Units [Member]

Summary of Stock Unit Activity

Performance unit activity in 2017 was as follows:

	Number of Shares			Weighted Average Award Date Fair Value per Share
Performance units outstanding at December 31, 2016		685,000		$	7.00
Performance units granted		—			—
Performance units released		(171,250	)		7.00
Performance units forfeited		(342,500	)		7.00
Performance units outstanding at December 31, 2017		171,250		$	7.00

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Reconciliation of Statutory Federal Income Tax Rate to Effective Tax Rate

The following is a reconciliation of the statutory federal income tax rate to the Company’s effective tax rate:

	Years Ended December 31,
	2017			2016			2015
Tax at federal statutory tax rate		(34	)%		34	%		(34	)%
State income tax, net of federal tax benefit		—			2	%		0	%
State Apportionment		—			(7	)%		0	%
Tax credits (net)		(8	)%		(32	)%		(7	)%
Federal statutory rate reduction		51	%		—			—
Deferred tax assets (utilized) not benefited		(10	)%		(15	)%		37	%
Stock-based compensation		—			7	%		2	%
NOL Expiration		—			9	%		2	%
Other		1	%		2	%		0	%
Total		0	%		0	%		0	%

Schedule of Components of Income (Loss) Before Taxes

	Years Ended December 31,
	2017			2016		2015
United States	$	(127,235	)	$	16,453	$	(37,501	)
Foreign		(555	)		—		—
Total	$	(127,790	)	$	16,453	$	(37,501	)

Company's Deferred Tax Assets and Liabilities

The significant components of the Company’s deferred tax assets and liabilities were as follows (in thousands):

	As of December 31,
	2017			2016			2015
Deferred tax assets:
Net operating loss ("NOL") carryforwards	$	98,630		$	146,961		$	153,251
Tax credits		64,185			46,998			38,742
Liability related to sale of future royalties		24,593			—			—
Reserves and accruals		10,524			10,258			12,899
Capitalized R&D		6,432			11,675			13,150
Depreciation and amortization		546			766			769
Total deferred tax assets		204,910			216,658			218,811
Less: Valuation allowance		(204,910	)		(216,658	)		(218,811	)
Net deferred tax assets	$	—		$	—		$	—

Schedule of Activity Related to our Gross Unrecognized Tax Benefits

Activity related to the Company’s gross unrecognized tax benefits were (in thousands):

	Years Ended December 31,
	2017		2016
Balance at the beginning of the year	$	7,565	$	6,715
Decrease related to prior year tax positions		-		5
Increase related to current year tax positions		1,800		845
Balance at the end of the year	$	9,365	$	7,565

Quarterly Financial Data (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Summary of Quarterly Results

Quarterly results were as follows (in thousands, except per share data):

	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
2017
Total revenues, net	$	4,153		$	3,053		$	6,180		$	(18	)
Net loss		(25,867	)		(29,081	)		(32,357	)		(40,484	)
Net loss per share — basic and diluted	$	(0.62	)	$	(0.60	)	$	(0.60	)	$	(0.75	)
2016
Total revenues, net	$	8,421		$	5,802		$	59,047		$	33,138
Net income (loss)		(12,455	)		(11,611	)		33,362			7,157
Net income (loss) per share — basic	$	(0.31	)	$	(0.29	)	$	0.84		$	0.18
Net income (loss) per share —diluted	$	(0.31	)	$	(0.29	)	$	0.77		$	0.16

Organization and Significant Accounting Policies - Additional Information (Detail) $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($) Financial_Institution	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Financial_Institution	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit incurred		$ (646,081)				$ (518,291)				$ (646,081)	$ (518,291)
Net loss		(40,484)	$ (32,357)	$ (29,081)	$ (25,867)	7,157	$ 33,362	$ (11,611)	$ (12,455)	(127,790)	16,453	$ (37,501)
Net cash provided by (used in) operating activities										(101,759)	36,982	$ 4,883
Cash, cash equivalents and investments		$ 285,400				$ 163,900				$ 285,400	$ 163,900
Cash requirements term										12 months
Number of major financial institutions \| Financial_Institution		3								3
Excess tax benefit recognized	$ 700
Accounting Standards Update 2016-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment to retained earnings		$ 0								$ 0
Computer Equipment and Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives										3 years
Laboratory Equipment and Office Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives										5 years
Furniture and Fixtures [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives										7 years
Leasehold Improvements [Member] \| Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives										3 years
Leasehold Improvements [Member] \| Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives										7 years

