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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2017	Jul. 27, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q2
Trading Symbol	CYTK
Entity Registrant Name	CYTOKINETICS INC
Entity Central Index Key	0001061983
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		53,666,761

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 100,711	$ 66,874
Short-term investments	211,340	89,375
Accounts receivable		24
Prepaid and other current assets	4,945	2,360
Total current assets	316,996	158,633
Long-term investments	20,087	7,672
Property and equipment, net	3,268	3,637
Other assets	279	200
Total assets	340,630	170,142
Current liabilities:
Accounts payable	1,783	4,236
Accrued liabilities	13,545	18,047
Deferred revenue, current	7,942	8,060
Current portion of long-term debt	7,315	2,500
Other current liabilities	474	415
Total current liabilities	31,059	33,258
Long-term debt, net	22,844	27,381
Liability related to the sale of future royalties, net	96,657	0
Deferred revenue, non-current	15,067	15,000
Other long-term liabilities	2	142
Total liabilities	165,629	75,781
Commitments and contingencies (Note 12)
Stockholders’ equity:
Preferred stock, $0.001 par value: Authorized: 10,000,000 shares; Issued and outstanding: Series A Convertible Preferred Stock — zero shares at June 30, 2017 and December 31, 2016	0	0
Common stock, $0.001 par value: Authorized: 163,000,000 shares; Issued and outstanding: 53,457,091 shares at June 30, 2017 and 40,646,595 shares at December 31, 2016	53	41
Additional paid-in capital	748,273	612,474
Accumulated other comprehensive income (loss)	(86)	137
Accumulated deficit	(573,239)	(518,291)
Total stockholders’ equity	175,001	94,361
Total liabilities and stockholders’ equity	$ 340,630	$ 170,142

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 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	53,457,091	40,646,595
Common stock, shares outstanding	53,457,091	40,646,595

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues:
Research and development, grant and other revenues, net	$ (1,889)	$ 3,852	$ 818	$ 8,299
License revenues	4,942	1,950	6,388	5,923
Total revenues	3,053	5,802	7,206	14,222
Operating expenses:
Research and development	19,809	9,723	39,098	23,256
General and administrative	8,438	7,090	16,553	13,931
Total operating expenses	28,247	16,813	55,651	37,187
Operating loss	(25,194)	(11,011)	(48,445)	(22,965)
Interest expense	(782)	(707)	(1,540)	(1,271)
Non-cash interest expense on liability related to sale of future royalties	(3,717)		(6,012)
Interest and other income, net	612	107	1,049	170
Net loss	$ (29,081)	$ (11,611)	$ (54,948)	$ (24,066)
Net loss per share - basic and diluted	$ (0.60)	$ (0.29)	$ (1.22)	$ (0.61)
Weighted-average number of shares used in computing net loss per share — basic and diluted	48,218	39,666	44,910	39,629
Other comprehensive (loss) income:
Unrealized (loss) gain on available-for-sale securities, net	$ (78)	$ 73	$ (223)	$ 80
Comprehensive loss	$ (29,159)	$ (11,538)	$ (55,171)	$ (23,986)

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (54,948)	$ (24,066)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization of property and equipment	860	340
Gain on disposal of equipment	(82)	(2)
Stock-based compensation	4,141	3,400
Non-cash interest expense related to long-term debt	278	257
Non-cash interest expense on liability related to sale of future royalties	6,036	0
Changes in operating assets and liabilities:
Accounts receivable	24	(14)
Prepaid and other assets	(2,663)	(4,600)
Accounts payable	(1,888)	841
Accrued and other liabilities	(3,912)	1,143
Deferred revenue	(51)	(6,024)
Net cash used in operating activities	(52,205)	(28,725)
Cash flows from investing activities:
Purchases of investments	(201,531)	(70,709)
Proceeds from sales and maturities of investments	66,928	47,036
Proceeds from sale of property and equipment	0	32
Purchases of property and equipment	(1,646)	(436)
Net cash used in investing activities	(136,249)	(24,077)
Cash flows from financing activities:
Proceeds from public offerings of common stock, net of issuance costs	112,232	0
Proceeds from sale of future royalties, net of issuance costs	90,621	0
Proceeds from issuance of common stock related to sale of future royalties, net of issuance costs	7,560	0
Proceeds from long term debt, net of debt discount and issuance costs	0	14,996
Proceeds from stock based award activities and warrants, net	11,878	454
Net cash provided by financing activities	222,291	15,450
Net increase (decrease) in cash and cash equivalents	33,837	(37,352)
Cash and cash equivalents, beginning of period	66,874	65,076
Cash and cash equivalents, end of period	$ 100,711	$ 27,724

Organization and Significant Accounting Policies	6 Months Ended
Organization and Significant Accounting Policies	Jun. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Significant Accounting Policies	Note 1 — Organization and Significant Accounting Policies 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 $573.2 million since inception and there can be no assurance that the Company will attain profitability. The Company had a net loss of $54.9 million and net cash used in operations of $52.2 million for the six months ended June 30, 2017. Cash, cash equivalents and investments increased to $332.1 million at June 30, 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, contract payments under its collaboration agreements, sale of future royalties, debt financing arrangements, sales of its convertible preferred stock, 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, from the filing date of this Quarterly Report on Form 10-Q. 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 condensed consolidated financial statements include the accounts of Cytokinetics and its wholly owned subsidiary. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“GAAP”) for interim financial information and the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. The financial statements include all adjustments (consisting only of normal recurring adjustments) that management believes are necessary for the fair statement of the Company’s position at June 30, 2017, and the results of operations for the three and six months ended June 30, 2017 and the cash flows for the six months ended June 30, 2017. These interim financial statement results are not necessarily indicative of results to be expected for the full fiscal year or any future interim period. The balance sheet at December 31, 2016 has been derived from the audited financial statements at that date, but does not include all of the information and footnotes required by GAAP for complete financial statements. The financial statements and related disclosures have been prepared with the presumption that users of the interim financial statements have read or have access to the audited financial statements for the preceding fiscal year. Accordingly, these financial statements should be read in conjunction with the audited financial statements and notes thereto contained in the Company’s Form 10-K for the year ended December 31, 2016, as filed with the SEC on March 6, 2017. 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. Stock-Based Compensation The Company accounts for stock-based payment awards made to employees and directors, including employee stock options and employee stock purchases by measuring the stock-based compensation cost at the grant date based on the calculated fair value of the award, and recognizing expense on a straight-line basis over the employee’s 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, from time to time it will likely change the valuation assumptions it uses to value stock based awards granted in future periods. The assumptions used in calculating the fair value of share-based payment awards represent management’s best estimates at the time, but these estimates involve inherent uncertainties and the application of management judgment. As a result, if conditions change and the management uses different assumptions, the Company’s stock-based compensation expense could be materially different in the future. In addition, the Company will continue to maintain the current forfeiture policy to estimate the expected forfeiture rate and recognize expense only for those shares expected to vest. If the actual forfeiture rate is materially different from management’s estimate, stock-based compensation expense could be significantly different from what has been recorded in the current period. Non-Cash Interest Expense on Liabilities Related to Sale of Future Royalties The Company accounted for Liabilities related to sale of future royalties as a debt financing for accounting purposes, to be amortized under the effective interest rate method over the life of the related royalty stream when the Company has a significant continuing involvement in the generation of royalty streams. Liabilities 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 liabilities 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 May 2017, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (“ASU”) 2017-09, Compensation – Stock Compensation (Topic 718): Scope of Modification Accounting to clarify when to account for a change to the terms or conditions of a share-based payment award as a modification. Under this new guidance, modification accounting is required if the fair value, vesting conditions, or classification of the award changes as a result of the change in terms or conditions. ASU 2017-09 is effective for annual reporting periods beginning after December 15, 2017, including interim reporting periods within each annual reporting period. The Company does not expect the adoption of this guidance to have a material impact on its financial statements or disclosures. 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 March 2016, the FASB issued ASU No. 2016-09 — Improvements to Employee Share-Based Payment Accounting which simplifies various aspects of accounting for share-based payments and presentation in the financial statements. ASU 2016-09 is effective for annual and interim reporting periods beginning after December 15, 2016 and early adoption is permitted. 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 June 30, 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 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 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. In March 2016, the FASB amended the principal-versus-agent implementation guidance and illustrations in the new standard. In April 2016, the FASB amended the guidance on identifying performance obligations and the implementation guidance on licensing in the new standard. In May 2016, the FASB amended the guidance on collectability, noncash consideration, presentation of sales tax and transition in the new standard. In December 2016, the FASB issued ASU No. 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers, which amends certain narrow aspects of the guidance issued in ASU 2014-09. The new standard will become effective starting on January 1, 2018. Early application is permitted to the original effective date of January 1, 2017. 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. While the Company is continuing to assess all potential impacts of the standard, the Company believes the most significant accounting impact will relate to the timing of the recognition of our license, collaboration, and milestone revenues.

