Cytokinetics (CYTK) 10-K 7 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Mar. 05, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	CYTK
Entity Registrant Name	CYTOKINETICS INC
Entity Central Index Key	1,061,983
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		54,888,369
Entity Public Float			$ 449.7

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 42,256	$ 125,206
Short-term investments	156,475	143,685
Accounts receivable	2,231	1,112
Contract assets	4,554	0
Prepaid and other current assets	2,158	4,292
Total current assets	207,674	274,295
Long-term investments	0	16,518
Property and equipment, net	3,204	3,568
Other assets	300	429
Total assets	211,178	294,810
Current liabilities:
Accounts payable	3,764	5,253
Accrued liabilities	15,757	17,392
Deferred revenue, current	0	9,572
Current portion of long-term debt	2,607	0
Other current liabilities	66	227
Total current liabilities	22,194	32,444
Long-term debt	39,806	31,777
Liability related to the sale of future royalties, net	122,473	104,650
Deferred revenue, non-current	0	15,000
Other long-term liabilities	771	1,097
Total liabilities	185,244	184,968
Commitments and contingencies	0	0
Stockholders’ equity:
Preferred stock, $0.001 par value: Authorized: 10,000,000 shares; Issued and outstanding: none	0	0
Common stock, $0.001 par value: Authorized: 163,000,000 shares Issued and outstanding: 54,717,906 shares at December 31, 2018 and 53,960,832 shares at December 31, 2017	55	54
Additional paid-in capital	768,703	755,526
Accumulated other comprehensive income	500	343
Accumulated deficit	(743,324)	(646,081)
Total stockholders’ equity	25,934	109,842
Total liabilities and stockholders’ equity	$ 211,178	$ 294,810

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
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	54,717,906	53,960,832
Common stock, shares outstanding	54,717,906	53,960,832

Consolidated Statement of Opera

Consolidated Statement of Operations And Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Total revenues	$ 31,501	$ 13,368
Operating expenses:
Research and development	89,135	90,296
General and administrative	31,282	36,468
Total operating expenses	120,417	126,764
Operating loss	(88,916)	(113,396)
Interest expense	(3,797)	(3,016)
Non-cash interest expense on liability related to sale of future royalties	(17,767)	(13,980)
Interest and other income, net	4,191	2,602
Net loss	$ (106,289)	$ (127,790)
Net loss income per share — basic and diluted	$ (1.95)	$ (2.59)
Weighted-average number of shares used in computing net loss per share — basic and diluted	54,420	49,404
Other comprehensive loss:
Unrealized gains on available-for-sale securities, net	$ 157	$ 206
Comprehensive loss	(106,132)	(127,584)
Research and Development, Grant and Other Revenues, Net [Member]
Revenues:
Total revenues	26,368	4,569
License Revenues [Member]
Revenues:
Total revenues	$ 5,133	$ 8,799

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2016	$ 94,361	$ 41	$ 612,474	$ 137	$ (518,291)
Beginning Balance, shares at Dec. 31, 2016		40,646,595
Exercise of stock options, value	1,918	$ 0	1,918	0	0
Exercise of stock options, shares		264,164
Issuance under Employee Stock Purchase Plan, value	1,167	$ 0	1,167	0	0
Issuance under Employee Stock Purchase Plan, shares		120,959
Vesting of restricted stock units, net of taxes withheld, value	(904)	$ 0	(904)	0	0
Vesting of restricted stock units, net of taxes withheld, shares		128,711
Exercise of warrants, value	12,071	$ 3	12,068	0	0
Exercise of warrants, shares		3,450,122
Issuance under secondary offering net of issuance costs	82,370	$ 6	82,364	0	0
Issuance under secondary offering, shares		6,049,000
Issuance net of commission and issuance costs	29,855	$ 3	29,852	0	0
Issuance net of commission and issuance costs, shares		2,425,625
Issuance pursuant to Royalty Purchase Agreement	7,560	$ 1	7,559	0	0
Issuance pursuant to Royalty Purchase Agreement, shares		875,656
Stock-based compensation	9,028	$ 0	9,028	0	0
Other comprehensive loss	206	0	0	206	0
Net loss	(127,790)	0	0	0	(127,790)
Ending Balance at Dec. 31, 2017	109,842	$ 54	755,526	343	(646,081)
Ending Balance, shares at Dec. 31, 2017		53,960,832
Exercise of stock options, value	3,173	$ 1	3,172	0	0
Exercise of stock options, shares		422,819
Issuance under Employee Stock Purchase Plan, value	928	$ 0	928	0	0
Issuance under Employee Stock Purchase Plan, shares		144,822
Vesting of restricted stock units, net of taxes withheld, value	(866)	$ 0	(866)	0	0
Vesting of restricted stock units, net of taxes withheld, shares		189,433
Issuance of warrants	182	$ 0	182	0	0
Stock-based compensation	9,761	0	9,761	0	0
ASC 606 Adoption | ASC 606 [Member]	9,046	0	0	0	9,046
Other comprehensive loss	157	0	0	157	0
Net loss	(106,289)	0	0	0	(106,289)
Ending Balance at Dec. 31, 2018	$ 25,934	$ 55	$ 768,703	$ 500	$ (743,324)
Ending Balance, shares at Dec. 31, 2018		54,717,906

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (106,289)	$ (127,790)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash interest expense on liability related to sale of future royalties	17,767	14,028
Stock-based compensation	9,761	9,028
Depreciation and amortization of property and equipment	1,239	1,920
Interest receivable and amortization on investments	(1,677)	0
Net gain on disposal of equipment	0	(67)
Non-cash interest expense related to long-term debt	920	635
Changes in operating assets and liabilities:
Accounts receivable	(1,119)	(1,088)
Contract assets	5,154	0
Prepaid and other assets	1,817	(2,161)
Accounts payable	(1,490)	1,457
Accrued and other liabilities	(2,063)	766
Contract liabilities	(18,750)	0
Deferred revenue	(6,485)	1,513
Net cash used in operating activities	(101,215)	(101,759)
Cash flows from investing activities:
Purchases of investments	(240,224)	(240,413)
Sales and maturities of investments	246,232	177,462
Purchases of property and equipment	(889)	(2,877)
Sales of property and equipment	14	0
Net cash provided by (used in) investing activities	5,133	(65,828)
Cash flows from financing activities:
Proceeds from public offerings of common stock, net of issuance costs	0	112,224
Proceeds from sale of future royalties, net of issuance costs	0	90,621
Proceeds from issuance of common stock related to sale of future royalties, net of issuance costs	0	7,560
Net proceeds from long-term debt, net of debt discount and issuance costs	9,898	1,261
Proceeds from stock based award activities and warrants, net	3,234	14,253
Net cash provided by financing activities	13,132	225,919
Net (decrease) increase in cash and cash equivalents	(82,950)	58,332
Cash and cash equivalents, beginning of period	125,206	66,874
Cash and cash equivalents, end of period	42,256	125,206
Supplemental disclosure of cash flow information
Cash paid for interest	2,877	2,128
Cash paid for taxes	$ 1	$ 1