Net (Loss) Income Per Share - Calculation of Basic and Diluted Net (Loss) Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net (loss) income	$ (40,484)	$ (32,357)	$ (29,081)	$ (25,867)	$ 7,157	$ 33,362	$ (11,611)	$ (12,455)	$ (127,790)	$ 16,453	$ (37,501)
Weighted-average number of shares used in computing net (loss) income per share — basic									49,404	39,943	38,814
Effect of dilutive securities:
Warrants to purchase common stock									0	2,019	0
Options to purchase common stock									0	409	0
Restricted stock units									0	181	0
Shares issuable related to the ESPP									0	9	0
Dilutive potential common shares									0	2,618	0
Weighted-average shares used in computing net (loss) income per share — diluted									49,404	42,561	38,814
Net (loss) income per share — basic					$ 0.18	$ 0.84	$ (0.29)	$ (0.31)	$ (2.59)	$ 0.41	$ (0.97)
Net (loss) income per share — diluted					$ 0.16	$ 0.77	$ (0.29)	$ (0.31)	$ (2.59)	$ 0.39	$ (0.97)

Net (Loss) Income Per Share - Instruments Excluded from the Computation of Diluted Net (Loss) Income Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	6,534	3,688	11,249
Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	5,957	3,688	4,835
Warrants to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	100	0	5,641
Restricted and Performance Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	457	0	757
ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	20	0	16

Cash Equivalents and Investments - Amortized Cost and Fair Value of Cash Equivalents and Available for Sale Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash Equivalents [Member] \| US Treasury Securities and Money Market Funds [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	$ 111,501	$ 55,658
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	111,501	55,658
Short-term Investments [Member] \| U.S. Treasury Securities and Agency Bonds [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	143,895
Unrealized Losses	(210)
Fair Value	143,685
Short-term Investments [Member] \| U.S. Treasury Securities [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost		89,396
Unrealized Gains		2
Unrealized Losses		(23)
Fair Value		89,375
Long-term Investments [Member] \| Equity and U.S. Treasury Securities [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	16,538	7,513
Unrealized Gains		176
Unrealized Losses	(20)	(17)
Fair Value	$ 16,518	$ 7,672

Cash Equivalents and Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($)
Cash And Cash Equivalents [Line Items]
Investments in continuous unrealized loss position for 12 months or longer	$ 0
U.S. Treasury Securities [Member]
Cash And Cash Equivalents [Line Items]
Maturity term of available for sale securities	1 year 6 months

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Long term debt, fair value	$ 31,800,000	$ 29,900,000
Fair value of liabilities transferred from level 1 to level 2	0
Fair value of liabilities transferred from level 2 to level 1	0
Fair value of liabilities transferred into level 3	0
Fair value of liabilities transferred from level 3	$ 0

Balance Sheet Components - Summary of Property and Equipment Balances (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment	$ 26,194	$ 24,755
Less: Accumulated depreciation and amortization	(22,626)	(21,118)
Total property and equipment, net	3,568	3,637
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	17,100	16,742
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	2,890	2,699
Office Equipment, Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,137	856
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 5,067	$ 4,458

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Depreciation expense	$ 1.9	$ 0.7	$ 0.6
Employer contributions under the plan	$ 0.5	$ 0.5	$ 0.4

Balance Sheet Components - Summary of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Clinical and preclinical costs	$ 8,370	$ 10,092
Bonus	4,054	3,800
Other payroll related	2,207	1,888
Other accrued expenses	1,426	897
Consulting and professional fees	1,335	698
Leasehold improvements	0	672
Total accrued liabilities	$ 17,392	$ 18,047