Net Loss Per Share

6 Months Ended

Jun. 30, 2017

Earnings Per Share [Abstract]

Net Loss Per Share

Note 2 — Net Loss Per Share

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

	Three Months Ended						Six Months Ended
	June 30, 2017			June 30, 2016			June 30, 2017			June 30, 2016
Net loss	$	(29,081	)	$	(11,611	)	$	(54,948	)	$	(24,066	)
Weighted-average shares used in computing net loss per share — basic and diluted		48,218			39,666			44,910			39,629
Net loss per share — basic and diluted	$	(0.60	)	$	(0.29	)	$	(1.22	)	$	(0.61	)

Basic net loss per share is computed by dividing net loss by the weighted average number of vested common shares outstanding during the period. Diluted net loss per share is computed by giving effect to all potentially dilutive common shares, including outstanding stock options, unvested restricted stock units, warrants, and shares issuable under the Company’s Employee Stock Purchase Plan (“ESPP”), by applying the treasury stock method, if they have a dilutive effect. The following instruments were excluded from the computation of diluted net income (loss) per share because their effect would have been antidilutive (in thousands):

	Three and Six Months Ended
	June 30, 2017		June 30, 2016
Options to purchase common stock		6,170		5,996
Warrants to purchase common stock		310		5,710
Restricted and Performance stock units		461		757
Shares issuable related to the ESPP		18		24
Total shares		6,959		12,487

Supplemental Cash Flow Data

6 Months Ended

Jun. 30, 2017

Supplemental Cash Flow Elements [Abstract]

Supplemental Cash Flow Data

Note 3 — Supplemental Cash Flow Data

Supplemental cash flow data was as follows (in thousands):

	Six Months Ended
	June 30, 2017		June 30, 2016
Cash paid for interest	$	1,270	$	951
Cash paid for taxes		1		1
Significant non-cash investing and financing activities:
Debt discount netted against proceeds from long term debt, recorded in equity	—			288
Interest paid on the long-term debt, at inception	—			63
Purchases of property and equipment through accounts payable		484		234
Purchases of property and equipment through accrued liabilities		670		(76	)

Research and Development Arrangements

6 Months Ended

Jun. 30, 2017

Research And Development [Abstract]

Research and Development Arrangements

Note 4 — Research and Development Arrangements

Amgen Inc. (“Amgen”)

The Company and Amgen continue activities 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 between the Company and Amgen, as amended (the “Amgen Agreement”). The Company has recognized research and development revenue from Amgen for reimbursements 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 December 2016, the Company provided notice of its exercise of its option under the Amgen Agreement to co-invest in the Phase 3 development program of omecamtiv mecarbil at the level of $10.0 million in exchange for an incremental royalty from Amgen of up to 1% on increasing worldwide sales of omecamtiv mecarbil outside Japan. In February 2017, the Company provided notice to Amgen of its further exercise of its co-invest option in the additional amount of $30.0 million (i.e. 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.

The Company made co-investment payments of $6.3 million and $7.5 million during the three and six months ended June 30, 2017, respectively. Because these payments are contingent on Amgen continuing the Phase 3 development program of omecamtiv mecarbil and the benefit to be received in exchange for the payments is not sufficiently separable from the Amgen Agreement the Company reduced research and development revenues by the amount of these payments.

Revenue from Amgen was as follows (in thousands):

	Three Months Ended					Six Months Ended
	June 30, 2017			June 30, 2016		June 30, 2017			June 30, 2016
Research and development revenues
Reimbursement of internal costs	$	388		$	616	$	1,279		$	1,233
Co-investment option payment		(6,250	)		—		(7,500	)		—
Total revenues from Amgen	$	(5,862	)	$	616	$	(6,221	)	$	1,233

There were no accounts receivables due from Amgen as of June 30, 2017 and December 31, 2016.

Under the Amgen Agreement, the Company is eligible to receive over $300.0 million in additional 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 up to $300.0 million in 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 solely upon the results of Amgen’s development and commercialization activities.

In 2013, in conjunction with the Amgen Agreement, the Company sold 1,404,100 shares of its common stock to Amgen, subject to certain trading restrictions. In prior periods, 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 to be 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”)

The Company and Astellas continue activities focused on the research, development, and commercialization of skeletal muscle activators, including CK-2127107, as novel drug candidates for diseases and medical conditions associated with muscle weakness under the Amended and Restated License and Collaboration Agreement dated December 22, 2014, as amended (the “Astellas Agreement”). The Astellas Agreement was further amended effective April 1, 2017 to adjust the payment mechanism under the Astellas Agreement because Astellas will also be incurring a portion of the development costs for ALS. This amendment had no effect on the accounting for the Astellas Agreement.

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 connection with the Astellas Agreement, in 2015, 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 CK-2127107 into Phase 2 clinical development. The Company determined that the license and the research and development services relating to the Astellas Agreement are a single unit of accounting as the license was determined to not have stand-alone value. Accordingly, the Company is recognizing this revenue over the research term of the Astellas Agreement using the proportional performance model.

In 2016, in connection with an amendment to the Astellas Agreement (the “2016 Astellas Amendment”). Astellas paid the Company a $35.0 million non-refundable upfront amendment fee and an accelerated $15.0 million milestone payment for the initiation of the first Phase 2 clinical trial of CK-2127107 in ALS that was otherwise provided for in the Astellas Agreement, as if such milestone had been achieved upon the execution of the 2016 Astellas Amendment, and committed research and development consideration of $44.2 million (total consideration of $94.2 million), which the Company allocated between units of accounting for license fees and research and development services. The Company allocated $24.9 million of research and development consideration to the license and $19.3 million of the research and development consideration to research and development services, to be recognized as revenue as research and development services are performed.

Astellas’ Option on Tirasemtiv

In 2016, in connection with the 2016 Astellas Amendment, 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”). Unless exercised, the Option on Tirasemtiv expires following the receipt of the approval letter for tirasemtiv from the FDA.