Organization and Accounting Pol

Organization and Accounting Policies	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Accounting Policies	Note 1 — Organization and Accounting Policies Organization Cytokinetics, Incorporated (the “Company”, “we” or “our”) was incorporated under the laws of the state of Delaware on August 5, 1997. We are 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. Our financial statements contemplate the conduct of our operations in the normal course of business. We have incurred an accumulated deficit of $743.3 million since inception and there can be no assurance that we will attain profitability. We had a net loss of $106.3 million and net cash used in operations of $101.2 million for the year ended December 31, 2018. Cash, cash equivalents and investments increased to $198.7 million at December 31, 2018 from $285.4 million at December 31, 2017. We anticipate that we will have operating losses and net cash outflows in future periods. We are 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 our future plans. Our liquidity will be impaired if sufficient additional capital is not available on terms acceptable to us. We have funded our operations and capital expenditures with proceeds primarily from private and public sales of our equity securities, a royalty monetization agreement, strategic alliances, long-term debt, other financings, interest on investments and grants. Until we achieve profitable operations, we intend to continue to fund operations through payments from strategic collaborations, additional sales of equity securities, grants and debt financings. We have never generated revenues from commercial sales of our drugs and may not have drugs to market for at least several years, if ever. Our success is dependent on our ability to enter into new strategic collaborations and/or raise additional capital and to successfully develop and market one or more of our drug candidates. As a result, we may choose to raise additional capital through equity or debt financings to continue to fund our operations in the future. We 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 our 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 our future financial results, financial position and cash flows. Based on the current status of our research and development plans, we believe that our existing cash, cash equivalents and investments will be sufficient to fund our cash requirements for at least the next 12 months after the issuance of the consolidated financial statements. If, at any time, our prospects for financing our research and development programs decline, we may decide to reduce research and development expenses by delaying, discontinuing or reducing our funding of one or more of our research or development programs. Alternatively, we might raise funds through strategic collaborations, public or private financings or other arrangements. Such funding, if needed, may not be available on favorable terms, or at all. The financial statements do not include any adjustments that might result from the outcome of this uncertainty. Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Basis of Presentation The consolidated financial statements include the accounts of Cytokinetics Incorporated and its wholly owned subsidiary and have been prepared in accordance with U.S. generally accepted accounting principles (“US GAAP”). Intercompany transactions and balances have been eliminated in consolidation. Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject us to concentrations of risk consist principally of cash and cash equivalents, investments, long-term debt and accounts receivable. Our cash, cash equivalents and investments are invested in deposits with two major financial institutions in the United States. Deposits in these banks may exceed the amount of insurance provided on such deposits. Our exposure to credit risk associated with non-payment is limited to our strategic partners Amgen Inc. (“Amgen”) and Astellas Pharma Inc. (“Astellas”) and any material non-payment from our partners would result in a material breach of the agreements underlying our strategic partnerships. Drug candidates we develop may require approvals or clearances from the U.S. Food and Drug Administration (“FDA”) or other regulatory agencies prior to commercial sales. There can be no assurance that our drug candidates will receive any of the required approvals or clearances. If we were to be denied approval, or clearance or any such approval or clearance was to be delayed, it would have a material adverse impact on us. Cash and Cash Equivalents We consider all highly liquid investments with a maturity of three months or less at the time of purchase to be cash equivalents. Investments Available-for-sale investments. Our investments consist of U.S. Treasury securities, agency bonds, commercial paper, corporate debt and money market funds. We designate all investments as available-for-sale and report them at fair value, based on quoted marked prices, with unrealized gains and losses recorded in accumulated other comprehensive loss. The cost of securities sold is based on the specific-identification method. Investments with original maturities greater than three months and remaining maturities of one year or less are classified as short-term investments. Investments with remaining maturities greater than one year are classified as long-term investments. The amortized cost of debt securities in this category is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization is included in interest income. Recognized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in other income or expense. Interest and dividends on securities classified as available-for-sale are included in Interest and other, net. Other-than-temporary impairment. All of our available-for-sale investments are subject to a periodic impairment review. We recognize an impairment charge when a decline in the fair value of investments below the cost basis is judged to be other-than-temporary. Factors we consider in assessing whether an other-than-temporary impairment has occurred include: the nature of the investment; whether the decline in fair value is attributable to specific adverse conditions affecting the investment; the financial condition of the investee; the severity and the duration of the impairment; and whether we have the intent and ability to hold the investment to maturity. When we determine that an other-than-temporary impairment has occurred, the investment is written down to its market value at the end of the period in which it is determined that an other-than-temporary decline has occurred. Property and Equipment, net Property and equipment are stated at cost less accumulated depreciation and are depreciated on a straight-line basis over the estimated useful lives of the related assets, which are generally three years for computer equipment and software, five years for laboratory equipment and office equipment, and seven years for furniture and fixtures. Amortization of leasehold improvements is computed using the straight-line method over the shorter of the remaining lease term or the estimated useful life of the related assets, typically ranging from three to seven years. Upon sale or retirement of assets, the costs and related accumulated depreciation and amortization are removed from the balance sheet and the resulting gain or loss is reflected in operations. Impairment of Long-lived Assets We review long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Impairment is measured as the amount by which the carrying amount of a long-lived asset exceeds its fair value. We would recognize an impairment loss when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are materially less than its carrying amount. Revenue Recognition On January 1, 2018, we adopted Accounting Standards Codification 606, Revenue from Contracts with Customers (“ASC 606”), using the modified retrospective method. On January 1, 2018, for contracts within the scope of ASC 606, we recognized a contract asset or liability and reduced our accumulated deficit for the effect of adopting ASC 606 and did not revise our prior period financial statements. Pursuant to ASC 606, to recognize revenue from a contract with a customer, we: (i) identify our contracts with our customers; (ii) identify our distinct performance obligations in each contract; (iii) determine the transaction price of each contract; (iv) allocate the transaction price to the performance obligations; and (v) recognize revenue as we satisfy our performance obligations. At contract inception, we assess the goods or services promised within each contract and assess whether each promised good or service is distinct and determine those that are performance obligations. We then recognize as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. Collaborative Arrangements We enter into collaborative arrangements with partners that typically include payment to us for one of more of the following: (i) license fees; (ii) milestone payments related to the achievement of developmental, regulatory, or commercial goals; and (iii) royalties on net sales of licensed products. Each of these payments results in collaboration or other revenues. Where a portion of non-refundable up-front fees or other payments received are allocated to continuing performance obligations under the terms of a collaborative arrangement, they are recorded as deferred revenue and recognized as revenue when (or as) the underlying performance obligation is satisfied. As part of the accounting for these arrangements, we must develop estimates and assumptions that require judgment to determine the underlying stand-alone selling price for each performance obligation which determines how the transaction price is allocated among the performance obligation. The stand-alone selling price may include such items as, forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success, to determine the transaction price to allocate to each performance obligation. For our collaboration agreements that include more than one performance obligation, such as a license combined with a commitment to perform research and development services, we make judgments to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, up-front fees. We evaluate our progress each reporting period and, if necessary, adjust the measure of a performance obligation and related revenue recognition. License Fees : If a license to our intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, we recognize revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promises, we utilize judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, up-front license fees. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. Milestone : We use judgement to determine whether a milestone is considered probable of being reached. Using the most likely amount method, we include the value of a milestone payment in the consideration for a contract at inception if we then conclude achieving the milestone is more likely than not. Otherwise, we exclude the value of a milestone payment from contract consideration at inception and recognize revenue for a milestone at a later date, when we judge that it is more likely than not that the milestone will be achieved. If we conclude it is probable that a significant revenue reversal would not occur, the associated milestone is included in the transaction price. We then allocate the transaction price to each performance obligation on a relative stand-alone selling price basis, for which we recognize revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, we re-evaluate the probability of achievement of such milestones and any related constraint, and if necessary, adjust our estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration and other revenues and earnings in the period of adjustment. Royalties : For contracts that include sales-based royalties, we recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied. To date, we have not recognized any royalty revenues resulting from contracts. Research and Development Cost Reimbursements : Our Astellas and Amgen arrangements include promises of research and development services. We have determined that these services collectively are distinct from the licenses provided to Astellas and Amgen and as such, these promises are accounted for as a separate performance obligation recorded over time. We record revenue for these services as the performance obligations are satisfied, which we estimate using internal development costs incurred. Accrued Research and Development Expenditures A substantial portion of our preclinical studies and all of our clinical trials have been performed by third-party contract research organizations (“CROs”) and other vendors and our accruals for expenses for preclinical studies and clinical trials may be significant. For preclinical studies, the significant factors used in estimating accruals include the percentage of work completed to date and contract milestones achieved. For clinical trial expenses, the significant factors used in estimating accruals include the number of patients enrolled, duration of enrollment, milestones achieved and percentage of work completed to date. We monitor patient enrollment levels and related activities to the extent practicable through internal reviews, correspondence and status meetings with CROs, and review of contractual terms. We depend on the timeliness and accuracy of data provided by its CROs and other vendors to accrue expenses. If we receive and rely on incomplete or inaccurate data, accruals and expenses may be too high or too low at a given point in time and corresponding adjustments to accruals and expenses would be made in future periods when the actual expense becomes known. Research and Development Expenditures Research and development costs are charged to operations as incurred. Research and development expenses consist primarily of clinical manufacturing costs, preclinical study expenses, consulting and other third-party costs, employee compensation, supplies and materials, allocation of overhead and occupancy costs, facilities costs and depreciation of equipment. Income Taxes We account for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to affect taxable income. Valuation allowances are established when necessary to reduce deferred tax assets to the amounts expected to be realized. We recognize uncertain tax positions taken or expected to be taken on a tax return. Tax positions are initially recognized when it is We recognize interest accrued related to unrecognized tax benefits and penalties as income tax expense. Stock-Based Compensation We maintain equity incentive plans under which incentive stock options may be granted to employees and nonqualified stock options, restricted stock awards, restricted stock units and stock appreciation rights may be granted to employees, directors, consultants and advisors. In addition, we maintain an employee stock purchase plan (“ESPP”) under which employees may purchase shares of our common stock through payroll deductions. Stock-based compensation expense related to stock options granted to employees and directors is recognized based on the grant date estimated fair values, net of an estimated forfeiture rate, using the Black Scholes option pricing model. The value of the portion of the award that is ultimately expected to vest is recognized as expense ratably over the requisite service period. We estimate our forfeiture rate based on an analysis of our actual forfeitures and the experience of other companies in the same industry, and we will continue to evaluate the adequacy of the forfeiture rate assumption based on actual forfeitures, analysis of employee turnover and other related factors. Stock-based compensation expense related to restricted stock units granted to employees is recognized based on the grant-date fair value of each award and recorded as expense over the vesting period using the straight-line method, net of estimated forfeitures. Stock-based compensation expense related to the ESPP is recognized based on the fair value of each award estimated on the first day of the offering period using the Black Scholes option pricing model and recorded as expense over the service period using the straight-line method. Amortization of Debt Discount and Issuance Costs Debt discount and issuance costs, consisting of legal and other fees directly related to the debt, are offset against gross proceeds from the issuance of debt and are amortized to interest expense over the estimated life of the debt based on the effective interest method. Liability Related to Sale of Future Royalties We treat the Liability related to sale of future royalties as a debt financing, to be amortized under the effective interest rate method over the life of the related royalty stream. The Liability related to sale of future royalties and the debt amortization are based on our current estimates of future royalties expected to be paid over the life of the arrangement. We will periodically assess the expected royalty payments using a combination of internal projections and forecasts from external sources. To the extent our 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, we will adjust the Liability related to sale of future royalties and prospectively recognize related non-cash interest expense. Recent Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, ‘Financial Instruments — Credit Losses — Measurement of Credit Losses on Financial Instruments. We are in the process of evaluating the impact the adoption of this standard would have on our financial statements and disclosures. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). . and believe that adoption will be material to our balance sheet as a result of the recognition of a right-to-use asset and corresponding liability for our building leases but will not have a material impact on our results of operations In November 2018, the Financial Accounting Standards Board (“FASB”) issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (“ASU 2018-18”), which make targeted improvements to clarify the interaction between Topic 808, Collaborative Arrangements, and Topic 606, Revenue from Contracts with Customers. ASU 2018-18 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted. We are currently evaluating the impact of adopting ASU 2018-18.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss Per Share	Note 2 — Net Loss Per Share 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) income per share is computed by giving effect to all potentially dilutive common shares, including outstanding stock options, unvested restricted stock, warrants, convertible preferred stock and shares issuable under our Employee Stock Purchase Plan (“ESPP”), by applying the treasury stock method. The following instruments were excluded from the computation of diluted net loss per share for the periods presented because their effect would have been antidilutive (in thousands): December 31, 2018 2017 Options to purchase common stock 5,476 5,957 Warrants to purchase common stock 116 100 Restricted and Performance stock units 547 457 Shares issuable related to the ESPP 107 20 Total shares 6,246 6,534

Revenue Recognition

Revenue Recognition	12 Months Ended
	Dec. 31, 2018
Revenue Recognition [Abstract]
Revenue Recognition	Note 3 – Revenue Recognition We believe recognizing revenue as research and development services are performed provides a faithful depiction of the transfer of the services because completion of clinical programs results in data useful to determine satisfaction of our promise. We may fund research and development in advance of the performance of the services. When we complete our performance obligation, if we have received more than we incurred, we are obligated to return unused advance funding. We recognize these advance payments as deferred revenue until we perform the related services. As discussed in Note 1, prior period amounts continue to be reported in accordance with our historic accounting under previous revenue recognition guidance, ASC 605. Our revenue for 2018 was affected by adopting ASC 606 as follows (in thousands): 2018 Research and development revenue using guidance in effect prior to ASC 606 $ 18,672 Impact of adoption of ASC 606 7,696 Research and development revenue $ 26,368 License revenue using guidance in effect prior to ASC 606 $ (5,347 ) Impact of adoption of ASC 606 10,480 License revenue $ 5,133 The impact of adoption of ASC 606 on our net loss per share was as follows (in thousands): 2018 Net loss per share using guidance in effect prior to ASC 606 $ (2.28 ) Impact of adoption of ASC 606 (0.33 ) Net loss per share $ (1.95 ) We recognized contract assets and contract liabilities for our Co-Invest Option from the Amgen Agreement, the 2014 Astellas Amendment and the 2016 Astellas Amendment, further described in Note 7. Research and Development Arrangements, below. In 2018, we completed our performance obligations for the Co-Invest Option and the 2014 Astellas Amendment. We expect to complete our performance obligations for the 2016 Astellas Amendment in 2019. Our contract assets and liabilities changed during the period, as follows (in thousands): 2018 Contract liability from the Amgen Agreement for the Co-Invest Option Balance at beginning of period $ 18,750 Payments made for the Co-Invest Option (18,750 ) Balance at end of period $ — Contract asset from the 2016 Astellas Amendment Balance at beginning of period $ 9,708 Services performed $ 11,713 Cash received for services $ (16,867 ) Balance at end of period $ 4,554 Contract liability from the 2014 Astellas Amendment Balance at beginning of period $ 6,288 Services performed (6,288 ) Balance at end of period $ —