Research and Development Arrangements - Additional Information (Details)	3 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) ClinicalTrial	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable		$ 1,112,000	$ 1,112,000	$ 24,000
Revenue recognized for milestones achieved			$ 11,000,000
Percentage of shared costs			75.00%
2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable option fee				15,000,000
Reldesemtiv [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of clinical trials conducted \| ClinicalTrial			2
Galactic HF [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized for milestones achieved			$ 10,000,000
Amgen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Co-invest option exercised amount			$ 40,000,000
Percentage of incremental royalty receivable on annual net sales			4.00%
Co-invest option payment			$ 21,300,000
Accounts receivable		1,000,000	1,000,000	0
Revenue recognized for milestones achieved			11,000,000	26,666,000	$ 0
Remaining deliverables		0	0
Amgen [Member] \| Cardiac Muscle Activator [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized for milestones achieved		1,000,000
Amgen [Member] \| Galactic HF [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized for milestones achieved				26,700,000
Astellas [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			11,934,000	15,110,000	$ 12,184,000
Upfront revenue recognition			50,000,000
Allocated Consideration			$ 94,200,000
Percentage of shared costs			25.00%
Astellas [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research milestone payments			$ 2,000,000
Astellas [Member] \| ALS License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront revenue recognition			50,000,000
Allocated Consideration			74,900,000
Astellas [Member] \| Research and ALS Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront revenue recognition			0
Allocated Consideration			$ 19,300,000
Astellas [Member] \| 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Premium percentage			100.00%
Amount received as milestone payment			$ 15,000,000	15,000,000
Upfront revenue recognition				50,000,000
Revenue recognition, Non-refundable upfront amendment fee				35,000,000
Allocated Consideration				94,200,000
Upfront revenue recognition			50,000,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement			600,000,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| Minimum [Member] \| Tirasemtiv Option [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option exercise payment to be received			25,000,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| Maximum [Member] \| Tirasemtiv Option [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount received as milestone payment			30,000,000
Option exercise payment to be received			80,000,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| ALS License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront revenue recognition			50,000,000
Allocated Consideration			74,900,000
Revenue Recognition over Performance Period		24,900,000	24,900,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| Research and ALS Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue Recognition over Performance Period		19,300,000	19,300,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| Tirasemtiv License [Member] \| Tirasemtiv Option [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized for milestones achieved			15,000,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| Non- neuromuscular Indications [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement			95,000,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| SMA and Other Neuromuscular Indications [Member] \| Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement			100,000,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| Commercial Milestones [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement			200,000,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| Additional Research Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue Recognition over Performance Period				5,100,000
Astellas [Member] \| 2016 Astellas Amendment [Member] \| ALS Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue Recognition over Performance Period				$ 39,100,000
Astellas [Member] \| 2014 Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received						$ 30,000,000
Amount received as milestone payment						$ 15,000,000
Astellas [Member] \| Initial Indication [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments to be received		100,000,000	100,000,000
Astellas [Member] \| Subsequent Indication [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments to be received		$ 50,000,000	$ 50,000,000
Scenario, Forecast [Member] \| Amgen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Co-invest option remaining	$ 18,800,000

Research and Development Arrangements - Revenues (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development milestone fees	$ 11,000
License revenues	8,799	$ 62,171	$ 13,918
Total revenues	13,368	106,407	28,658
Amgen [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Reimbursement of internal costs	1,279	2,466	2,460
Research and development milestone fees	11,000	26,666	0
Co-invest option payment	(20,000)	(1,250)	0
Allocated consideration	0	0	21
Total revenues	(7,721)	27,882	2,481
Astellas [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
License revenues	8,799	62,171	13,918
Research and development revenues	11,934	15,110	12,184
Total revenues	$ 20,733	$ 77,281	$ 26,102

Research and Development Arrangements - Schedule Represents Allocation of Arrangement Consideration, and Revenue Recognition (Details) - Astellas [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Deferred Revenue Arrangement [Line Items]
Allocated Consideration	$ 94.2
Upfront Revenue Recognition	50.0
Revenue Recognition over Performance Period	44.2
ALS License [Member]
Deferred Revenue Arrangement [Line Items]
Allocated Consideration	74.9
Upfront Revenue Recognition	50.0
Revenue Recognition over Performance Period	24.9
Research and ALS Development Services [Member]
Deferred Revenue Arrangement [Line Items]
Allocated Consideration	19.3
Upfront Revenue Recognition	0.0
Revenue Recognition over Performance Period	$ 19.3