Prior to Astellas’ exercise of the Option on Tirasemtiv, the Company will continue the development of tirasemtiv, including VITALITY-ALS, at its own expense to support regulatory approval in the U.S., EU and certain other jurisdictions and will retain the final decision making authority on the development of tirasemtiv. Therefore, the Company concluded that there was no obligation related to any development services during the option period.

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.

The Company concluded that the Option on Tirasemtiv is 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 was as follows (in thousands):

	Three Months Ended June 30, 2017		Three Months Ended June 30, 2016		Six Months Ended June 30, 2017		Six Months Ended June 30, 2016
License revenues	$	4,942	$	1,950	$	6,388	$	5,923
Research and development revenues		3,973		2,898		6,698		6,578
Total Revenue from Astellas	$	8,915	$	4,848	$	13,086	$	12,501

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):

	June 30, 2017		December 31, 2016
Deferred revenue, current	$	7,942	$	8,060
Deferred revenue, non-current	$	15,067	$	15,000

There were no accounts receivable due from Astellas at June 30, 2017 and December 31, 2016.

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 CK-2127107 in non-neuromuscular indications, and over $100.0 million related to CK-2127107 in each of spinal muscular atrophy (“SMA”), amyotrophic lateral sclerosis (“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 are 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 and each milestone will result in additional payments to the Company. 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.

In conjunction with the Astellas Agreement in December 2014, the Company also sold 2,040,816 shares of its common stock to Astellas at a price per share of $4.90 and an aggregate purchase price of $10.0 million, subject to certain trading restrictions. In prior periods, the Company considered Astellas to be a related party, due in part to Astellas’ equity ownership percentage, and reported revenue under the Astellas Agreement to be revenues from a related party. Effective April 1, 2017, in part due to a decrease in Astellas’ equity ownership percentage, the Company no longer considers Astellas to be a related party.

Cash Equivalents and Investments

6 Months Ended

Jun. 30, 2017

Investments Debt And Equity Securities [Abstract]

Cash Equivalents and Investments

Note 5 — 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 June 30, 2017 and December 31, 2016 were as follows (in thousands):

	June 30, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value		Maturity Dates
Cash equivalents — U.S. Treasury and money market funds	$	93,824	$	1	$	—		$	93,825
Short-term investments — U.S. Treasury securities and Agency bonds	$	211,564	$	1	$	(225	)	$	211,340	7/2017 - 6/2018
Long-term investments — Equity, U.S. Treasury securities and Agency bonds	$	19,950	$	180	$	(43	)	$	20,087	7/2018 - 8/2018

	December 31, 2016
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value		Maturity Dates
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	1/2017 – 12/2017
Long-term investments — Equity and U.S. Treasury securities	$	7,513	$	176	$	(17	)	$	7,672	2/2018 – 3/2018

At June 30, 2017 there were no investments that had been in a continuous unrealized loss position for 12 months or longer.

Interest income was as follows (in thousands):

Three Months Ended

Six Months Ended

June 30,

2017

June 30,

2016

June 30,

2017

June 30,

2016

Interest income

694

105

1,183

168

Fair Value Measurements

6 Months Ended

Jun. 30, 2017

Fair Value Disclosures [Abstract]

Fair Value Measurements

Note 6 — Fair Value Measurements

The Company follows the fair value accounting guidance to value its financial assets and liabilities. Fair value is 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.

Fair value of financial assets:

Financial assets measured at fair value on a recurring basis as of June 30, 2017 and December 31, 2016 are classified in the table below in one of the three categories described above (in thousands):

	June 30, 2017
	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Assets At Fair Value
Assets:
Money market funds	$	43,829	$	—	$	—	$	43,829
U.S. Treasury securities		179,411		—		—		179,411
Agency bonds		—		101,834		—		101,834
Equity securities		178		—		—		178
Total	$	223,418	$	101,834	$	—	$	325,252
Amounts included in:
Cash and cash equivalents	$	93,825	$	-	$	—	$	93,825
Short-term investments		114,481		96,859		—		211,340
Long-term investments		15,112		4,975		—		20,087
Total	$	223,418	$	101,834	$	—	$	325,252

	December 31, 2016
	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Assets 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 valuation technique used to measure fair value for the Company’s Level 1 assets is a market approach, using prices and other relevant information generated by market transactions involving identical assets. When quoted market prices are not available for the specific security, then the Company estimates fair value by using benchmark yields, reported trades, broker/dealer quotes, and issuer spreads; these securities are classified as Level 2. As of June 30, 2017 and December 31, 2016, the Company had no financial assets measured at fair value on a recurring basis using significant Level 3 inputs. 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 June 30, 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 $30.0 million and $29.9 million, respectively, because it is carried at a market observable interest rate, which are considered Level 2.

As of June 30, 2017, the fair value of liabilities related to the sale of future royalties is based on the Company’s current estimates of future royalties expected to be paid to RPI over the life of the arrangement, which are considered Level 3 (See Note 9 – “Liability Related to Sale of Future Royalties”).

- 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

6 Months Ended

Jun. 30, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Balance Sheet Components

Note 7 — Balance Sheet Components

Accrued liabilities were as follows (in thousands):

	June 30, 2017		December 31, 2016
Accrued liabilities:
Clinical and preclinical costs	$	6,782	$	10,092
Bonus		2,311		3,800
Other payroll related		2,145		1,888
Consulting and professional fees		1,605		698
Other accrued expenses		702		897
Leasehold improvements	—			672
Total accrued liabilities	$	13,545	$	18,047

Long-Term Debt

6 Months Ended

Jun. 30, 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):

	June 30, 2017			December 31 2016
Notes payable, gross	$	30,000		$	30,000
Less: Unamortized debt discount		(372	)		(472	)
Accretion of final payment fee		531			353
Carrying value of notes payable	$	30,159		$	29,881
Less: Current portion of long-term debt		(7,315	)		(2,500	)
Long-term debt	$	22,844		$	27,381

The Company entered into a loan and security agreement (the “Loan Agreement”) with Oxford Finance LLC (“Oxford”) and Silicon Valley Bank (“SVB”) (Oxford and SVB, collectively the “Lenders”) to fund its working capital and other general corporate needs. The Loan Agreement provides for term loans of up to $40.0 million in aggregate and warrants that are 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.

Under the Loan Agreement, the Company drew down $15.0 million in October 2016 and an additional $15.0 million in February 2016 and issued warrants to purchase 65,189 shares of the Company’s common stock at an exercise price of $6.90 and warrants to purchase 68,285 shares of the Company’s common stock at an exercise price of $6.59 per share. These draw downs bear interest at a rate of 7.5% per annum.

The Company is required to repay the outstanding principal in 36 equal installments beginning October 2017 through October 2020 and to make a final payment fee of 4.0% of the amounts of the Term Loans drawn payable on the earlier of (i) the prepayment of the Term Loans or (ii) the Maturity Date. The loan carries prepayment penalties of 3.0% and 2.0% for prepayment within one and two years, respectively, of the loan origination and 1.0% thereafter.

The Company allocated a portion of the gross proceeds from each draw down under the Loan Agreement to the underlying warrants, using the relative fair value method. This resulted in the allocation of $0.6 million of the draw down proceeds to the warrants, which was accounted for as debt discount. Debt discount is being amortized over the term of the debt, and recorded in interest expense in the statement of operations. The fair value of the warrants was determined using the Black-Scholes pricing model and are classified as equity.

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 Loan 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 Loan Agreement are secured by substantially all of the Company’s current and future assets, other than its intellectual property.