Cash Equivalents and Investment

Cash Equivalents and Investments	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Cash Equivalents and Investments	Note 4 — Cash Equivalents and Investments Cash Equivalents and Available for Sale Investments The amortized cost and fair value of cash equivalents and available for sale investments at December 31, 2018 and 2017 were as follows (in thousands): December 31, 2018 Cash and Investments available for sale Amortized Cost Unrealized Gains Unrealized Losses Fair Value Money market funds $ 34,771 $ — $ — $ 34,771 U.S. Treasury securities 56,999 — (41 ) 56,958 Agency bonds 61,792 1 (14 ) 61,779 Commercial paper 19,448 — (13 ) 19,435 Corporate obligations 17,644 2 (8 ) 17,638 $ 190,654 $ 3 $ (76 ) $ 190,581 December 31, 2017 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents — U. S. Treasury securities and money market funds $ 111,501 $ — $ — $ 111,501 Short-term investments — U.S. Treasury securities and Agency bonds $ 143,895 $ — $ (210 ) $ 143,685 Long-term investments — Equity and U.S. Treasury securities $ 16,538 $ — $ (20 ) $ 16,518 As of December 31, 2018, none of the investments were other-than-temporarily impaired, no investment was in a continuous unrealized loss position for more than one year, unrealized losses were not due to change in credit risk and we believe investments with an unrealized loss would be held until maturity.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	Note 5 — Fair Value Measurements We value our financial assets and liabilities at fair value, defined as the price that would be received for assets when sold or paid to transfer a liability in an orderly transaction between market participants at the measurement date (exit price). We utilize market data or assumptions that we believe 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. We primarily apply the market approach for recurring fair value measurements and endeavors to utilize the best information reasonably available. Accordingly, we utilize 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. We classify the determined fair value based on the observability of those inputs. Fair value accounting guidance establishes a fair value hierarchy that prioritizes the inputs used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurement) and the lowest priority to unobservable inputs (Level 3 measurement). The three defined levels of the fair value hierarchy are as follows: Level 1 — Observable inputs, such as quoted prices in active markets for identical assets or liabilities; Level 2 — Inputs, other than the quoted prices in active markets, that are observable either directly or through corroboration with observable market data; and Level 3 — Unobservable inputs, for which there is little or no market data for the assets or liabilities, such as internally-developed valuation models. Financial assets measured at fair value on a recurring basis as of December 31, 2018 and 2017 are classified in the table below in one of the three categories described above (in thousands): December 31, 2018 Fair Value Measurements Using Assets Level 1 Level 2 Level 3 At Fair Value Assets: Money market funds $ 34,771 $ — $ — $ 34,771 U.S. Treasury securities 56,958 — — 56,958 Agency bonds — 61,779 — 61,779 Commercial paper — 19,435 — 19,435 Corporate obligations — 17,638 — 17,638 Total $ 91,729 $ 98,852 $ — $ 190,581 December 31, 2017 Fair Value Measurements Using Assets Level 1 Level 2 Level 3 At Fair Value Assets: Money market funds $ 51,001 $ — $ — $ 51,001 U.S. Treasury securities 165,801 — — 165,801 Agency bonds — 54,329 54,329 Equity securities 573 — — 573 $ 217,375 $ 54,329 $ — $ 271,704 Investments available for sale at December 31, 2018 excludes an investment in equity classified as a Level 1 investment in our short-term investments with a fair value and unrealized gain of $0.7 million. At December 31, 2018, there were no investments that had been in a continuous unrealized loss position for 12 months or longer. The carrying amount of our accounts receivable and accounts payable approximates fair value due to the short-term nature of these instruments. Fair value of financial liabilities: As of December 31, 2018 and 2017, the fair value of the long-term debt, payable in installments through 2020, approximated its carrying value of $42.1 million and $31.8 million, respectively, because it is carried at a market observable interest rate, which is a Level 2 input. As of December 31, 2018, the fair value of the Liability related to the sale of future royalties is based on our current estimates of future royalties expected to be paid to RPI Finance Trust (“RPI”), an entity related to Royalty Pharma, over the life of the arrangement, which are considered Level 3 inputs (See Note 9 – “Liability Related to Sale of Future Royalties”). There were no transfers between Level 1, Level 2, and Level 3 during the periods presented.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Balance Sheet Components	Note 6 — Balance Sheet Components Our property and equipment consisted of (in thousands): December 31, 2018 2017 Property and equipment, net: Laboratory equipment $ 17,916 $ 17,100 Computer equipment and software 2,882 2,890 Office equipment, furniture and fixtures 1,137 1,137 Leasehold improvements 5,130 5,067 Total property and equipment 27,065 26,194 Less: Accumulated depreciation and amortization (23,861 ) (22,626 ) $ 3,204 $ 3,568 Depreciation expense was $1.2 million for 2018 and $1.9 million for 2017. Our accrued liabilities were (in thousands): December 31, 2018 2017 Accrued liabilities: Clinical and preclinical costs $ 8,618 $ 8,370 Compensation related 6,118 6,261 Other accrued expenses 1,021 2,761 $ 15,757 $ 17,392 We sponsor a 401(k) defined contribution plan covering all employees and contributed $0.5 million to this plan in both 2018 and 2017.

Research and Development Arrang

Research and Development Arrangements	12 Months Ended
	Dec. 31, 2018
Research And Development [Abstract]
Research and Development Arrangements	Note 7 — Research and Development Arrangements Amgen Inc. (“Amgen”) We and Amgen continue activities related to novel small molecule therapeutics, including omecamtiv mecarbil, that activate cardiac muscle contractility for potential applications in the treatment of heart failure under our collaboration and option agreement with Amgen dated December 29, 2006, as amended (the “Amgen Agreement”). We recognize research and development revenue for reimbursements from Amgen of both internal costs of certain FTEs and other costs related to the Amgen Agreement. Under the Amgen Agreement, we are eligible to receive over $300.0 million in additional development milestone payments 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, we are 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, we cannot estimate if and when these milestone payments could be achieved or become due and, accordingly, we consider the milestone payments to be constrained and exclude the milestone payments from the transaction price. We paid Amgen $18.8 million in 2018 and $20.0 million in 2017 and have exercised our option under the Amgen Agreement to co-invest $40.0 million in the Phase 3 development program of omecamtiv mecarbil in exchange for a total incremental royalty from Amgen of up to 4% on increasing worldwide sales of omecamtiv mecarbil outside Japan (the “Co-Invest Option”). Adoption of ASC 606 We determined that the Amgen Agreement was within the scope of ASC 606. As of January 1, 2018, all the performance obligations under the Amgen Agreement were complete. On January 1, 2018, we recognized a contract liability for $18.8 million with a corresponding increase in accumulated deficit for the Co-Invest Option. We paid Amgen $18.8 million for the Co-Invest Option during 2018. Research and development revenues from Amgen for December 31, 2018 and 2017 were as follows (in thousands): Years Ended December 31, 2018 2017 Reimbursements $ 1,915 $ 1,279 Milestone fees — 11,000 Co-Invest Option payments — (20,000 ) $ 1,915 $ (7,721 ) Co-Invest Option payments in 2017 reduced our revenues in that year (prior to adopting ASC 606). Milestone fees in 2017 of $11.0 million consisted of $10.0 million related to the start in Japan of GALACTIC-HF, the Phase 3 cardiovascular outcomes clinical trial of omecamtiv mecarbil Astellas Pharma Inc. (“Astellas”) We and Astellas continue activities focused on the research, development, and commercialization of skeletal muscle activators, including reldesemtiv, 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”). We recognize research and development revenue for reimbursements from Astellas of internal costs of both FTEs and other costs related to Astellas Agreement. In 2014, we and Astellas amended and restated the license and collaboration agreement (the “2014 Astellas Amendment”) and expanded the objective of the collaboration to include spinal muscular atrophy (“SMA”) and potentially other neuromuscular indications for reldesemtiv and other fast skeletal muscle troponin activators (“FSTAs”); in connection therewith, Astellas paid us a $30.0 million non-refundable upfront license fee and a $15.0 million milestone payment. We determined at that time that the license for the expanded SMA rights did not have stand-alone value and the license and research and development services were a single unit of accounting and recognized revenue for these payments using the proportional performance model. In 2016, we and Astellas amended the Astellas Agreement (the “2016 Astellas Amendment”) to expand the collaboration to include the development of reldesemtiv for the potential treatment of amyotrophic lateral sclerosis (“ALS”), as well as the possible development in ALS of other FSTAs previously licensed by us to Astellas, and Astellas paid us 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 reldesemtiv 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, for total consideration of $94.2 million. We allocated the consideration to the license and to the research and development services, and recognized license revenue and research and development revenue using the proportional performance model. In addition, Astellas paid us a $15.0 million non-refundable million fee for the option for a global collaboration for the development and commercialization of tirasemtiv, our first-generation FSTA Under the Astellas Agreement, additional research and early and late state development milestone payments for 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 and include up to $95.0 million relating to reldesemtiv in non-neuromuscular indications, and over $100.0 million related to reldesemtiv in each of SMA, ALS and other neuromuscular indications. Additionally, $200.0 million in commercial milestones could be received under the Astellas Agreement provided certain sales targets are met. We are eligible to receive up to $2.0 million in research milestone payments under the collaboration for each future potential drug candidate. 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, and accordingly, are constrained and not included in the transaction price. In collaboration with Astellas, we are conducting a Phase 2 clinical trial of reldesemtiv in patients with ALS, called FORTITUDE-ALS ( F O R T I T U D E ALS Adoption of ASC 606 On January 1, 2018, in adopting ASC 606, we concluded: (i) that the original agreement with Astellas in 2013 was outside the scope of ASC 606, since all performance obligations thereunder were completed prior to entering into the 2014 Astellas Amendment and the 2014 Astellas Amendment was not an amendment of the original agreement, (ii) the 2014 Astellas Amendment is a separate agreement within the scope of ASC 606 with no effect on the ongoing accounting for the related license and research and development service deliverables and (iii) the 2016 Astellas Amendment is a separate agreement within the scope of ASC 606. In adopting ASC 606 for the 2016 Amendment, we determined: • Our performance obligations were the delivery of the license and performance of research and development services; • The transaction price included the $65.0 million in non-refundable fees and $35.6 million in then-committed research and development fees; • The consideration allocated to the license resulted in a contract asset of $16.7 million, with a corresponding decrease to accumulated deficit on January 1, 2018, and to be realized using the proportional performance model; and • Research services we perform under the Astellas Agreement in 2018 and beyond are a separate contract. The transaction price above was allocated to the license (approximately $83 million) and to the services (approximately $18 million) based on their respective stand-alone prices. License revenues and research and development revenues from Astellas for 2018 and 2017 were as follows (in thousands): Years Ended December 31, 2018 2017 License revenues $ 5,133 $ 8,799 Reimbursements 22,253 11,934 Milestone fees 2,000 — $ 29,386 $ 20,733 Of the revenue recognized in 2018, $9.6 million was included in the contract liability at the end of 2017 as a result of adopting ASC 606. This revenue includes the cumulative effect of changes made during 2018 in the estimated costs of research and development services to be incurred to satisfy the related deliverable. Prior period amounts continue to be reported in accordance with our historic accounting under previous revenue recognition guidance, ASC 605. See note 3 “Revenue Recognition” above, for more information about the impact of the adoption on ASC 606. In 2018, we completed all our deliverables for the 2014 Astellas Amendment and have recognized as revenue all the consideration under that agreement. We expect to complete all our deliverables for the 2016 Astellas Amendment in 2019. We had accounts receivable from Astellas of $0.3 million at December 31, 2018 and none at December 31, 2017. Deferred revenue, current at December 31, 2017 of $9.6 million reflected the unrecognized portion of the license revenue and reimbursements and Deferred revenue, non-current at December 31, 2017 of $15.0 million reflected the Option on Tirasemtiv (prior to adopting ASC 606). MyoKardia Inc. (“MyoKardia”) In July 2018, we received $0.2 million for achievement of a clinical milestone pursuant to our collaboration agreement with MyoKardia.