Research and Development Arrangements - Deferred Revenue (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Revenue Arrangement [Line Items]
Deferred revenue, current	$ 9,572	$ 8,060
Deferred revenue, non-current	15,000	15,000
Astellas [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue, current	9,572	8,060
Deferred revenue, non-current	$ 15,000	$ 15,000

- Definition

The carrying amount of consideration received or receivable as of the balance sheet date on potential earnings that were not recognized as revenue in conformity with GAAP, and which are expected to be recognized as such within one year or the normal operating cycle, if longer, including sales, license fees, and royalties, but excluding interest income.

+ References + Details

- Definition

The noncurrent portion of deferred revenue amount as of balance sheet date. Deferred revenue is a liability related to a revenue producing activity for which revenue has not yet been recognized, and is not expected to be recognized in the next twelve months. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

+ References + Details

Other Research and Development Revenue Arrangements Grants - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jul. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 28, 2015
Research and Development Expenses [Line Items]
Grant to support research and development	$ 1,500,000
Milestone payment					$ 500,000
ALSA Grant Revenue [Member]
Research and Development Expenses [Line Items]
Grant revenue		$ 300,000	$ 1,100,000	$ 100,000
Deferred revenue		$ 0

Long-Term Debt - Schedule of Long Term Debt and Unamortized Debt Discount (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Notes payable, gross	$ 32,000	$ 30,000
Less: Unamortized debt discount	(325)	(472)
Accretion of final exit fee	102	353
Carrying value of notes payable	31,777	29,881
Long-term Debt, by current and noncurrent
Carrying value of notes payable	31,777	29,881
Less: Current portion of long-term debt	0	(2,500)
Long-term debt	$ 31,777	$ 27,381

Long-Term Debt - Additional Information (Detail) $ / shares in Units, $ in Millions		1 Months Ended	12 Months Ended
	Oct. 27, 2017 USD ($) Installment	Jan. 31, 2017 shares	Dec. 31, 2017 shares	Dec. 31, 2016 shares	Feb. 29, 2016 $ / shares shares	Oct. 31, 2015 $ / shares shares
Debt Instrument [Line Items]
Number of issued shares of common stock related to cashless exercise of warrants \| shares			3,450,122	834,998
Amended Loan Agreement [Member] \| Oxford and Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Proceeds from term loan	$ 32.0
Retirement of debt outstanding	30.0
Payment of accrued portion of final payment fee	0.5
Outstanding loan balance	$ 32.0
Loan repayment terms	Payments on the new outstanding loan balance of $32.0 million will be interest only through May 2019, followed by 41 months of equal monthly payments of interest and principal.
Number of installments description	41 months of equal monthly payments of interest and principal
Number of installments \| Installment	41
Final payment fee percentage	6.50%
Debt instrument, base interest rate for scenario 2	6.81%
Debt instrument, applicable interest rate for scenario 1	8.05%
Interest rate description	The interest rate under the Amended Loan Agreement is the greater of (a) 8.05% or (b) the sum of 6.81% plus the 30-day U.S. LIBOR rate
Loan and Security Agreement [Member] \| Warrants to Purchase Common Stock [Member]
Debt Instrument [Line Items]
Number of issued shares of common stock related to cashless exercise of warrants \| shares		16,126
Loan and Security Agreement [Member] \| Oxford and Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Warrants outstanding to purchase upon exercise of common stock \| shares					68,285	65,189
Warrants exercise price \| $ / shares					$ 6.59	$ 6.90
Warrant exercisable term			5 years
Loan and Security Agreement [Member] \| Oxford and Silicon Valley Bank [Member] \| Within One Year [Member]
Debt Instrument [Line Items]
Percentage of prepayment fee			3.00%
Loan and Security Agreement [Member] \| Oxford and Silicon Valley Bank [Member] \| Within Two Year [Member]
Debt Instrument [Line Items]
Percentage of prepayment fee			2.00%
Loan and Security Agreement [Member] \| Oxford and Silicon Valley Bank [Member] \| Thereafter [Member]
Debt Instrument [Line Items]
Percentage of prepayment fee			1.00%