The Company recorded interest on principal, amortization of the debt discount and debt issuance costs, and the accretion of the final payments as interest expense of $0.8 million and $0.7 million for the three months ended June 30, 2017 and 2016, respectively and $1.5 million and $1.3 million for the six months ended June 30, 2017 and 2016, respectively. The effective interest rate on the Loan Agreement, including the amortization of the debt discount and issuance cost, and the accretion of the final payment, was 9.3% for both the three and six months ended June 30, 2017 and 2016.

Future minimum payments under the Loan Agreement, as of June 30, 2017 are as follows (in thousands):

Remainder of 2017	$	3,635
2018		11,743
2019		10,982
2020		8,938
Total minimum payments		35,298
Less: Interest and final payment		(5,298	)
Notes payable, gross	$	30,000

Liabilities Related to Sale of Future Royalties

6 Months Ended

Jun. 30, 2017

Deferred Revenue Disclosure [Abstract]

Note 9 - Liabilities Related to Sale of Future Royalties

In February 2017, the Company entered into a Royalty Purchase Agreement (the “Royalty Agreement”) with RPI Finance Trust (“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 Liabilities 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.

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 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, with an effective annual non-cash interest rate of 17%. 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 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 liabilities related to sale of future royalties during the six months ended June 30, 2017 (in thousands):

Liability related to sale of future royalties at February 1, 2017	$	92,300
Non-cash interest expense recognized		6,012
Liability related to sale of future royalties at June 30, 2017		98,312
Less: Unamortized transaction costs		(1,655	)
Carrying value of liability related to sale of future royalties at June 30, 2017		96,657

Stockholders' Equity

6 Months Ended

Jun. 30, 2017

Equity [Abstract]

Stockholders' Equity

Note 10 — Stockholders’ Equity

During the second quarter of 2017, the Company completed a secondary offering of its common stock and issued 6,049,000 shares for net proceeds of $82.8 million, before expenses.

Accumulated Other Comprehensive Loss

Comprehensive loss is comprised of net loss and other comprehensive loss. Other comprehensive loss is comprised of unrealized holding gains and losses on the Company’s available-for-sale securities that are excluded from net loss and reported separately in stockholders’ equity.

In the first six months of 2017 and 2016, the Company recorded insignificant amounts of unrealized gains (losses) in available-for-sale securities in accumulated other comprehensive loss.

Warrants

In June 2012, the Company issued warrants in connection with two separate, concurrent offerings for our securities. These warrants had an expiration date of June 25, 2017. During the six months ended June 30, 2017, the Company issued 3,240,549 shares of Common stock for exercises of these 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.

Committed Equity Offering

In September 2015, the Company and Cantor Fitzgerald & Co. entered into a Committed Equity Offering (the “CE Offering”) that is an at-the-market issuance sales agreement (the “Cantor Fitzgerald Agreement”) pursuant to which the Company could issue and sell shares of common stock having an aggregate offering price of up to $40.0 million. During the three and six months ended June 30, 2017, the Company issued 987,068 shares and 2,425,625 shares under the Cantor Fitzgerald Agreement for net proceeds totaling $12.5 million and $29.9 million, respectively, completing the sale of all common stock subject to the Cantor Fitzgerald Agreement.

Equity Incentive Plan

In May 2017, the Company’s stockholders approved an amendment to the Amended and Restated 2004 Equity Incentive Plan (the “2004 Equity Incentive Plan”) to increase the number of authorized shares reserved for issuance under the 2004 Equity Incentive Plan by 3.9 million shares. As of June 30, 2017, 3.8 million authorized shares were available for grant under the 2004 Equity Incentive Plan.

Total employee stock-based compensation expenses were $2.2 million and $1.8 million for the three months ended June 30, 2017 and 2016, respectively and $4.1 million and $3.4 million for the six months ended June 30, 2017 and 2016, respectively.

Stock Options

Stock option activity under the 2004 Equity Incentive Plan, for the six months ended June 30, 2017, was as follows:

	Stock Options Outstanding			Weighted Average Exercise Price per Share of Stock Options
Balance at December 31, 2016		5,192,813		$	9.27
Options granted		1,169,624			11.49
Options exercised		(24,055	)		7.09
Options forfeited/expired		(168,294	)		34.25
Balance at June 30, 2017		6,170,088		$	9.02

Restricted Stock Units

Restricted stock unit activity for the six months ended June 30, 2017 was as follows:

	Number of Shares			Weighted Average Award Date Fair Value per Share
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		(500	)		6.67
Unvested restricted stock units outstanding at June 30, 2017		289,502		$	10.44

Restricted Stock Units that Contain Performance Conditions

Performance stock unit activity was as follows:

	Number of Shares			Weighted Average Award Date Fair Value per Share
Performance stock units outstanding at December 31, 2016		685,000		$	7.00
Restricted stock units granted	—			—
Restricted stock units released		(171,250	)		7.00
Restricted stock units forfeited		(342,500	)		7.00
Performance stock units outstanding at June 30, 2017		171,250		$	7.00

Interest and Other Income, Net	6 Months Ended
Interest and Other Income, Net	Jun. 30, 2017
Other Income And Expenses Abstract
Interest and Other Income, Net	Note 11 — Interest and Other Income, Net Interest and other income, net for the three and six months ended June 30, 2017 and 2016 primarily consisted of interest income generated from the Company’s cash, cash equivalents and investments.

Commitments and Contingencies

6 Months Ended

Jun. 30, 2017

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

Note 12 — Commitments and Contingencies

Commitments

Operating Lease

The Company leases office space and equipment under a non-cancelable operating lease that expires in 2018, with an option to extend the lease for an additional three-year period. The lease terms provide for rental payments on a graduated scale and the Company’s payment of certain operating expenses. During March 2016, the Company amended the lease agreement to include certain additional operating expenses, related to the replacement of two boilers. The Company recognizes rent expense on a straight-line basis over the lease period. Rent expense was $0.9 million and $0.8 million, respectively, for the three months ended June 30, 2017 and 2016 and $1.8 million and $1.7 million, respectively, for the six months ended June 30, 2017 and 2016.

Co-invest option

In December 2016, the Company agreed to exercise its option to co-invest $10.0 million in the Phase 3 development program of omecamtiv mecarbil under the Amgen Agreement. In February 2017, the Company provided notice to Amgen of its further exercise of its co-investment option in the additional amount of $30.0 million (i.e. to co-invest $40.0 million) in the Phase 3 development program of omecamtiv mecarbil under the Amgen Agreement. By exercising its option and fully co-funding $40.0 million, the Company will be eligible to receive a total incremental royalty of up to 4% on increasing worldwide sales of omecamtiv mecarbil outside of Japan and have 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 June 30, 2017, future minimum payments due to Amgen were as follows (in thousands):

Remainder of 2017	$	12,500
2018		18,750
Total	$	31,250

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.

Income Taxes	6 Months Ended
Income Taxes	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Note 13 — Income Taxes The Company did not record a provision for income tax for the three and six months ended June 30, 2017 because the Company expects to report a net tax loss for the year ending December 31, 2017. The Company defines the threshold for recognizing the benefits of tax return positions in the financial statements as “more-likely-than-not” to be sustained by the taxing authorities based solely on the technical merits of the position. If the recognition threshold is met, the tax benefit is measured and recognized as the largest amount of tax benefit that, in the Company’s judgment, is greater than 50% likely to be realized.