Long-Term Debt

Long-Term Debt	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Long-Term Debt	Note 8 — Long-Term Debt We have 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 our working capital and other general corporate needs. In October 2017, we entered into a Second Amendment to Loan and Security Agreement (the “Amended Loan Agreement”) with Oxford and SVB, drew $32.0 million and retired our then-outstanding debt of $30.0 million, and $0.5 million related to the accrued portion of the final payment fee under the Loan Agreement. following the satisfaction of certain conditions specified in the Loan Agreement Long-term debt and unamortized debt discount balances are as follows (in thousands): December 31, 2018 2017 Notes payable, gross $ 42,000 $ 32,000 Unamortized debt discount and interest payable (135 ) (325 ) Accretion of final exit fee 548 102 Carrying value of notes payable 42,413 31,777 Less: Current portion of long-term debt 2,607 — Long-term debt $ 39,806 $ 31,777 Payments on Long-term debt will be interest only through November 2019, followed by 35 months of equal monthly payments of interest and principal. We are required to make a final payment upon loan maturity of 6.5% of the amounts advanced. The interest rate under the Amended Loan Agreement is the greater of (a) 8.05% or (b) the sum of 6.81% plus the 30-day U.S. LIBOR rate. The Loan Agreement contains customary representations and warranties and customary affirmative and negative covenants applicable to us and includes customary events of default, including but not limited to the nonpayment of principal or interest, violations of covenants and material adverse changes. Upon an event of default, the Lenders may, among other things, accelerate the loans and foreclose on the collateral. Our obligations under the Amended Loan Agreement are secured by substantially all our current and future assets, other than our intellectual property. Future minimum payments under the Loan Agreement loan, as of December 31, 2018 are as follows (in thousands): 2019 $ 5,166 2020 17,636 2021 16,267 2022 15,237 Total minimum payments 54,306 Less: Interest and final payment (12,306 ) Notes payable, gross $ 42,000

Liability Related to Sale of Fu

Liability Related to Sale of Future Royalties	12 Months Ended
	Dec. 31, 2018
Deferred Revenue Disclosure [Abstract]
Liability Related to Sale of Future Royalties	Note 9 - Liability Related to Sale of Future Royalties In February 2017, we and Royalty Pharma Inc. (“RPI”) entered into a Royalty Purchase Agreement (the “Royalty Agreement”), under which we sold to RPI for $90.0 million (the “Royalty Monetization”). 875,656 $10.0 We 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 sale of the RPI Common Stock. We 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% based on our estimate of the cash flows to be received over the life of the Royalty Agreement. We determined that the fair value of the RPI Common Stock was $8.1 million at the time we entered into the Royalty Agreement. We determined the fair value at $131.6 million at December 31, 2017 after considering the new statutory effective tax rate of 21% in 2018. We allocated the consideration of $100.0 million and related transaction costs of $1.8 million on a relative fair value basis to the liability for $92.3 million and the common stock for $7.7 million. Through December 31, 2018, we accreted the Liability related to sale of future royalties using the interest method with an annual pre-tax interest rate of 17% and recognized non-cash interest expense on liability related to sale of future royalties of $17.8 million in 2018 and $14.0 million in 2017. Transaction costs are amortized to interest expense over the estimated term of the Royalty Agreement. Payments made to RPI pursuant to the Royalty Monetization, if any, would reduce this liability.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	Note 10 — Commitments and Contingencies Commitments - Operating Lease We lease office space under a non-cancelable operating lease that expires in June 2021. The lease provides for rental payments on a graduated scale and our payment of certain operating expenses. We recognize rent expense on a straight-line basis over the lease period. Rent expense was $5.0 million in 2018, $3.6 million in 2017 and $3.4 million in 2016. As of December 31, 2018, future minimum lease payments under noncancelable operating leases were $4.7 million in 2019, $4.8 million in 2020 and $2.5 million in 2021. Contingencies In the ordinary course of business, we 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 our breach of such agreements, services to be provided by or on behalf of us, or from intellectual property infringement claims made by third parties. In addition, we have indemnification agreements with our directors and certain of our officers and employees that will require us, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors, officers or employees. We maintain director and officer insurance, which may cover certain liabilities arising from our obligation to indemnify our directors and certain of our officers and employees, and former officers and directors in certain circumstances. We maintain product liability insurance and comprehensive general liability insurance, which may cover certain liabilities arising from our 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. We are not currently aware of any matters that could have a material adverse effect on our financial position, results of operations or cash flows.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Stockholders' Equity	Note 11 — Stockholders’ Equity Equity Incentive Plan Our amended and restated 2004 Equity Incentive Plan (the “2004 Plan”) provides for us to grant incentive stock options, nonstatutory stock options, restricted stock, stock appreciation rights, restricted stock units, performance shares and performance units to employees, directors and consultants. We may grant options for terms of up to ten years at prices not lower than 100% of the fair market value of our common stock on the date of grant. Options granted to new employees generally vest 25% after one year and monthly thereafter over a period of four years. Options granted to existing employees generally vest monthly over a period of four years. As of December 31, 2018, we have 2.4 million shares of common stock reserved and available for issuance under the 2004 Plan. Stock option activity in 2018 was as follows: Stock Options Outstanding Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Life (in years) Aggregate Intrinsic Value (in thousands) Balance at December 31, 2017 5,957,458 $ 9.19 Granted 1,804,047 8.04 Exercised (422,819 ) 7.51 Forfeited (884,649 ) 11.06 Balance at December 31, 2018 6,454,037 $ 8.72 6.6 $ 412 Exercisable at December 31, 2018 4,461,463 $ 8.72 5.7 $ 411 We expect all outstanding options to vest. The intrinsic value of stock options exercised, calculated based on the difference between the market value at the date of exercise and the exercise price, was $0.7 million for 2018 and $1.8 million for 2017. The intrinsic value of stock options outstanding at December 31, 2017 was $4.3 million. Restricted stock and Performance unit activity in 2018 was as follows: Number of Restricted Stock Units Weighted Average Award Date Fair Value per Share Balance at December 31, 2017 456,752 $ 9.08 Granted 466,500 $ 7.80 Released (301,000 ) $ 8.29 Forfeited (75,752 ) $ 8.29 Balance at December 31, 2018 546,500 $ 8.53 Restricted stock units generally vest monthly over three to four years. For 2018, the fair value of restricted stock and performance units vested, calculated based on the units vested multiplied by the closing price of our common stock on the date of vesting, was $2.3 million. During 2015, we granted 685,000 Performance Units with a grant date fair value of $7.00 per share. In 2017, performance criteria for 342,500 Performance Units were met, 171,250 of those units vested. In March 2018, the remaining 171,250 of these Performance Units vested. In 2017, the other 342,500 Performance Units granted in 2015 were forfeited when we determined that the performance criteria for those Performance Units would not be met. Employee Stock Purchase Plan Under our 2015 Employee Stock Purchase Plan (the “ESPP”), employees may purchase common stock up to a specified maximum amount at a price equal to 85% of the fair market value at certain plan-defined dates. We issued 144,822 shares at an average price of $6.40 during 2018 and 120,959 shares at an average price of $9.65 in 2017 pursuant to the ESPP. At December 31, 2018, we have 253,617 shares of common stock reserved for issuance under the ESPP. Stock-Based Compensation Expense We use the Black-Scholes option pricing model to determine the fair value of stock option grants to employees and directors and employee stock purchase plan shares. The fair value of share-based payments was estimated on the date of grant based on the following assumptions: Year Ended December 31, 2018 Year Ended December 31, 2017 Options ESPP Options ESPP Risk-free interest rate 2.3% to 3.0% 1.5% to 2.5% 2.2% 1.3% Volatility 73% to 74% 73% to 74% 74% 74% Expected term in years 6.5 0.5 6.5 0.5 Expected dividend yield 0% 0% 0% 0% We use the U.S. Treasury zero-coupon issues with remaining terms similar to the expected terms of the options for the risk-free interest rate. We use our own volatility history based on its stock’s trading history and our own historical exercise and forfeiture activity to estimate expected term for option grants. We do not anticipate paying dividends in the foreseeable future and use an expected dividend yield of zero. We reviewed the impact of estimating forfeitures on our stock-based compensation, determined the impact was immaterial and stopped estimating forfeitures in 2018. Prior to 2018, we estimated forfeitures at the time of grant and revised those estimates in subsequent periods if actual forfeitures differ from our estimates using historical forfeiture data and recorded stock-based compensation expense on those awards that were expected to vest. We measure compensation expense for restricted stock units at fair value on the date of grant and recognizes the expense over the expected vesting period. We recognize stock-based compensation expense on a straight-line basis over the requisite service period, generally the vesting period of the award for share-based awards. Stock-based compensation expense for 2018 and 2017 was as follows (in thousands): Years Ended December 31, 2018 2017 Research and development $ 5,101 $ 5,656 General and administrative 4,660 3,372 $ 9,761 $ 9,028 Non-cash stock-based compensation expense for share-based awards to non-employees was $0.1 million in 2018 and $0.5 million in 2017. As of December 31, 2018, we expect to recognize $13.8 million of unrecognized compensation cost related to unvested stock options over a weighted-average period of 2.4 years and $3.5 million of unrecognized compensation cost related to unvested restricted stock over a weighted-average period of 1.8 years. Warrants Pursuant to the Loan Agreement described in Note 8 “Long-Term Debt,” we issued warrants to purchase 65,189 shares of our common stock at an exercise price of $6.90 per share and additional warrants to purchase 68,285 shares of our common stock at an exercise price of $6.59 per share. In 2017, we issued 16,126 shares of common stock related to cashless exercises of some of these warrants. In August 2018, we issued 42,253 warrants with a weighted average of $7.10 per share. At December 31, 2018, 142,359 warrants with a weighted average exercise price of $6.85 per share were outstanding. In June 2012, we issued warrants with expiration in June 2017 pursuant to public offerings of our securities in 2012. In 2017, we issued 3,450,122 shares of common stock for exercise of these warrants, respectively. Controlled Equity Offering During 2017, we issued 2,425,625 shares of common stock under a Controlled Equity Offering Sales Agreement, an at-the-market issuance sales agreement, for net proceeds of $29.9 million and completed the offering.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Note 12 — Income Taxes We did not record an income tax provision in 2018 and 2017 because we had net taxable losses. The following reconciles the statutory federal income tax rate to our effective tax rate: Years Ended December 31, 2018 2017 Tax at federal statutory tax rate 21 % 34 % Tax credits (net) 1 % 8 % Federal statutory rate reduction — (51 )% Change in valuation allowance (17 )% 10 % Stock-based compensation (1 )% — Other (3 )% (1 )% Total 0 % 0 % Our significant jurisdictions are the United States and California. We are subject to income tax examination for all fiscal years since inception. Income (loss) before taxes includes the following components (in thousands): Years Ended December 31, 2018 2017 United States $ (106,289 ) $ (127,235 ) Foreign — (555 ) Total $ (106,289 ) $ (127,790 ) Deferred tax assets, net, reflecting the net tax effect of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes, were as follows (in thousands): As of December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 121,748 $ 98,630 Tax credits 64,797 64,185 Liability related to sale of future royalties 26,294 24,593 Reserves and accruals 5,772 10,524 Capitalized R&D 4,614 6,432 Depreciation and amortization 586 546 Total noncurrent deferred tax assets 223,811 204,910 Deferred tax liabilities: Accounting method change (2,682 ) — Other (20 ) — Total noncurrent deferred tax liabilities (2,702 ) — Less: Valuation allowance (221,109 ) (204,910 ) Net deferred tax assets $ — $ — At December 31, 2018, federal NOL carryforwards were $490.1 million and apportioned state NOL carryforwards before federal benefits were $252.8 million. If not utilized, federal and state operating loss carryforwards incurred prior to 2018 will begin to expire in various amounts beginning 2022 and 2028, respectively. At December 31, 2018, tax credits of $61.6 million and $15.0 million for federal and state income tax purposes, respectively consisted of Research and Development Credits and Orphan Drug Credits. If not utilized, the federal carryforwards will expire in various amounts beginning in 2021. California based credit carryforwards do not expire. Realization of deferred tax assets is dependent upon future earnings, if any, the timing and amount of which are uncertain. Based upon the weight of available evidence, which includes our historical operating performance, reported cumulative net losses since inception, expected future losses, and difficulty in accurately forecasting our future results and an assessment of both positive and negative evidence when determining whether it is more likely than not that deferred tax assets are recoverable, we maintained a full valuation allowance on the net deferred tax assets as of December 31, 2018 and 2017. The valuation allowance increased by $16.2 million in 2018 and decreased by $11.7 million in 2017. In general, under Section 382 of the Internal Revenue Code (“Section 382”), a corporation that undergoes an ‘ownership change’ is subject to limitations on its ability to utilize its pre-change net operating losses and tax credits to offset future taxable income. We do not believe it has experienced an ownership change since 2006 expect a portion of its NOLs and tax credits from prior to 2007 will be subject to limitations under Section 382. Activity related to our gross unrecognized tax benefits were (in thousands): Years Ended December 31, 2018 2017 Balance at the beginning of the year $ 9,365 $ 7,565 Decrease related to prior year tax positions - - Increase related to current year tax positions 110 1,800 Balance at the end of the year $ 9,475 $ 9,365 We are subject to income tax examination for all fiscal years since inception. Included in the balance of unrecognized tax benefits as of December 31, 2018 and 2017 are $8.6 million and $8.1 million of tax benefits, respectively, that, if recognized, would result in adjustments to other tax accounts, primarily deferred taxes. Tax Reform The Tax Cuts and Jobs Act of 2017 (the “Tax Act”) made significant changes to the Internal Revenue Code. Changes include, but are not limited to, a corporate tax rate decrease from 34% to 21% (the “Rate Reduction”) effective for tax years beginning after December 31, 2017. We reduced deferred tax assets at December 31, 2017 for the effect of the Rate Reduction. The Rate Reduction did not impact our provision for income taxes for 2017 due to the full valuation allowance on deferred tax assets. Due to the complexities of implementing the provisions of the Tax Act, the staff of the U.S. Securities and Exchange Commission issued Staff Accounting Bulletin 118 (“SAB 118”), which provides guidance on accounting for tax effects of the Tax Act and permits a measurement period not to exceed one year from the enactment date for companies to complete the required analyses and accounting. As permitted under SAB 118, the adjustments we recorded due to the Tax Act, including the remeasurement of deferred tax assets and liabilities and the transition tax, were based on reasonable estimates and were considered provisional during the year. In the fourth quarter of 2018, we completed our analysis to determine the effect of the Tax Act and recorded immaterial adjustments as of December 31, 2018. The Company has considered and completed all applicable elements of tax reform under the remeasurement period.