Long-Term Debt - Schedule of Future Minimum Payments under Loan (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying value of notes payable	$ 31,777	$ 29,881
Long-term debt, alternative
Carrying value of notes payable	31,777	29,881
Notes payable, gross	32,000	$ 30,000
Loan and Security Agreement [Member] \| Oxford and Silicon Valley Bank [Member]
Debt Instrument [Line Items]
2018	2,615
2019	7,963
2020	11,187
2021	10,417
2022	10,176
Carrying value of notes payable	42,358
Long-term debt, alternative
Carrying value of notes payable	42,358
Less: Interest and final payment	(10,358)
Notes payable, gross	$ 32,000

Liability Related to Sale of Future Royalties - Additional Information (Detail) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	Mar. 31, 2017	Feb. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability Related to Sale of Future Royalties [Line Items]
Effective pre-tax annual interest rate in calculating liability related to sale of future royalties		17.00%
Proceeds from issuance of common stock				$ 112,224	$ 0	$ 8,673
Effective tax rate				34.00%
Transaction costs incurred in connection with the Royalty Monetization and RPI Common stock				$ 1,800
Scenario, Forecast [Member]
Liability Related to Sale of Future Royalties [Line Items]
Effective tax rate			21.00%
Royalty Purchase Agreement [Member]
Liability Related to Sale of Future Royalties [Line Items]
Cash payment under Royalty Agreement		$ 90,000		90,000
Common stock, shares issued in period		875,656
Proceeds from issuance of common stock		$ 10,000		10,000
Common stock fair value	$ 8,100
Fair value for the liability related to sale of future royalties				$ 131,600
Effective tax rate				35.00%
Royalty Purchase Agreement [Member] \| Scenario, Forecast [Member]
Liability Related to Sale of Future Royalties [Line Items]
Effective tax rate			21.00%
Royalty Purchase Agreement [Member] \| Scenario Previously Reported [Member]
Liability Related to Sale of Future Royalties [Line Items]
Fair value for the liability related to sale of future royalties				$ 96,700

Liability Related to Sale of Future Royalties - Schedule Represents Allocation of Transaction Consideration on a Relative Fair Value Basis to the Liability and the Common Stock (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Liability Related to Sale of Future Royalties [Line Items]
Allocated Consideration	$ 100.0
Liability Related to Sale of Future Royalties [Member]
Liability Related to Sale of Future Royalties [Line Items]
Allocated Consideration	92.3
Common Stock [Member]
Liability Related to Sale of Future Royalties [Line Items]
Allocated Consideration	$ 7.7

Liability Related to Sale of Future Royalties - Schedule Represents Activity Within Liability Related to Sale of Future Royalties (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability Related to Sale of Future Royalties [Line Items]
Non-cash interest expense recognized	$ (14,028)	$ 0	$ 0
Royalty Purchase Agreement [Member]
Liability Related to Sale of Future Royalties [Line Items]
Liability related to sale of future royalties beginning balance	92,300
Non-cash interest expense recognized	13,980
Liability related to sale of future royalties ending balance	106,280	$ 92,300
Less: Unamortized transaction costs	(1,630)
Carrying value of liability related to sale of future royalties at December 31, 2017	$ 104,650

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Non-cancelable operating lease expiration year	2021
Settlement agreement amount		$ 4.5
Amgen [Member]
Other Commitments [Line Items]
Co-invest option exercised amount	$ 40.0
Percentage of incremental royalty receivable on annual net sales	4.00%
Contractual obligation	$ 18.8

Commitments and Contingencies - Rent Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Rent expense	$ 3,627	$ 3,448	$ 3,297

Commitments and Contingencies - Future Minimum Lease Payments under Noncancelable Operating Leases (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2018	$ 3,789
2019	4,682
2020	4,846
2021	2,465
Total	$ 15,782