Organization and Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
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 condensed consolidated financial statements include the accounts of Cytokinetics and its wholly owned subsidiary. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“GAAP”) for interim financial information and the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. The financial statements include all adjustments (consisting only of normal recurring adjustments) that management believes are necessary for the fair statement of the Company’s position at June 30, 2017, and the results of operations for the three and six months ended June 30, 2017 and the cash flows for the six months ended June 30, 2017. These interim financial statement results are not necessarily indicative of results to be expected for the full fiscal year or any future interim period. The balance sheet at December 31, 2016 has been derived from the audited financial statements at that date, but does not include all of the information and footnotes required by GAAP for complete financial statements. The financial statements and related disclosures have been prepared with the presumption that users of the interim financial statements have read or have access to the audited financial statements for the preceding fiscal year. Accordingly, these financial statements should be read in conjunction with the audited financial statements and notes thereto contained in the Company’s Form 10-K for the year ended December 31, 2016, as filed with the SEC on March 6, 2017. 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.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock-based payment awards made to employees and directors, including employee stock options and employee stock purchases by measuring the stock-based compensation cost at the grant date based on the calculated fair value of the award, and recognizing expense on a straight-line basis over the employee’s 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, from time to time it will likely change the valuation assumptions it uses to value stock based awards granted in future periods. The assumptions used in calculating the fair value of share-based payment awards represent management’s best estimates at the time, but these estimates involve inherent uncertainties and the application of management judgment. As a result, if conditions change and the management uses different assumptions, the Company’s stock-based compensation expense could be materially different in the future. In addition, the Company will continue to maintain the current forfeiture policy to estimate the expected forfeiture rate and recognize expense only for those shares expected to vest. If the actual forfeiture rate is materially different from management’s estimate, stock-based compensation expense could be significantly different from what has been recorded in the current period.
Non-Cash Interest Expense on Liabilities Related to Sale of Future Royalties	Non-Cash Interest Expense on Liabilities Related to Sale of Future Royalties The Company accounted for Liabilities related to sale of future royalties as a debt financing for accounting purposes, to be amortized under the effective interest rate method over the life of the related royalty stream when the Company has a significant continuing involvement in the generation of royalty streams. Liabilities 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 liabilities 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 May 2017, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (“ASU”) 2017-09, Compensation – Stock Compensation (Topic 718): Scope of Modification Accounting to clarify when to account for a change to the terms or conditions of a share-based payment award as a modification. Under this new guidance, modification accounting is required if the fair value, vesting conditions, or classification of the award changes as a result of the change in terms or conditions. ASU 2017-09 is effective for annual reporting periods beginning after December 15, 2017, including interim reporting periods within each annual reporting period. The Company does not expect the adoption of this guidance to have a material impact on its financial statements or disclosures. 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 March 2016, the FASB issued ASU No. 2016-09 — Improvements to Employee Share-Based Payment Accounting which simplifies various aspects of accounting for share-based payments and presentation in the financial statements. ASU 2016-09 is effective for annual and interim reporting periods beginning after December 15, 2016 and early adoption is permitted. 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 June 30, 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 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 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. In March 2016, the FASB amended the principal-versus-agent implementation guidance and illustrations in the new standard. In April 2016, the FASB amended the guidance on identifying performance obligations and the implementation guidance on licensing in the new standard. In May 2016, the FASB amended the guidance on collectability, noncash consideration, presentation of sales tax and transition in the new standard. In December 2016, the FASB issued ASU No. 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers, which amends certain narrow aspects of the guidance issued in ASU 2014-09. The new standard will become effective starting on January 1, 2018. Early application is permitted to the original effective date of January 1, 2017. 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. While the Company is continuing to assess all potential impacts of the standard, the Company believes the most significant accounting impact will relate to the timing of the recognition of our license, collaboration, and milestone revenues.

Net Loss Per Share (Tables)

6 Months Ended

Jun. 30, 2017

Earnings Per Share [Abstract]

Calculation of Basic and Diluted Net Loss Per Share

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

	Three Months Ended						Six Months Ended
	June 30, 2017			June 30, 2016			June 30, 2017			June 30, 2016
Net loss	$	(29,081	)	$	(11,611	)	$	(54,948	)	$	(24,066	)
Weighted-average shares used in computing net loss per share — basic and diluted		48,218			39,666			44,910			39,629
Net loss per share — basic and diluted	$	(0.60	)	$	(0.29	)	$	(1.22	)	$	(0.61	)

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

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

	Three and Six Months Ended
	June 30, 2017		June 30, 2016
Options to purchase common stock		6,170		5,996
Warrants to purchase common stock		310		5,710
Restricted and Performance stock units		461		757
Shares issuable related to the ESPP		18		24
Total shares		6,959		12,487

Supplemental Cash Flow Data (Tables)

6 Months Ended

Jun. 30, 2017

Supplemental Cash Flow Elements [Abstract]

Supplemental Cash Flow Data

Supplemental cash flow data was as follows (in thousands):

	Six Months Ended
	June 30, 2017		June 30, 2016
Cash paid for interest	$	1,270	$	951
Cash paid for taxes		1		1
Significant non-cash investing and financing activities:
Debt discount netted against proceeds from long term debt, recorded in equity	—			288
Interest paid on the long-term debt, at inception	—			63
Purchases of property and equipment through accounts payable		484		234
Purchases of property and equipment through accrued liabilities		670		(76	)

Research and Development Arrangements (Tables)

6 Months Ended

Jun. 30, 2017

Amgen [Member]

Summary of Research and Development Revenue

Revenue from Amgen was as follows (in thousands):

	Three Months Ended					Six Months Ended
	June 30, 2017			June 30, 2016		June 30, 2017			June 30, 2016
Research and development revenues
Reimbursement of internal costs	$	388		$	616	$	1,279		$	1,233
Co-investment option payment		(6,250	)		—		(7,500	)		—
Total revenues from Amgen	$	(5,862	)	$	616	$	(6,221	)	$	1,233

Astellas [Member]

Summary of Research and Development Revenue

Research and development revenue from Astellas was as follows (in thousands):

	Three Months Ended June 30, 2017		Three Months Ended June 30, 2016		Six Months Ended June 30, 2017		Six Months Ended June 30, 2016
License revenues	$	4,942	$	1,950	$	6,388	$	5,923
Research and development revenues		3,973		2,898		6,698		6,578
Total Revenue from Astellas	$	8,915	$	4,848	$	13,086	$	12,501

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):

	June 30, 2017		December 31, 2016
Deferred revenue, current	$	7,942	$	8,060
Deferred revenue, non-current	$	15,067	$	15,000

Cash Equivalents and Investments (Tables)

6 Months Ended

Jun. 30, 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 June 30, 2017 and December 31, 2016 were as follows (in thousands):

	June 30, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value		Maturity Dates
Cash equivalents — U.S. Treasury and money market funds	$	93,824	$	1	$	—		$	93,825
Short-term investments — U.S. Treasury securities and Agency bonds	$	211,564	$	1	$	(225	)	$	211,340	7/2017 - 6/2018
Long-term investments — Equity, U.S. Treasury securities and Agency bonds	$	19,950	$	180	$	(43	)	$	20,087	7/2018 - 8/2018

	December 31, 2016
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value		Maturity Dates
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	1/2017 – 12/2017
Long-term investments — Equity and U.S. Treasury securities	$	7,513	$	176	$	(17	)	$	7,672	2/2018 – 3/2018

Summary of Interest Income

At June 30, 2017 there were no investments that had been in a continuous unrealized loss position for 12 months or longer.