Interest and Other Income, Net

Interest and Other Income, Net	12 Months Ended
	Dec. 31, 2018
Other Income And Expenses [Abstract]
Interest and Other Income, Net	Note 14 — Interest and Other Income, Net Interest and other income, net for the years ended December 31, 2018 and 2017 primarily consisted of interest income generated from our cash, cash equivalents and investments.

Organization and Accounting P_2

Organization and Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	Organization Cytokinetics, Incorporated (the “Company”, “we” or “our”) was incorporated under the laws of the state of Delaware on August 5, 1997. We are 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. Our financial statements contemplate the conduct of our operations in the normal course of business. We have incurred an accumulated deficit of $743.3 million since inception and there can be no assurance that we will attain profitability. We had a net loss of $106.3 million and net cash used in operations of $101.2 million for the year ended December 31, 2018. Cash, cash equivalents and investments increased to $198.7 million at December 31, 2018 from $285.4 million at December 31, 2017. We anticipate that we will have operating losses and net cash outflows in future periods. We are 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 our future plans. Our liquidity will be impaired if sufficient additional capital is not available on terms acceptable to us. We have funded our operations and capital expenditures with proceeds primarily from private and public sales of our equity securities, a royalty monetization agreement, strategic alliances, long-term debt, other financings, interest on investments and grants. Until we achieve profitable operations, we intend to continue to fund operations through payments from strategic collaborations, additional sales of equity securities, grants and debt financings. We have never generated revenues from commercial sales of our drugs and may not have drugs to market for at least several years, if ever. Our success is dependent on our ability to enter into new strategic collaborations and/or raise additional capital and to successfully develop and market one or more of our drug candidates. As a result, we may choose to raise additional capital through equity or debt financings to continue to fund our operations in the future. We 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 our 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 our future financial results, financial position and cash flows. Based on the current status of our research and development plans, we believe that our existing cash, cash equivalents and investments will be sufficient to fund our cash requirements for at least the next 12 months after the issuance of the consolidated financial statements. If, at any time, our prospects for financing our research and development programs decline, we may decide to reduce research and development expenses by delaying, discontinuing or reducing our funding of one or more of our research or development programs. Alternatively, we might raise funds through strategic collaborations, public or private financings or other arrangements. Such funding, if needed, may not be available on favorable terms, or at all. The financial statements do not include any adjustments that might result from the outcome of this uncertainty.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of Cytokinetics Incorporated and its wholly owned subsidiary and have been prepared in accordance with U.S. generally accepted accounting principles (“US GAAP”). Intercompany transactions and balances have been eliminated in consolidation.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject us to concentrations of risk consist principally of cash and cash equivalents, investments, long-term debt and accounts receivable. Our cash, cash equivalents and investments are invested in deposits with two major financial institutions in the United States. Deposits in these banks may exceed the amount of insurance provided on such deposits. Our exposure to credit risk associated with non-payment is limited to our strategic partners Amgen Inc. (“Amgen”) and Astellas Pharma Inc. (“Astellas”) and any material non-payment from our partners would result in a material breach of the agreements underlying our strategic partnerships. Drug candidates we develop may require approvals or clearances from the U.S. Food and Drug Administration (“FDA”) or other regulatory agencies prior to commercial sales. There can be no assurance that our drug candidates will receive any of the required approvals or clearances. If we were to be denied approval, or clearance or any such approval or clearance was to be delayed, it would have a material adverse impact on us.
Cash and Cash Equivalents	Cash and Cash Equivalents We consider all highly liquid investments with a maturity of three months or less at the time of purchase to be cash equivalents.
Investments	Investments Available-for-sale investments. Our investments consist of U.S. Treasury securities, agency bonds, commercial paper, corporate debt and money market funds. We designate all investments as available-for-sale and report them at fair value, based on quoted marked prices, with unrealized gains and losses recorded in accumulated other comprehensive loss. The cost of securities sold is based on the specific-identification method. Investments with original maturities greater than three months and remaining maturities of one year or less are classified as short-term investments. Investments with remaining maturities greater than one year are classified as long-term investments. The amortized cost of debt securities in this category is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization is included in interest income. Recognized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in other income or expense. Interest and dividends on securities classified as available-for-sale are included in Interest and other, net. Other-than-temporary impairment. All of our available-for-sale investments are subject to a periodic impairment review. We recognize an impairment charge when a decline in the fair value of investments below the cost basis is judged to be other-than-temporary. Factors we consider in assessing whether an other-than-temporary impairment has occurred include: the nature of the investment; whether the decline in fair value is attributable to specific adverse conditions affecting the investment; the financial condition of the investee; the severity and the duration of the impairment; and whether we have the intent and ability to hold the investment to maturity. When we determine that an other-than-temporary impairment has occurred, the investment is written down to its market value at the end of the period in which it is determined that an other-than-temporary decline has occurred.
Property and Equipment, net	Property and Equipment, net Property and equipment are stated at cost less accumulated depreciation and are depreciated on a straight-line basis over the estimated useful lives of the related assets, which are generally three years for computer equipment and software, five years for laboratory equipment and office equipment, and seven years for furniture and fixtures. Amortization of leasehold improvements is computed using the straight-line method over the shorter of the remaining lease term or the estimated useful life of the related assets, typically ranging from three to seven years. Upon sale or retirement of assets, the costs and related accumulated depreciation and amortization are removed from the balance sheet and the resulting gain or loss is reflected in operations.
Impairment of Long-lived Assets	Impairment of Long-lived Assets We review long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Impairment is measured as the amount by which the carrying amount of a long-lived asset exceeds its fair value. We would recognize an impairment loss when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are materially less than its carrying amount.
Revenue Recognition	Revenue Recognition On January 1, 2018, we adopted Accounting Standards Codification 606, Revenue from Contracts with Customers (“ASC 606”), using the modified retrospective method. On January 1, 2018, for contracts within the scope of ASC 606, we recognized a contract asset or liability and reduced our accumulated deficit for the effect of adopting ASC 606 and did not revise our prior period financial statements. Pursuant to ASC 606, to recognize revenue from a contract with a customer, we: (i) identify our contracts with our customers; (ii) identify our distinct performance obligations in each contract; (iii) determine the transaction price of each contract; (iv) allocate the transaction price to the performance obligations; and (v) recognize revenue as we satisfy our performance obligations. At contract inception, we assess the goods or services promised within each contract and assess whether each promised good or service is distinct and determine those that are performance obligations. We then recognize as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. Collaborative Arrangements We enter into collaborative arrangements with partners that typically include payment to us for one of more of the following: (i) license fees; (ii) milestone payments related to the achievement of developmental, regulatory, or commercial goals; and (iii) royalties on net sales of licensed products. Each of these payments results in collaboration or other revenues. Where a portion of non-refundable up-front fees or other payments received are allocated to continuing performance obligations under the terms of a collaborative arrangement, they are recorded as deferred revenue and recognized as revenue when (or as) the underlying performance obligation is satisfied. As part of the accounting for these arrangements, we must develop estimates and assumptions that require judgment to determine the underlying stand-alone selling price for each performance obligation which determines how the transaction price is allocated among the performance obligation. The stand-alone selling price may include such items as, forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success, to determine the transaction price to allocate to each performance obligation. For our collaboration agreements that include more than one performance obligation, such as a license combined with a commitment to perform research and development services, we make judgments to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, up-front fees. We evaluate our progress each reporting period and, if necessary, adjust the measure of a performance obligation and related revenue recognition. License Fees : If a license to our intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, we recognize revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promises, we utilize judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, up-front license fees. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. Milestone : We use judgement to determine whether a milestone is considered probable of being reached. Using the most likely amount method, we include the value of a milestone payment in the consideration for a contract at inception if we then conclude achieving the milestone is more likely than not. Otherwise, we exclude the value of a milestone payment from contract consideration at inception and recognize revenue for a milestone at a later date, when we judge that it is more likely than not that the milestone will be achieved. If we conclude it is probable that a significant revenue reversal would not occur, the associated milestone is included in the transaction price. We then allocate the transaction price to each performance obligation on a relative stand-alone selling price basis, for which we recognize revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, we re-evaluate the probability of achievement of such milestones and any related constraint, and if necessary, adjust our estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration and other revenues and earnings in the period of adjustment. Royalties : For contracts that include sales-based royalties, we recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied. To date, we have not recognized any royalty revenues resulting from contracts. Research and Development Cost Reimbursements : Our Astellas and Amgen arrangements include promises of research and development services. We have determined that these services collectively are distinct from the licenses provided to Astellas and Amgen and as such, these promises are accounted for as a separate performance obligation recorded over time. We record revenue for these services as the performance obligations are satisfied, which we estimate using internal development costs incurred.
Accrued Research and Development Expenditures	Accrued Research and Development Expenditures A substantial portion of our preclinical studies and all of our clinical trials have been performed by third-party contract research organizations (“CROs”) and other vendors and our accruals for expenses for preclinical studies and clinical trials may be significant. For preclinical studies, the significant factors used in estimating accruals include the percentage of work completed to date and contract milestones achieved. For clinical trial expenses, the significant factors used in estimating accruals include the number of patients enrolled, duration of enrollment, milestones achieved and percentage of work completed to date. We monitor patient enrollment levels and related activities to the extent practicable through internal reviews, correspondence and status meetings with CROs, and review of contractual terms. We depend on the timeliness and accuracy of data provided by its CROs and other vendors to accrue expenses. If we receive and rely on incomplete or inaccurate data, accruals and expenses may be too high or too low at a given point in time and corresponding adjustments to accruals and expenses would be made in future periods when the actual expense becomes known.
Research and Development Expenditures	Research and Development Expenditures Research and development costs are charged to operations as incurred. Research and development expenses consist primarily of clinical manufacturing costs, preclinical study expenses, consulting and other third-party costs, employee compensation, supplies and materials, allocation of overhead and occupancy costs, facilities costs and depreciation of equipment.
Income Taxes	Income Taxes We account for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to affect taxable income. Valuation allowances are established when necessary to reduce deferred tax assets to the amounts expected to be realized. We recognize uncertain tax positions taken or expected to be taken on a tax return. Tax positions are initially recognized when it is We recognize interest accrued related to unrecognized tax benefits and penalties as income tax expense.
Stock-Based Compensation	Stock-Based Compensation We maintain equity incentive plans under which incentive stock options may be granted to employees and nonqualified stock options, restricted stock awards, restricted stock units and stock appreciation rights may be granted to employees, directors, consultants and advisors. In addition, we maintain an employee stock purchase plan (“ESPP”) under which employees may purchase shares of our common stock through payroll deductions. Stock-based compensation expense related to stock options granted to employees and directors is recognized based on the grant date estimated fair values, net of an estimated forfeiture rate, using the Black Scholes option pricing model. The value of the portion of the award that is ultimately expected to vest is recognized as expense ratably over the requisite service period. We estimate our forfeiture rate based on an analysis of our actual forfeitures and the experience of other companies in the same industry, and we will continue to evaluate the adequacy of the forfeiture rate assumption based on actual forfeitures, analysis of employee turnover and other related factors. Stock-based compensation expense related to restricted stock units granted to employees is recognized based on the grant-date fair value of each award and recorded as expense over the vesting period using the straight-line method, net of estimated forfeitures. Stock-based compensation expense related to the ESPP is recognized based on the fair value of each award estimated on the first day of the offering period using the Black Scholes option pricing model and recorded as expense over the service period using the straight-line method.
Amortization of Debt Discount and Issuance Costs	Amortization of Debt Discount and Issuance Costs Debt discount and issuance costs, consisting of legal and other fees directly related to the debt, are offset against gross proceeds from the issuance of debt and are amortized to interest expense over the estimated life of the debt based on the effective interest method.
Liabilities Related to Sale of Future Royalties	Liability Related to Sale of Future Royalties We treat the Liability related to sale of future royalties as a debt financing, to be amortized under the effective interest rate method over the life of the related royalty stream. The Liability related to sale of future royalties and the debt amortization are based on our current estimates of future royalties expected to be paid over the life of the arrangement. We will periodically assess the expected royalty payments using a combination of internal projections and forecasts from external sources. To the extent our 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, we will adjust the Liability related to sale of future royalties and prospectively recognize related non-cash interest expense.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, ‘Financial Instruments — Credit Losses — Measurement of Credit Losses on Financial Instruments. We are in the process of evaluating the impact the adoption of this standard would have on our financial statements and disclosures. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). . and believe that adoption will be material to our balance sheet as a result of the recognition of a right-to-use asset and corresponding liability for our building leases but will not have a material impact on our results of operations In November 2018, the Financial Accounting Standards Board (“FASB”) issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (“ASU 2018-18”), which make targeted improvements to clarify the interaction between Topic 808, Collaborative Arrangements, and Topic 606, Revenue from Contracts with Customers. ASU 2018-18 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted. We are currently evaluating the impact of adopting ASU 2018-18.