Stockholders' Equity - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	May 31, 2017	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 29, 2016	Oct. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net proceeds of issuance of common stock			$ 112,224,000	$ 0	$ 8,673,000
Number of issued shares of common stock related to cashless exercise of warrants			3,450,122	834,998
Class of warrant outstanding			100,106
Class of warrant or right outstanding weighted average exercise price			$ 6.74
Warrant expiration date			2017-06
Allocated stock-based compensation expense			$ 9,028,000	$ 7,146,000	$ 4,567,000
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			48 months
Share based compensation, vested restricted stock units, total fair value			$ 1,500,000
Performance stock units, remaining			285,502	64,502
Performance stock units, forfeited			4,500
Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant date fair value of options granted					$ 7.00
Stock options granted					685,000
Number of Performance stock unit for which performance criteria were met			342,500
Performance stock units, vested			171,250
Performance stock units, remaining			171,250	685,000
Performance stock units, expected to vest			2018-03
Performance stock units, forfeited			342,500
Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost			$ 11,100,000
Weighted-average period			2 years 4 months 24 days
Restricted Stock Units and Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost			$ 2,100,000
Weighted-average period			1 year 3 months 18 days
2004 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option grant prices as percentage of the fair market value of the common stock			100.00%
Term to grant nonstatutory stock options and incentive stock options			10 years
Percentage of options grant to new employees			25.00%
Increased in number of shares of common stock	3,900,000
Shares of common stock reserved for issuance			3,800,000
Shares of common stock available for issuance			3,800,000
Share based compensation, options exercised, total intrinsic value			$ 1,800,000	$ 200,000	$ 100,000
Per share price of common stock			$ 8.15
Weighted-average grant date fair value of options granted			$ 7.95	$ 4.77	$ 5.35
Grant date fair value of option shares vested			$ 6,500,000	$ 4,900,000	$ 3,600,000
2004 Plan [Member] \| New Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting right			Options granted to new employees generally vest 25% after one year and monthly thereafter over a period of four years
Period from percentage of stock option vested			1 year
Vesting period			4 years
2004 Plan [Member] \| Existing Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting right			Options granted to existing employees generally vest monthly over a period of four years.
Vesting period			4 years
2004 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option grant prices as percentage of the fair market value of the common stock			85.00%
2015 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option grant prices as percentage of the fair market value of the common stock			85.00%
Shares of common stock reserved for issuance			398,439
Issuance of common stock pursuant to ESPP, shares			120,959	129,604	21,167
Issuance of common stock pursuant to ESPP, per share			$ 9.65	$ 7.08	$ 3.24
Non-employee Stock-Based Compensation [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated stock-based compensation expense			$ 532,000	$ 147,000	$ 27,000
Loan and Security Agreement [Member] \| Warrants to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of issued shares of common stock related to cashless exercise of warrants		16,126
Oxford and Silicon Valley Bank [Member] \| Loan and Security Agreement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants outstanding to purchase upon exercise of common stock						68,285	65,189
Warrants exercise price						$ 6.59	$ 6.90
Cantor Fitzgerald Agreement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares issued in period			2,425,625		808,193
Net proceeds of issuance of common stock			$ 29,900,000		$ 8,900,000

Stockholders' Equity - Summary of Equity Incentive Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Options Outstanding, Beginning Balance	5,192,813
Stock Options Outstanding, Options granted	1,310,674
Stock Options Outstanding, Options forfeited/expired	(281,865)
Stock Options Outstanding, Ending Balance	5,957,458	5,192,813
Stock Options Outstanding, Exercisable	4,019,836
Stock Options Outstanding, Vested and expected to vest	5,866,326
Weighted Average Exercise Price per Share - Stock Options, Beginning Balance	$ 9.27
Weighted Average Exercise Price per Share - Stock Options, Options granted	11.74
Weighted Average Exercise Price per Share - Stock Options, Options exercised	7.26
Weighted Average Exercise Price per Share - Stock Options, Options forfeited/expired	24.45
Weighted Average Exercise Price per Share - Stock Options, Ending Balance	9.19	$ 9.27
Weighted Average Exercise Price per Share - Stock Options, Exercisable	9.01
Weighted Average Exercise Price per Share - Stock Options, Vested and expected to vest, Outstanding	$ 9.18
Weighted Average Remaining Contractual Life	6 years 7 months 9 days
Weighted Average Remaining Contractual Life, Exercisable at December 31, 2017	5 years 7 months 24 days
Weighted Average Remaining Contractual Life, Vested and expected to vest as of December 31, 2017	6 years 6 months 25 days
Aggregate Intrinsic Value	$ 4,287
Aggregate Intrinsic Value, Exercisable at December 31, 2017	3,300
Aggregate Intrinsic Value, Vested and expected to vest as of December 31, 2017	$ 4,250
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Options Outstanding, Options exercised	(264,164)	(74,556)	(68,635)