Interest income was as follows (in thousands):

Three Months Ended

Six Months Ended

June 30,

2017

June 30,

2016

June 30,

2017

June 30,

2016

Interest income

694

105

1,183

168

Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 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 June 30, 2017 and December 31, 2016 are classified in the table below in one of the three categories described above (in thousands):

	June 30, 2017
	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Assets At Fair Value
Assets:
Money market funds	$	43,829	$	—	$	—	$	43,829
U.S. Treasury securities		179,411		—		—		179,411
Agency bonds		—		101,834		—		101,834
Equity securities		178		—		—		178
Total	$	223,418	$	101,834	$	—	$	325,252
Amounts included in:
Cash and cash equivalents	$	93,825	$	-	$	—	$	93,825
Short-term investments		114,481		96,859		—		211,340
Long-term investments		15,112		4,975		—		20,087
Total	$	223,418	$	101,834	$	—	$	325,252

	December 31, 2016
	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Assets 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)

6 Months Ended

Jun. 30, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Summary of Accrued Liabilities

Accrued liabilities were as follows (in thousands):

	June 30, 2017		December 31, 2016
Accrued liabilities:
Clinical and preclinical costs	$	6,782	$	10,092
Bonus		2,311		3,800
Other payroll related		2,145		1,888
Consulting and professional fees		1,605		698
Other accrued expenses		702		897
Leasehold improvements	—			672
Total accrued liabilities	$	13,545	$	18,047

Long-Term Debt (Tables)

6 Months Ended

Jun. 30, 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):

	June 30, 2017			December 31 2016
Notes payable, gross	$	30,000		$	30,000
Less: Unamortized debt discount		(372	)		(472	)
Accretion of final payment fee		531			353
Carrying value of notes payable	$	30,159		$	29,881
Less: Current portion of long-term debt		(7,315	)		(2,500	)
Long-term debt	$	22,844		$	27,381

Schedule of Future Minimum Payments under Loan Agreement

Future minimum payments under the Loan Agreement, as of June 30, 2017 are as follows (in thousands):

Remainder of 2017	$	3,635
2018		11,743
2019		10,982
2020		8,938
Total minimum payments		35,298
Less: Interest and final payment		(5,298	)
Notes payable, gross	$	30,000

Liabilities Related to Sale of Future Royalties (Tables)

6 Months Ended

Jun. 30, 2017

Deferred Revenue Disclosure [Abstract]

Schedule of activity of allocated transaction consideration on fair value basis liability and common stock

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 Liabilities Related to Sale of Future Royalties

The following table shows the activity within liabilities related to sale of future royalties during the six months ended June 30, 2017 (in thousands):

Liability related to sale of future royalties at February 1, 2017	$	92,300
Non-cash interest expense recognized		6,012
Liability related to sale of future royalties at June 30, 2017		98,312
Less: Unamortized transaction costs		(1,655	)
Carrying value of liability related to sale of future royalties at June 30, 2017		96,657

Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2017

Summary of Stock Option Activity

Stock option activity under the 2004 Equity Incentive Plan, for the six months ended June 30, 2017, was as follows:

	Stock Options Outstanding			Weighted Average Exercise Price per Share of Stock Options
Balance at December 31, 2016		5,192,813		$	9.27
Options granted		1,169,624			11.49
Options exercised		(24,055	)		7.09
Options forfeited/expired		(168,294	)		34.25
Balance at June 30, 2017		6,170,088		$	9.02

Restricted Stock Units [Member]

Summary of Stock Unit Activity

Restricted stock unit activity for the six months ended June 30, 2017 was as follows:

	Number of Shares			Weighted Average Award Date Fair Value per Share
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		(500	)		6.67
Unvested restricted stock units outstanding at June 30, 2017		289,502		$	10.44

Performance Restricted Stock Units [Member]

Summary of Stock Unit Activity

Performance stock unit activity was as follows:

	Number of Shares			Weighted Average Award Date Fair Value per Share
Performance stock units outstanding at December 31, 2016		685,000		$	7.00
Restricted stock units granted	—			—
Restricted stock units released		(171,250	)		7.00
Restricted stock units forfeited		(342,500	)		7.00
Performance stock units outstanding at June 30, 2017		171,250		$	7.00

Commitments and Contingencies (Tables)

6 Months Ended

Jun. 30, 2017

Amgen [Member]

Future Minimum Payments for Co-investment

Remainder of 2017	$	12,500
2018		18,750
Total	$	31,250

Organization and Significant Accounting Policies - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended
	Jan. 01, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit incurred		$ (573,239,000)		$ (573,239,000)		$ (518,291,000)
Net loss		(29,081,000)	$ (11,611,000)	(54,948,000)	$ (24,066,000)
Net cash used in operating activities				(52,205,000)	$ (28,725,000)
Cash, cash equivalents and investments		332,100,000		$ 332,100,000		$ 163,900,000
Cash requirements term				12 months
Effect on previously reported net income due to reclassifications				$ 0
Excess tax benefit recognized	$ 700,000
Accounting Standards Update 2016-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment to retained earnings		$ 0		$ 0

Net Loss Per Share - Calculation of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net loss	$ (29,081)	$ (11,611)	$ (54,948)	$ (24,066)
Weighted-average shares used in computing net loss per share — basic and diluted	48,218	39,666	44,910	39,629
Net loss per share - basic and diluted	$ (0.60)	$ (0.29)	$ (1.22)	$ (0.61)

Net Loss Per Share - Instruments Excluded from the Computation of Diluted Net Income (Loss) Per Share (Detail) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	6,959	12,487	6,959	12,487
Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	6,170	5,996	6,170	5,996
Warrants to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	310	5,710	310	5,710
Restricted and Performance Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	461	757	461	757
ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	18	24	18	24

Supplemental Cash Flow Data - Supplemental Cash Flow Data (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Supplemental Cash Flow Elements [Abstract]
Cash paid for interest	$ 1,270	$ 951
Cash paid for taxes	1	1
Significant non-cash investing and financing activities:
Debt discount netted against proceeds from long term debt, recorded in equity	0	288
Interest paid on the long-term debt, at inception	0	63
Purchases of property and equipment through accounts payable	484	234
Purchases of property and equipment through accrued liabilities	$ 670	$ (76)

Research and Development Arrangements - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 28, 2017	Dec. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable		$ 24,000					$ 24,000
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
Amgen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Co-invest option exercised amount	$ 30,000,000	10,000,000					$ 10,000,000
Percentage of incremental royalty receivable on annual net sales	4.00%						1.00%
Co-fund costs requirement value	$ 40,000,000				$ 40,000,000
Co-invest option payment			$ 6,300,000		7,500,000
Accounts receivable		0	0		0		$ 0
Pre-commercialization milestone payments eligible to receive			300,000,000		300,000,000
Common stock, shares issued in period										1,404,100
Amgen [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Commercialization milestone payments eligible to receive			300,000,000		300,000,000
Astellas [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable		0	0		0		0
Upfront payment received			3,973,000	$ 2,898,000	$ 6,698,000	$ 6,578,000
Percentage of shared costs					25.00%
Astellas [Member] \| ALS License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue Recognition over Performance Period		24,900,000					24,900,000
Astellas [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] \| 2014 Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, shares issued in period									2,040,816
Upfront payment received								$ 30,000,000
Amount received as milestone payment								$ 15,000,000
Common stock issued, price per share									$ 4.90
Common stock issued, value									$ 10,000,000
Astellas [Member] \| 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount received as milestone payment					$ 15,000,000		15,000,000
Revenue recognition, Non-refundable upfront amendment fee							35,000,000
Revenue Recognition over Performance Period		$ 44,200,000					44,200,000
Allocated Consideration							$ 94,200,000
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] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research milestone payments					2,000,000
Astellas [Member] \| Maximum [Member] \| 2016 Astellas Amendment [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] \| Maximum [Member] \| 2016 Astellas Amendment [Member] \| Non- neuromuscular Indications [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement					95,000,000
Astellas [Member] \| Maximum [Member] \| 2016 Astellas Amendment [Member] \| Commercial Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement					200,000,000
Astellas [Member] \| Maximum [Member] \| Initial Indication [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments to be received			100,000,000		100,000,000
Astellas [Member] \| Maximum [Member] \| Subsequent Indication [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments to be received			$ 50,000,000		50,000,000
Astellas [Member] \| Minimum [Member] \| 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement					600,000,000
Astellas [Member] \| Minimum [Member] \| 2016 Astellas Amendment [Member] \| Tirasemtiv Option [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option exercise payment to be received					25,000,000
Astellas [Member] \| Minimum [Member] \| 2016 Astellas Amendment [Member] \| SMA and Other Neuromuscular Indications [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement					$ 100,000,000