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Instruments Excluded from the Computation of Diluted Net Loss Per Share	The following instruments were excluded from the computation of diluted net loss per share for the periods presented because their effect would have been antidilutive (in thousands): December 31, 2018 2017 Options to purchase common stock 5,476 5,957 Warrants to purchase common stock 116 100 Restricted and Performance stock units 547 457 Shares issuable related to the ESPP 107 20 Total shares 6,246 6,534

Revenue Recognition (Tables)

Revenue Recognition (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Schedule of Revenue Affected by Adopting ASC 606	Our revenue for 2018 was affected by adopting ASC 606 as follows (in thousands): 2018 Research and development revenue using guidance in effect prior to ASC 606 $ 18,672 Impact of adoption of ASC 606 7,696 Research and development revenue $ 26,368 License revenue using guidance in effect prior to ASC 606 $ (5,347 ) Impact of adoption of ASC 606 10,480 License revenue $ 5,133
Schedule of Changes in Contract Assets and Liabilities	Our contract assets and liabilities changed during the period, as follows (in thousands): 2018 Contract liability from the Amgen Agreement for the Co-Invest Option Balance at beginning of period $ 18,750 Payments made for the Co-Invest Option (18,750 ) Balance at end of period $ — Contract asset from the 2016 Astellas Amendment Balance at beginning of period $ 9,708 Services performed $ 11,713 Cash received for services $ (16,867 ) Balance at end of period $ 4,554 Contract liability from the 2014 Astellas Amendment Balance at beginning of period $ 6,288 Services performed (6,288 ) Balance at end of period $ —
Accounting Standards Update 2014-09 [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Schedule of Revenue Impact of Adoption of ASC 606 on Net Loss Per Share	The impact of adoption of ASC 606 on our net loss per share was as follows (in thousands): 2018 Net loss per share using guidance in effect prior to ASC 606 $ (2.28 ) Impact of adoption of ASC 606 (0.33 ) Net loss per share $ (1.95 )

Cash Equivalents and Investme_2

Cash Equivalents and Investments (Tables)	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Amortized Cost and Fair Value of Cash Equivalents and Available for Sale Investments	The amortized cost and fair value of cash equivalents and available for sale investments at December 31, 2018 and 2017 were as follows (in thousands): December 31, 2018 Cash and Investments available for sale Amortized Cost Unrealized Gains Unrealized Losses Fair Value Money market funds $ 34,771 $ — $ — $ 34,771 U.S. Treasury securities 56,999 — (41 ) 56,958 Agency bonds 61,792 1 (14 ) 61,779 Commercial paper 19,448 — (13 ) 19,435 Corporate obligations 17,644 2 (8 ) 17,638 $ 190,654 $ 3 $ (76 ) $ 190,581 December 31, 2017 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents — U. S. Treasury securities and money market funds $ 111,501 $ — $ — $ 111,501 Short-term investments — U.S. Treasury securities and Agency bonds $ 143,895 $ — $ (210 ) $ 143,685 Long-term investments — Equity and U.S. Treasury securities $ 16,538 $ — $ (20 ) $ 16,518

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured at Fair Value on Recurring Basis	Financial assets measured at fair value on a recurring basis as of December 31, 2018 and 2017 are classified in the table below in one of the three categories described above (in thousands): December 31, 2018 Fair Value Measurements Using Assets Level 1 Level 2 Level 3 At Fair Value Assets: Money market funds $ 34,771 $ — $ — $ 34,771 U.S. Treasury securities 56,958 — — 56,958 Agency bonds — 61,779 — 61,779 Commercial paper — 19,435 — 19,435 Corporate obligations — 17,638 — 17,638 Total $ 91,729 $ 98,852 $ — $ 190,581 December 31, 2017 Fair Value Measurements Using Assets Level 1 Level 2 Level 3 At Fair Value Assets: Money market funds $ 51,001 $ — $ — $ 51,001 U.S. Treasury securities 165,801 — — 165,801 Agency bonds — 54,329 54,329 Equity securities 573 — — 573 $ 217,375 $ 54,329 $ — $ 271,704

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Summary of Property and Equipment	Our property and equipment consisted of (in thousands): December 31, 2018 2017 Property and equipment, net: Laboratory equipment $ 17,916 $ 17,100 Computer equipment and software 2,882 2,890 Office equipment, furniture and fixtures 1,137 1,137 Leasehold improvements 5,130 5,067 Total property and equipment 27,065 26,194 Less: Accumulated depreciation and amortization (23,861 ) (22,626 ) $ 3,204 $ 3,568
Summary of Accrued Liabilities	Our accrued liabilities were (in thousands): December 31, 2018 2017 Accrued liabilities: Clinical and preclinical costs $ 8,618 $ 8,370 Compensation related 6,118 6,261 Other accrued expenses 1,021 2,761 $ 15,757 $ 17,392

Research and Development Arra_2

Research and Development Arrangements (Tables)	12 Months Ended
	Dec. 31, 2018
Amgen [Member]
Summary of Research and Development Revenue	Research and development revenues from Amgen for December 31, 2018 and 2017 were as follows (in thousands): Years Ended December 31, 2018 2017 Reimbursements $ 1,915 $ 1,279 Milestone fees — 11,000 Co-Invest Option payments — (20,000 ) $ 1,915 $ (7,721 )
Astellas [Member]
Summary of License Revenues and Research and Development Revenues	License revenues and research and development revenues from Astellas for 2018 and 2017 were as follows (in thousands): Years Ended December 31, 2018 2017 License revenues $ 5,133 $ 8,799 Reimbursements 22,253 11,934 Milestone fees 2,000 — $ 29,386 $ 20,733

Long-Term Debt (Tables)

Long-Term Debt (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule Of Long Term Debt And Unamortized Debt Discount	Long-term debt and unamortized debt discount balances are as follows (in thousands): December 31, 2018 2017 Notes payable, gross $ 42,000 $ 32,000 Unamortized debt discount and interest payable (135 ) (325 ) Accretion of final exit fee 548 102 Carrying value of notes payable 42,413 31,777 Less: Current portion of long-term debt 2,607 — Long-term debt $ 39,806 $ 31,777
Schedule of Future Minimum Payments under Loan Agreement	Future minimum payments under the Loan Agreement loan, as of December 31, 2018 are as follows (in thousands): 2019 $ 5,166 2020 17,636 2021 16,267 2022 15,237 Total minimum payments 54,306 Less: Interest and final payment (12,306 ) Notes payable, gross $ 42,000

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Summary of Stock Option Activity	Stock option activity in 2018 was as follows: Stock Options Outstanding Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Life (in years) Aggregate Intrinsic Value (in thousands) Balance at December 31, 2017 5,957,458 $ 9.19 Granted 1,804,047 8.04 Exercised (422,819 ) 7.51 Forfeited (884,649 ) 11.06 Balance at December 31, 2018 6,454,037 $ 8.72 6.6 $ 412 Exercisable at December 31, 2018 4,461,463 $ 8.72 5.7 $ 411
Summary of Restricted Stock and Performance Unit Activity	Restricted stock and Performance unit activity in 2018 was as follows: Number of Restricted Stock Units Weighted Average Award Date Fair Value per Share Balance at December 31, 2017 456,752 $ 9.08 Granted 466,500 $ 7.80 Released (301,000 ) $ 8.29 Forfeited (75,752 ) $ 8.29 Balance at December 31, 2018 546,500 $ 8.53
Fair Value of Share-Based Payments was Estimated on Date of Grant Based on Assumptions	We use the Black-Scholes option pricing model to determine the fair value of stock option grants to employees and directors and employee stock purchase plan shares. The fair value of share-based payments was estimated on the date of grant based on the following assumptions: Year Ended December 31, 2018 Year Ended December 31, 2017 Options ESPP Options ESPP Risk-free interest rate 2.3% to 3.0% 1.5% to 2.5% 2.2% 1.3% Volatility 73% to 74% 73% to 74% 74% 74% Expected term in years 6.5 0.5 6.5 0.5 Expected dividend yield 0% 0% 0% 0%
Summary of Stock-Based Compensation Expense	Stock-based compensation expense for 2018 and 2017 was as follows (in thousands): Years Ended December 31, 2018 2017 Research and development $ 5,101 $ 5,656 General and administrative 4,660 3,372 $ 9,761 $ 9,028