Stockholders' Equity - Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units [Member]	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units outstanding, Beginning Balance \| shares	64,502
Units granted \| shares	269,000
Units released \| shares	(43,500)
Units forfeited \| shares	(4,500)
Units outstanding, Ending Balance \| shares	285,502
Units outstanding, Weighted Average Award Date Fair Value per Share, Beginning Balance \| $ / shares	$ 7.19
Units granted, Weighted Average Award Date Fair Value per Share \| $ / shares	10.60
Units released, Weighted Average Award Date Fair Value per Share \| $ / shares	6.67
Units forfeited, Weighted Average Award Date Fair Value per Share \| $ / shares	9.73
Units outstanding, Weighted Average Award Date Fair Value per Share, Ending Balance \| $ / shares	$ 10.44

Stockholders' Equity - Summary of Performance Stock Unit Activity (Detail) - Performance Units [Member]	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units outstanding, Beginning Balance \| shares	685,000
Units granted \| shares	0
Units released \| shares	(171,250)
Units forfeited \| shares	(342,500)
Units outstanding, Ending Balance \| shares	171,250
Units outstanding, Weighted Average Award Date Fair Value per Share, Beginning Balance \| $ / shares	$ 7.00
Units granted, Weighted Average Award Date Fair Value per Share \| $ / shares	0
Units released, Weighted Average Award Date Fair Value per Share \| $ / shares	7.00
Units forfeited, Weighted Average Award Date Fair Value per Share \| $ / shares	7.00
Units outstanding, Weighted Average Award Date Fair Value per Share, Ending Balance \| $ / shares	$ 7.00

Stockholders' Equity - Fair Value of Share-Based Payments was Estimated on Date of Grant Using Black-Scholes Option Pricing Model Based on Weighted Average Assumptions (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ESPP [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Risk-free interest rate	1.30%	0.50%	0.30%
Volatility	74.00%	74.00%	75.30%
Expected term in years	6 months	6 months	6 months 21 days
Expected dividend yield	0.00%	0.00%	0.00%
Options [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Risk-free interest rate	2.20%	1.90%	1.70%
Volatility	74.00%	74.00%	79.40%
Expected term in years	6 years 6 months 7 days	6 years 5 months 9 days	6 years 4 months 17 days
Expected dividend yield	0.00%	0.00%	0.00%

Stockholders' Equity - Stock-Based Compensation Related to Stock Options, Restricted Stock Awards, Restricted Stock Unit, and Employee Stock Purchases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 9,028	$ 7,146	$ 4,567
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	5,656	4,252	1,828
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 3,372	$ 2,894	$ 2,739

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Provision for income tax		$ 0	$ 0	$ 0
Research and development credits and orphan drug credits, federal carryforwards will expire		2021
Increase (decrease) in valuation allowance		$ (11,700)	(2,100,000)	13,900,000
Unrecognized tax benefits		$ 8,100,000	$ 6,300,000	$ 5,500,000
Effective tax rate		34.00%
Effect of Tax Cuts and Jobs Act, estimated reduction in deferred tax assets		$ 68,300,000
Scenario, Forecast [Member]
Income Tax Contingency [Line Items]
Effective tax rate	21.00%
Federal Tax [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards		$ 382,800,000
Net operating loss carryforwards expiration		2022
Credit carryforwards for federal and state		$ 61,100,000
Federal and State Tax [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards		$ 244,800,000
Net operating loss carryforwards expiration		2028
Credit carryforwards for federal and state		$ 14,700,000

Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax at federal statutory tax rate	(34.00%)	34.00%	(34.00%)
State income tax, net of federal tax benefit	0.00%	2.00%	0.00%
State Apportionment	0.00%	(7.00%)	0.00%
Tax credits (net)	(8.00%)	(32.00%)	(7.00%)
Federal statutory rate reduction	51.00%	0.00%	0.00%
Deferred tax assets (utilized) not benefited	(10.00%)	(15.00%)	37.00%
Stock-based compensation	0.00%	7.00%	2.00%
NOL Expiration	0.00%	9.00%	2.00%
Other	1.00%	2.00%	0.00%
Total	0.00%	0.00%	0.00%

Income Taxes - Schedule of Components of Income (Loss) Before Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States	$ (127,235)	$ 16,453	$ (37,501)
Foreign	(555)	0	0
Total	$ (127,790)	$ 16,453	$ (37,501)

Income Taxes - Company's Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss ("NOL") carryforwards	$ 98,630	$ 146,961	$ 153,251
Tax credits	64,185	46,998	38,742
Liability related to sale of future royalties	24,593	0	0
Reserves and accruals	10,524	10,258	12,899
Capitalized R&D	6,432	11,675	13,150
Depreciation and amortization	546	766	769
Total deferred tax assets	204,910	216,658	218,811
Less: Valuation allowance	(204,910)	(216,658)	(218,811)
Net deferred tax assets	$ 0	$ 0	$ 0

Income Taxes - Schedule of Activity Related to our Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Balance at the beginning of the year	$ 7,565	$ 6,715
Decrease related to prior year tax positions	0	5
Increase related to current year tax positions	1,800	845
Balance at the end of the year	$ 9,365	$ 7,565

Quarterly Financial Data - Summary of Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues, net	$ (18)	$ 6,180	$ 3,053	$ 4,153	$ 33,138	$ 59,047	$ 5,802	$ 8,421
Net loss	$ (40,484)	$ (32,357)	$ (29,081)	$ (25,867)	$ 7,157	$ 33,362	$ (11,611)	$ (12,455)	$ (127,790)	$ 16,453	$ (37,501)
Net loss per share — basic and diluted	$ (0.75)	$ (0.60)	$ (0.60)	$ (0.62)
Net income (loss) per share — basic					$ 0.18	$ 0.84	$ (0.29)	$ (0.31)	$ (2.59)	$ 0.41	$ (0.97)
Net income (loss) per share —diluted					$ 0.16	$ 0.77	$ (0.29)	$ (0.31)	$ (2.59)	$ 0.39	$ (0.97)

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	$ 271,704	$ 152,705
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	51,001	52,657
U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	165,801	99,872
Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	54,329
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	573	176
Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	16,518	7,672
Fair Value Measurements Using Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	217,375	152,705
Fair Value Measurements Using Level 1 [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	51,001	52,657
Fair Value Measurements Using Level 1 [Member] \| U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	165,801	99,872
Fair Value Measurements Using Level 1 [Member] \| Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0
Fair Value Measurements Using Level 1 [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	573	176
Fair Value Measurements Using Level 1 [Member] \| Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	16,518	7,672
Fair Value Measurements Using Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	54,329	0
Fair Value Measurements Using Level 2 [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Fair Value Measurements Using Level 2 [Member] \| U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Fair Value Measurements Using Level 2 [Member] \| Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	54,329
Fair Value Measurements Using Level 2 [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Fair Value Measurements Using Level 2 [Member] \| Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Fair Value Measurements Using Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Fair Value Measurements Using Level 3 [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Fair Value Measurements Using Level 3 [Member] \| U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Fair Value Measurements Using Level 3 [Member] \| Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0
Fair Value Measurements Using Level 3 [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Fair Value Measurements Using Level 3 [Member] \| Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Cash and Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	111,501	55,658
Cash and Cash Equivalents [Member] \| Fair Value Measurements Using Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	111,501	55,658
Cash and Cash Equivalents [Member] \| Fair Value Measurements Using Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Cash and Cash Equivalents [Member] \| Fair Value Measurements Using Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	0	0
Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	143,685	89,375
Short-term Investments [Member] \| Fair Value Measurements Using Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	89,356	89,375
Short-term Investments [Member] \| Fair Value Measurements Using Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	54,329	0
Short-term Investments [Member] \| Fair Value Measurements Using Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	$ 0	$ 0