Research and Development Arrangements - Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Research And Development Arrangement Contract To Perform For Others [Line Items]
License revenues	$ 4,942	$ 1,950	$ 6,388	$ 5,923
Total revenues	3,053	5,802	7,206	14,222
Amgen [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Reimbursement of internal costs	388	616	1,279	1,233
Co-investment option payment	(6,250)		(7,500)
Total revenues	(5,862)	616	(6,221)	1,233
Astellas [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
License revenues	4,942	1,950	6,388	5,923
Total research and development revenue from Astellas	3,973	2,898	6,698	6,578
Total revenues	$ 8,915	$ 4,848	$ 13,086	$ 12,501

Research and Development Arrangements - Deferred Revenue (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Deferred Revenue Arrangement [Line Items]
Deferred revenue, current	$ 7,942	$ 8,060
Deferred revenue, non-current	15,067	15,000
Astellas [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue, current	7,942	8,060
Deferred revenue, non-current	$ 15,067	$ 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

Cash Equivalents and Investments - Amortized Cost and Fair Value of Cash Equivalents and Available for Sale Investments (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Short-term Investments [Member] \| U.S. Treasury Securities and Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Amortized Cost	$ 211,564
Unrealized Gains	1
Unrealized Losses	(225)
Fair Value	211,340
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, U.S. Treasury Securities and Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Amortized Cost	19,950
Unrealized Gains	180
Unrealized Losses	(43)
Fair Value	20,087
Long-term Investments [Member] \| Equity and U.S. Treasury Securities [Member]
Cash and Cash Equivalents [Line Items]
Amortized Cost		7,513
Unrealized Gains		176
Unrealized Losses		(17)
Fair Value		7,672
Cash Equivalents [Member] \| US Treasury Securities and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Amortized Cost	93,824	55,658
Unrealized Gains	1	0
Unrealized Losses	0	0
Fair Value	$ 93,825	$ 55,658
Maximum [Member] \| Short-term Investments [Member] \| U.S. Treasury Securities and Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Maturity Dates	Jun. 30, 2018
Maximum [Member] \| Short-term Investments [Member] \| U.S. Treasury Securities [Member]
Cash and Cash Equivalents [Line Items]
Maturity Dates		Dec. 31, 2017
Maximum [Member] \| Long-term Investments [Member] \| Equity, U.S. Treasury Securities and Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Maturity Dates	Aug. 30, 2018
Maximum [Member] \| Long-term Investments [Member] \| Equity and U.S. Treasury Securities [Member]
Cash and Cash Equivalents [Line Items]
Maturity Dates		Mar. 31, 2018
Minimum [Member] \| Short-term Investments [Member] \| U.S. Treasury Securities and Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Maturity Dates	Jul. 31, 2017
Minimum [Member] \| Short-term Investments [Member] \| U.S. Treasury Securities [Member]
Cash and Cash Equivalents [Line Items]
Maturity Dates		Jan. 31, 2017
Minimum [Member] \| Long-term Investments [Member] \| Equity, U.S. Treasury Securities and Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Maturity Dates	Jul. 31, 2018
Minimum [Member] \| Long-term Investments [Member] \| Equity and U.S. Treasury Securities [Member]
Cash and Cash Equivalents [Line Items]
Maturity Dates		Feb. 28, 2018

Cash Equivalents and Investments - Additional Information (Detail)	Jun. 30, 2017 USD ($)
Cash And Cash Equivalents [Abstract]
Investments in continuous unrealized loss position for 12 months or longer	$ 0

Cash Equivalents and Investments - Summary of Interest Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Cash Cash Equivalents And Short Term Investments [Abstract]
Interest income	$ 694	$ 105	$ 1,183	$ 168

Fair Value Measurements - Additional Information (Detail) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long term debt, fair value	$ 30,000,000	$ 29,900,000
Fair Value Measurements Using Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets measured at fair value on a recurring basis	$ 0	$ 0

Balance Sheet Components - Summary of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Accrued liabilities:
Clinical and preclinical costs	$ 6,782	$ 10,092
Bonus	2,311	3,800
Other payroll related	2,145	1,888
Consulting and professional fees	1,605	698
Other accrued expenses	702	897
Leasehold improvements	0	672
Total accrued liabilities	$ 13,545	$ 18,047

Long-Term Debt - Schedule of Long Term Debt and Unamortized Debt Discount (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Notes payable, gross	$ 30,000	$ 30,000
Less: Unamortized debt discount	(372)	(472)
Accretion of final payment fee	531	353
Carrying value of notes payable	30,159	29,881
Long-term Debt, by current and noncurrent
Carrying value of notes payable	30,159	29,881
Less: Current portion of long-term debt	(7,315)	(2,500)
Long-term debt	$ 22,844	$ 27,381

Long-Term Debt - Additional Information (Detail)	1 Months Ended		3 Months Ended		6 Months Ended
Long-Term Debt - Additional Information (Detail)	Oct. 31, 2016 USD ($) $ / shares shares	Feb. 29, 2016 USD ($) $ / shares shares	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Installment	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Debt instrument, unamortized discount			$ 372,000		$ 372,000		$ 472,000
Interest expense			782,000	$ 707,000	1,540,000	$ 1,271,000
Warrants to Purchase Common Stock [Member]
Debt Instrument [Line Items]
Debt instrument, unamortized discount			600,000		600,000
Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Warrants outstanding to purchase upon exercise of common stock \| shares	65,189	68,285
Warrants exercise price \| $ / shares	$ 6.90	$ 6.59
Oxford and Silicon Valley Bank [Member] \| Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Term loan maximum borrowing capacity			$ 40,000,000		$ 40,000,000
Warrant exercisable term					5 years
Proceeds from term loan	$ 15,000,000	$ 15,000,000
Warrants outstanding to purchase upon exercise of common stock \| shares	65,189	68,285
Warrants exercise price \| $ / shares	$ 6.90	$ 6.59
Term loan interest rate	7.50%	7.50%
Loan repayment terms					The Company is required to repay the outstanding principal in 36 equal installments beginning October 2017 through October 2020
Number of installments description					36 equal installments beginning October 2017 through October 2020
Debt instrument, installment end date					2020-10
Number of installments \| Installment					36
Final payment fee percentage					4.00%
Effective interest rate on the amounts borrowed under the Agreement			9.30%	9.30%	9.30%	9.30%
Oxford and Silicon Valley Bank [Member] \| Loan and Security Agreement [Member] \| Within One Year [Member]
Debt Instrument [Line Items]
Percentage of prepayment fee					3.00%
Oxford and Silicon Valley Bank [Member] \| Loan and Security Agreement [Member] \| Within Two Year [Member]
Debt Instrument [Line Items]
Percentage of prepayment fee					2.00%
Oxford and Silicon Valley Bank [Member] \| Loan and Security Agreement [Member] \| Thereafter [Member]
Debt Instrument [Line Items]
Percentage of prepayment fee					1.00%