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Reconciliation of Statutory Federal Income Tax Rate to Effective Tax Rate	The following reconciles the statutory federal income tax rate to our effective tax rate: Years Ended December 31, 2018 2017 Tax at federal statutory tax rate 21 % 34 % Tax credits (net) 1 % 8 % Federal statutory rate reduction — (51 )% Change in valuation allowance (17 )% 10 % Stock-based compensation (1 )% — Other (3 )% (1 )% Total 0 % 0 %
Schedule of Components of Income (Loss) Before Taxes	Our significant jurisdictions are the United States and California. We are subject to income tax examination for all fiscal years since inception. Income (loss) before taxes includes the following components (in thousands): Years Ended December 31, 2018 2017 United States $ (106,289 ) $ (127,235 ) Foreign — (555 ) Total $ (106,289 ) $ (127,790 )
Summary of Deferred Tax Assets, Net	Deferred tax assets, net, reflecting the net tax effect of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes, were as follows (in thousands): As of December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 121,748 $ 98,630 Tax credits 64,797 64,185 Liability related to sale of future royalties 26,294 24,593 Reserves and accruals 5,772 10,524 Capitalized R&D 4,614 6,432 Depreciation and amortization 586 546 Total noncurrent deferred tax assets 223,811 204,910 Deferred tax liabilities: Accounting method change (2,682 ) — Other (20 ) — Total noncurrent deferred tax liabilities (2,702 ) — Less: Valuation allowance (221,109 ) (204,910 ) Net deferred tax assets $ — $ —
Schedule of Activity Related to our Gross Unrecognized Tax Benefits	Activity related to our gross unrecognized tax benefits were (in thousands): Years Ended December 31, 2018 2017 Balance at the beginning of the year $ 9,365 $ 7,565 Decrease related to prior year tax positions - - Increase related to current year tax positions 110 1,800 Balance at the end of the year $ 9,475 $ 9,365

Organization and Accounting P_3

Organization and Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)Financial_InstitutionObligation	Dec. 31, 2017USD ($)
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit incurred	$ (743,324)	$ (646,081)
Net loss	(106,289)	(127,790)
Net cash provided by (used in) operating activities	(101,215)	(101,759)
Cash, cash equivalents and investments	$ 198,700	$ 285,400
Cash requirements term	12 months
Number of major financial institutions | Financial_Institution	2
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of performance obligation | Obligation	1
Computer Equipment and Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives	3 years
Laboratory Equipment and Office Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives	5 years
Furniture and Fixtures [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives	7 years
Leasehold Improvements [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives	3 years
Leasehold Improvements [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives	7 years

Net Loss Per Share - Instrument

Net Loss Per Share - Instruments Excluded from the Computation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	6,246	6,534
Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	5,476	5,957
Warrants to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	116	100
Restricted and Performance Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	547	457
ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	107	20

Revenue Recognition - Schedule

Revenue Recognition - Schedule of Revenue Affected by Adopting ASC 606 (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenues	$ 31,501	$ 13,368
Research and Development Revenue [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenues	26,368	4,569
License Revenues [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenues	5,133	$ 8,799
Calculated Under Revenue Guidance in Effect Before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Research and Development Revenue [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenues	18,672
Calculated Under Revenue Guidance in Effect Before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | License Revenues [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenues	(5,347)
Difference Between Revenue Guidance in Effect Before and After Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Research and Development Revenue [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenues	7,696
Difference Between Revenue Guidance in Effect Before and After Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | License Revenues [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenues	$ 10,480

Revenue Recognition - Schedul_2

Revenue Recognition - Schedule of Revenue Impact of Adoption of ASC 606 on Net Loss Per Share (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss per share	$ (1.95)	$ (2.59)
Guidance in Effect Prior Topic to ASC 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss per share	(2.28)
Impact of Adoption of ASC 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss per share	$ (0.33)

Revenue Recognition - Schedul_3

Revenue Recognition - Schedule of Changes in Contract Assets and Liabilities (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Balance at beginning of period	$ 0
Balance at end of period	4,554
Amgen Agreement [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Balance at beginning of period	18,750
Payments made for the Co-Invest Option	(18,750)
Balance at end of period	0
2016 Astellas Agreement [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Balance at beginning of period	9,708
Services performed	11,713
Cash received for services	(16,867)
Balance at end of period	4,554
2014 Astellas Agreement [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Balance at beginning of period	6,288
Services performed	(6,288)
Balance at end of period	$ 0

Cash Equivalents and Investme_3

Cash Equivalents and Investments - Amortized Cost and Fair Value of Cash Equivalents and Available for Sale Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Line Items]
Amortized Cost	$ 42,256	$ 125,206
Unrealized Gains	700
Amortized Cost	190,654
Unrealized Gains	3
Unrealized Losses	(76)
Fair Value	190,581
Money Market Funds [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	34,771
Unrealized Gains	0
Unrealized Losses	0
Fair Value	34,771
U.S. Treasury Securities [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	56,999
Unrealized Gains	0
Unrealized Losses	(41)
Fair Value	56,958
Agency Bonds [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	61,792
Unrealized Gains	1
Unrealized Losses	(14)
Fair Value	61,779
Commercial Paper [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	19,448
Unrealized Gains	0
Unrealized Losses	(13)
Fair Value	19,435
Corporate Obligations [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	17,644
Unrealized Gains	2
Unrealized Losses	(8)
Fair Value	$ 17,638
US Treasury Securities and Money Market Funds [Member] | Cash Equivalents [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost		111,501
Unrealized Gains		0
Unrealized Losses		0
Fair Value		111,501
U.S. Treasury Securities and Agency Bonds [Member] | Short-term Investments [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost		143,895
Unrealized Gains		0
Unrealized Losses		(210)
Fair Value		143,685
Equity and U.S. Treasury Securities [Member] | Long-term Investments [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost		16,538
Unrealized Gains		0
Unrealized Losses		(20)
Fair Value		$ 16,518

Cash Equivalents and Investme_4

Cash Equivalents and Investments - Additional Information (Detail)	Dec. 31, 2018USD ($)
Cash And Cash Equivalents [Abstract]
Investments in continuous unrealized loss position for 12 months or longer	$ 0

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets at fair value	$ 190,581	$ 271,704
Fair Value Measurements Using Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets at fair value	91,729	217,375
Fair Value Measurements Using Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets at fair value	98,852	54,329
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
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets at fair value	34,771	51,001
Money Market Funds [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	34,771	51,001
Money Market Funds [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
Money Market Funds [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
U.S. Treasury Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets at fair value	56,958	165,801
U.S. Treasury Securities [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	56,958	165,801
U.S. Treasury Securities [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
U.S. Treasury Securities [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
Agency Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets at fair value	61,779	54,329
Agency Bonds [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	0	0
Agency Bonds [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	61,779	54,329
Agency Bonds [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
Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets at fair value	19,435
Commercial Paper [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	0
Commercial Paper [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	19,435
Commercial Paper [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
Corporate Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets at fair value	17,638
Corporate Obligations [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	0
Corporate Obligations [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	17,638
Corporate Obligations [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
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets at fair value		573
Equity Securities [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		573
Equity Securities [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
Equity Securities [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

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Available for sale investments, unrealized gain	$ 700,000
Available for sale investments in continous unrealized loss position for 12 months or longer	0
Long term debt, fair value	42,100,000	$ 31,800,000
Fair value of liabilities transferred from level 1 to level 2	0
Fair value of liabilities transferred from level 2 to level 1	0
Fair value of liabilities transferred into level 3	0
Fair value of liabilities transferred from level 3	$ 0

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property and equipment	$ 27,065	$ 26,194
Less: Accumulated depreciation and amortization	(23,861)	(22,626)
Total property and equipment, net	3,204	3,568
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	17,916	17,100
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	2,882	2,890
Office Equipment, Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,137	1,137
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 5,130	$ 5,067

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Depreciation expense	$ 1.2	$ 1.9
Employer contributions under the plan	$ 0.5	$ 0.5

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued liabilities:
Clinical and preclinical costs	$ 8,618	$ 8,370
Compensation related	6,118	6,261
Other accrued expenses	1,021	2,761
Total accrued liabilities	$ 15,757	$ 17,392

Research and Development Arra_3

Research and Development Arrangements - Additional Information (Details)	Jan. 01, 2018USD ($)	Jul. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)ClinicalTrial	Dec. 31, 2016USD ($)	Dec. 31, 2014USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accumulated deficit incurred			$ (646,081,000)	$ (743,324,000)	$ (646,081,000)
Total revenues				31,501,000	13,368,000
Accounts receivable			1,112,000	2,231,000	1,112,000
Research and development				89,135,000	90,296,000
Contract assets			0	4,554,000	0
Deferred revenue, current			9,572,000	0	9,572,000
Deferred revenue, non-current			15,000,000	0	$ 15,000,000
2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable option fee						$ 15,000,000
MyoKardia, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone clinical payment recived under collaboration agreement		$ 200,000
Reldesemtiv [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of clinical trials conducted | ClinicalTrial					2
Amgen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Co-invest option payment				18,800,000	$ 20,000,000
Co-invest option exercised amount				$ 40,000,000
Percentage of incremental royalty receivable on annual net sales				4.00%
Total revenues				$ 1,915,000	(7,721,000)
Amgen [Member] | Milestone Fees [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues				0	11,000,000
Astellas [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues				29,386,000	$ 20,733,000
Astellas [Member] | 2014 Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received							$ 30,000,000
Amount received as milestone payment							$ 15,000,000
Astellas [Member] | 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount received as milestone payment						15,000,000
Revenue recognition, Non-refundable upfront amendment fee						35,000,000
Revenue Recognition over Performance Period						44,200,000
Allocated Consideration						$ 94,200,000
Premium percentage					100.00%
Astellas [Member] | Milestone Fees [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues				2,000,000	$ 0
Astellas [Member] | Calculated Under Revenue Guidance in Effect Before Topic 606 [Member] | 2016 Astellas Amendment [Member] | Tirasemtiv License [Member] | Tirasemtiv Option [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, current			9,600,000		9,600,000
Deferred revenue, non-current			15,000,000		15,000,000
Maximum [Member] | 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research milestone payments				2,000,000
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
Maximum [Member] | Astellas [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
Minimum [Member] | Astellas [Member] | 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement				600,000,000
Minimum [Member] | Astellas [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
Accounting Standards Update 2014-09 [Member] | Difference Between Revenue Guidance in Effect Before and After Topic 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract liabilities				9,600,000
Accounting Standards Update 2014-09 [Member] | Difference Between Revenue Guidance in Effect Before and After Topic 606 [Member] | 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accumulated deficit incurred	$ 16,700,000
Non-refundable fees	65,000,000
Research and development	35,600,000
Contract assets	16,700,000
License transaction price	83,000,000
Services transaction price	18,000,000
Accounting Standards Update 2014-09 [Member] | Amgen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Co-invest option payment				18,800,000
Accounts receivable			1,000,000	1,900,000	1,000,000
Accounting Standards Update 2014-09 [Member] | Amgen [Member] | Milestone Fees [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues					11,000,000
Accounting Standards Update 2014-09 [Member] | Amgen [Member] | Milestone Fees [Member] | Galactic HF [Member] | Japan [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues					10,000,000
Accounting Standards Update 2014-09 [Member] | Amgen [Member] | Milestone Fees [Member] | Cardiac Muscle Activator [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues			1,000,000
Accounting Standards Update 2014-09 [Member] | Amgen [Member] | Difference Between Revenue Guidance in Effect Before and After Topic 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with Customer, Liability, Revenue Recognized	18,800,000
Accumulated deficit incurred	$ (18,800,000)
Accounting Standards Update 2014-09 [Member] | Astellas [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable			$ 0	300,000	$ 0
Accounting Standards Update 2014-09 [Member] | Amgen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Pre-commercialization milestone payments eligible to receive				300,000,000
Accounting Standards Update 2014-09 [Member] | Maximum [Member] | Amgen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Commercialization milestone payments eligible to receive				$ 300,000,000