Long-Term Debt - Schedule of Future Minimum Payments under Loan Agreement (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying value of notes payable	$ 30,159	$ 29,881
Long-term debt, alternative
Carrying value of notes payable	30,159	29,881
Notes payable, gross	30,000	$ 30,000
Loan and Security Agreement [Member] \| Oxford and Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Remainder of 2017	3,635
2018	11,743
2019	10,982
2020	8,938
Carrying value of notes payable	35,298
Long-term debt, alternative
Carrying value of notes payable	35,298
Less: Interest and final payment	(5,298)
Notes payable, gross	$ 30,000

Liabilities Related to Sale of Future Royalties - Additional Information (Detail) - USD ($) $ in Thousands		1 Months Ended	6 Months Ended
	Mar. 31, 2017	Feb. 28, 2017	Jun. 30, 2017	Jun. 30, 2016
Liability Related to Sale of Future Royalties [Line Items]
Proceeds from issuance of common stock			$ 112,232	$ 0
Transaction costs incurred in connection with the Royalty Monetization and RPI Common stock			1,800
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
Fair value for the liability related to sale of future royalties			$ 96,700
Effective annual non-cash interest rate in calculating liability related to sale of future royalties			17.00%
Common stock fair value	$ 8,100

Liabilities 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	6 Months Ended
	Jun. 30, 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

Liabilities Related to Sale of Future Royalties - Schedule Represents Activity Within Liabilities Related to Sale of Future Royalties (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Liability Related to Sale of Future Royalties [Line Items]
Non-cash interest expense recognized	$ (6,036)	$ 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	6,012
Liability related to sale of future royalties ending balance	98,312
Less: Unamortized transaction costs	(1,655)
Carrying value of liability related to sale of future royalties at June 30, 2017	$ 96,657

Stockholders' Equity - Additional Information (Detail) - USD ($)	1 Months Ended			3 Months Ended		6 Months Ended
	May 31, 2017	Jan. 31, 2017	Sep. 30, 2015	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Oct. 31, 2016	Feb. 29, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net proceeds of issuance of common stock						$ 112,232,000	$ 0
Warrant expiration date						Jun. 25, 2017
Number of issued shares of common stock related to cashless exercise of warrants						3,240,549
Equity incentive plan authorized shares	3,900,000
Shares available for grant of options				3,800,000		3,800,000
Employee stock-based compensation expenses				$ 2,200,000	$ 1,800,000	$ 4,100,000	$ 3,400,000
Cantor Fitzgerald Agreement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued				987,068		2,425,625
Net proceeds of issuance of common stock				$ 12,500,000		$ 29,900,000
Loan and Security Agreement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants purchase upon exercise of common stock								65,189	68,285
Warrants purchase upon exercise of common stock, exercise price								$ 6.90	$ 6.59
Warrants to Purchase Common Stock [Member] \| Loan and Security Agreement [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
Secondary Offering [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued				6,049,000
Net proceeds of issuance of common stock				$ 82,800,000
At-The-Market Issuance Sales Agreement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum aggregated offer value of saleable and issuable shares			$ 40,000,000

Stockholders' Equity - Summary of Equity Incentive Plan (Detail)	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Options Outstanding, Beginning Balance \| shares	5,192,813
Stock Options Outstanding, Options granted \| shares	1,169,624
Stock Options Outstanding, Options exercised \| shares	(24,055)
Stock Options Outstanding, Options forfeited/expired \| shares	(168,294)
Stock Options Outstanding, Ending Balance \| shares	6,170,088
Weighted Average Exercise Price per Share of Stock Options, Beginning Balance \| $ / shares	$ 9.27
Weighted Average Exercise Price per Share of Stock Options, Options granted \| $ / shares	11.49
Weighted Average Exercise Price per Share of Stock Options, Options exercised \| $ / shares	7.09
Weighted Average Exercise Price per Share of Stock Options, Options forfeited/expired \| $ / shares	34.25
Weighted Average Exercise Price per Share of Stock Options, Ending Balance \| $ / shares	$ 9.02

Stockholders' Equity - Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units [Member]	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding, Number of Shares, Beginning Balance \| shares	64,502
Restricted stock units granted, Number of Shares \| shares	269,000
Restricted stock units released, Number of Shares \| shares	(43,500)
Restricted stock units forfeited, Number of Shares \| shares	(500)
Restricted stock units outstanding, Number of Shares, Ending Balance \| shares	289,502
Restricted stock units outstanding, Weighted Average Award Date Fair Value per Share, Beginning Balance \| $ / shares	$ 7.19
Restricted stock units granted, Weighted Average Award Date Fair Value per Share \| $ / shares	10.60
Restricted stock units released, Weighted Average Award Date Fair Value per Share \| $ / shares	6.67
Restricted stock units forfeited, Weighted Average Award Date Fair Value per Share \| $ / shares	6.67
Restricted stock 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 Restricted Stock Units [Member]	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding, Number of Shares, Beginning Balance \| shares	685,000
Restricted stock units granted, Number of Shares \| shares	0
Restricted stock units released, Number of Shares \| shares	(171,250)
Restricted stock units forfeited, Number of Shares \| shares	(342,500)
Restricted stock units outstanding, Number of Shares, Ending Balance \| shares	171,250
Restricted stock units outstanding, Weighted Average Award Date Fair Value per Share, Beginning Balance \| $ / shares	$ 7.00
Performance stock units granted, Weighted Average Award Date Fair Value per Share \| $ / shares	0
Restricted stock units released, Weighted Average Award Date Fair Value per Share \| $ / shares	7.00
Restricted stock units forfeited, Weighted Average Award Date Fair Value per Share \| $ / shares	7.00
Restricted stock units outstanding, Weighted Average Award Date Fair Value per Share, Ending Balance \| $ / shares	$ 7.00

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 28, 2017	Dec. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Other Commitments [Line Items]
Non-cancelable operating lease expiration year					2018
Non-cancelable operating lease additional extension period					3 years
Rent expense			$ 0.9	$ 0.8	$ 1.8	$ 1.7
Amgen [Member]
Other Commitments [Line Items]
Co-invest option exercised amount	$ 30.0	$ 10.0					$ 10.0
Percentage of incremental royalty receivable on annual net sales	4.00%						1.00%
Co-fund costs requirement value	$ 40.0				$ 40.0

Commitments and Contingencies - Future Minimum Payments for Co-investment (Detail) - Amgen [Member] $ in Thousands	Jun. 30, 2017 USD ($)
Other Commitments [Line Items]
Remainder of 2017	$ 12,500
2018	18,750
Total	$ 31,250

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 0	$ 0
Percentage of income tax likely to be realized		50.00%

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	$ 325,252	$ 152,705
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	43,829	52,657
U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	179,411	99,872
Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	101,834
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	178	176
Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets at fair value	20,087	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	223,418	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	43,829	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	179,411	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	178	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	15,112	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	101,834	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	101,834
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	4,975	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	93,825	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	93,825	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	211,340	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	114,481	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	96,859	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