Research and Development Arra_4

Research and Development Arrangements - Summary of Research and Development Revenue (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	$ 31,501	$ 13,368
Amgen [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	1,915	(7,721)
Amgen [Member] | Reimbursements [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	1,915	1,279
Amgen [Member] | Milestone Fees [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	0	11,000
Amgen [Member] | Co-Invest Option Payments [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	$ 0	$ (20,000)

Research and Development Arra_5

Research and Development Arrangements - Summary of License Revenues and Research and Development Revenues (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	$ 31,501	$ 13,368
Astellas [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	29,386	20,733
License Revenues [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	5,133	8,799
License Revenues [Member] | Astellas [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	5,133	8,799
Reimbursements [Member] | Astellas [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	22,253	11,934
Milestone Fees [Member] | Astellas [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenues	$ 2,000	$ 0

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail) - Amended Loan Agreement [Member] - Oxford and Silicon Valley Bank [Member] $ in Millions	Oct. 27, 2017USD ($)	Aug. 31, 2018USD ($)	Dec. 31, 2018Installment
Debt Instrument [Line Items]
Proceeds from term loan	$ 32	$ 10
Retirement of debt outstanding	30
Payment of accrued portion of final payment fee	$ 0.5
Loan repayment terms			Payments on Long-term debt will be interest only through November 2019, followed by 35 months of equal monthly payments of interest and principal.
Number of installments description			35 months of monthly payments of interest and principal
Number of installments | Installment			35
Final payment fee percentage			6.50%
Debt instrument, base interest rate for scenario 2			6.81%
Debt instrument, applicable interest rate for scenario 1			8.05%
Interest rate description			The interest rate under the Amended Loan Agreement is the greater of (a) 8.05% or (b) the sum of 6.81% plus the 30-day U.S. LIBOR rate

Long-Term Debt - Schedule of Lo

Long-Term Debt - Schedule of Long Term Debt and Unamortized Debt Discount (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Notes payable, gross	$ 42,000	$ 32,000
Unamortized debt discount and interest payable	(135)	(325)
Accretion of final exit fee	548	102
Carrying value of notes payable	42,413	31,777
Less: Current portion of long-term debt	2,607	0
Long-term debt	$ 39,806	$ 31,777

Long-Term Debt - Schedule of Fu

Long-Term Debt - Schedule of Future Minimum Payments under Loan Agreement (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Carrying value of notes payable	$ 42,413	$ 31,777
Long-term debt, alternative
Carrying value of notes payable	42,413	31,777
Notes payable, gross	42,000	$ 32,000
Loan and Security Agreement [Member] | Oxford and Silicon Valley Bank [Member]
Debt Instrument [Line Items]
2,019	5,166
2,020	17,636
2,021	16,267
2,022	15,237
Carrying value of notes payable	54,306
Long-term debt, alternative
Carrying value of notes payable	54,306
Less: Interest and final payment	(12,306)
Notes payable, gross	$ 42,000

Liability Related to Sale of _2

Liability Related to Sale of Future Royalties - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2017	Dec. 31, 2018	Dec. 31, 2017
Liability Related to Sale of Future Royalties [Line Items]
Proceeds from issuance of common stock		$ 0	$ 112,224
Effective tax rate		21.00%	34.00%
Allocated Consideration		$ 100,000	$ 0
Transaction costs incurred in connection with the Royalty Monetization and RPI Common stock		1,800
Non-cash interest expense recognized		17,767	14,028
Liability Related to Sale of Future Royalties [Member]
Liability Related to Sale of Future Royalties [Line Items]
Allocated Consideration		92,300
Common Stock [Member]
Liability Related to Sale of Future Royalties [Line Items]
Allocated Consideration		$ 7,700
Royalty Purchase Agreement [Member]
Liability Related to Sale of Future Royalties [Line Items]
Cash payment under Royalty Agreement	$ 90,000
Common stock, shares issued in period	875,656
Proceeds from issuance of common stock	$ 10,000
Fair value for the liability related to sale of future royalties			131,600
Effective annual non-cash interest rate in calculating liability related to sale of future royalties		17.00%
Common stock fair value		$ 8,100
Effective tax rate		21.00%
Annual pre-tax interest rate in calculating liability related to sale of future royalties		17.00%
Non-cash interest expense recognized		$ 17,800	$ 14,000
Royalty Purchase Agreement [Member] | Scenario Previously Reported [Member]
Liability Related to Sale of Future Royalties [Line Items]
Fair value for the liability related to sale of future royalties		$ 96,700

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Non-cancelable operating lease expiration month and year	2021-06
Rent expense	$ 5	$ 3.6	$ 3.4
Future minimum lease payments under noncancelable operating leases in 2019	4.7
Future minimum lease payments under noncancelable operating leases in 2020	4.8
Future minimum lease payments under noncancelable operating leases in 2021	$ 2.5

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015	Aug. 31, 2018	Mar. 31, 2018	Feb. 29, 2016	Oct. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of stock options outstanding	$ 412
Non-cash stock-based compensation expense	$ 9,761	$ 9,028
Number of issued shares of common stock related to cashless exercise of warrants		3,450,122
Warrants issued				42,253
Weighted average exercise price of warrants issued				$ 7.10
Outstanding warrants	142,359
Outstanding warrants, weighted average exercise price	$ 6.85
Warrant expiration date		2017-06
Net proceeds of issuance of common stock	$ 0	$ 112,224
Cantor Fitzgerald Agreement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares issued in period		2,425,625
Net proceeds of issuance of common stock		$ 29,900
Loan and Security Agreement [Member] | Warrants to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of issued shares of common stock related to cashless exercise of warrants		16,126
Oxford and Silicon Valley Bank [Member] | Loan and Security Agreement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants outstanding to purchase upon exercise of common stock						68,285	65,189
Warrants exercise price						$ 6.59	$ 6.90
Restricted Stock and Performance Unit Activity
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, vested restricted stock and performance units, total fair value	$ 2,300
Performance stock units, remaining	546,500	456,752
Performance stock units, forfeited	75,752
Restricted Stock and Performance Unit Activity | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Restricted Stock and Performance Unit Activity | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted			685,000
Weighted-average grant date fair value of options granted			$ 7
Number of Performance stock unit for which performance criteria were met		342,500
Performance stock units, vested		171,250
Performance stock units, remaining					171,250
Performance stock units, expected to vest	2018-03
Performance stock units, forfeited		342,500
Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	$ 13,800
Weighted-average period	2 years 4 months 24 days
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	$ 3,500
Weighted-average period	1 year 9 months 18 days
2004 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option grant prices as percentage of the fair market value of the common stock	100.00%
Term to grant nonstatutory stock options and incentive stock options	10 years
Percentage of options grant to new employees	25.00%
Shares of common stock reserved for issuance	2,400,000
Shares of common stock available for issuance	2,400,000
Share based compensation, options exercised, total intrinsic value	$ 700	$ 1,800
Intrinsic value of stock options outstanding		$ 4,300
2004 Plan [Member] | New Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting right	Options granted to new employees generally vest 25% after one year and monthly thereafter over a period of four years
Period from percentage of stock option vested	1 year
Vesting period	4 years
2004 Plan [Member] | Existing Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting right	Options granted to existing employees generally vest monthly over a period of four years.
Vesting period	4 years
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option grant prices as percentage of the fair market value of the common stock	85.00%
Shares of common stock reserved for issuance	253,617
Issuance of common stock pursuant to ESPP, shares	144,822	120,959
Issuance of common stock pursuant to ESPP, per share	$ 6.40	$ 9.65
Non-employee Stock-Based Compensation [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-cash stock-based compensation expense	$ 100	$ 500

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Options Outstanding, Beginning Balance	5,957,458
Stock Options Outstanding, Granted	1,804,047
Stock Options Outstanding, Forfeited	(884,649)
Stock Options Outstanding, Ending Balance	6,454,037	5,957,458
Stock Options Outstanding, Exercisable	4,461,463
Weighted Average Exercise Price per Share, Beginning Balance	$ 9.19
Weighted Average Exercise Price per Share, Granted	8.04
Weighted Average Exercise Price per Share, Exercised	7.51
Weighted Average Exercise Price per Share, Forfeited	11.06
Weighted Average Exercise Price per Share, Ending Balance	8.72	$ 9.19
Weighted Average Exercise Price per Share, Exercisable	$ 8.72
Weighted Average Remaining Contractual Life	6 years 7 months 6 days
Weighted Average Remaining Contractual Life, Exercisable at December 31, 2018	5 years 8 months 12 days
Aggregate Intrinsic Value	$ 412
Aggregate Intrinsic Value, Exercisable at December 31, 2018	$ 411
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Options Outstanding, Exercised	(422,819)	(264,164)

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of Restricted Stock And Performance Unit Activity (Detail) - Restricted Stock and Performance Unit Activity	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Restricted Stock Units, Beginning Balance | shares	456,752
Number of Restricted Stock Units, Granted | shares	466,500
Number of Restricted Stock Units, Released | shares	(301,000)
Number of Restricted Stock Units, Forfeited | shares	(75,752)
Number of Restricted Stock Units, Ending Balance | shares	546,500
Weighted Average Award Date Fair Value per Share, Beginning Balance | $ / shares	$ 9.08
Weighted Average Award Date Fair Value per Share, Granted | $ / shares	7.80
Weighted Average Award Date Fair Value per Share, Released | $ / shares	8.29
Weighted Average Award Date Fair Value per Share, Forfeited | $ / shares	8.29
Weighted Average Award Date Fair Value per Share, Ending Balance | $ / shares	$ 8.53

Stockholders' Equity - Fair Val

Stockholders' Equity - Fair Value of Share-Based Payments was Estimated on Date of Grant Based on Assumptions (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Options [Member]
Deferred Compensation Arrangement With Individual Share Based Payments [Line Items]
Risk-free interest rate, minimum	2.30%
Risk-free interest rate, maximum	3.00%
Risk-free interest rate		2.20%
Volatility, minimum	73.00%
Volatility, maximum	74.00%
Volatility		74.00%
Expected term in years	6 years 6 months	6 years 6 months
Expected dividend yield	0.00%	0.00%
ESPP [Member]
Deferred Compensation Arrangement With Individual Share Based Payments [Line Items]
Risk-free interest rate, minimum	1.50%
Risk-free interest rate, maximum	2.50%
Risk-free interest rate		1.30%
Volatility, minimum	73.00%
Volatility, maximum	74.00%
Volatility		74.00%
Expected term in years	6 months	6 months
Expected dividend yield	0.00%	0.00%

Stockholders' Equity - Summar_3

Stockholders' Equity - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 9,761	$ 9,028
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	5,101	5,656
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 4,660	$ 3,372

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Provision for income tax	$ 0	$ 0
Research and development credits and orphan drug credits, federal carryforwards will expire	2,021
Increase (decrease) in valuation allowance	$ 16,200,000	(11,700,000)
Unrecognized tax benefits	$ 8,600,000	$ 8,100,000
Effective tax rate	21.00%	34.00%
Tax Cuts and Jobs Act, maximum measurement period permitted for companies to complete accounting	1 year
Federal Tax [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 490,100,000
Net operating loss carryforwards expiration	2,022
Credit carryforwards for federal and state	$ 61,600,000
Federal and State Tax [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 252,800,000
Net operating loss carryforwards expiration	2,028
Credit carryforwards for federal and state	$ 15,000,000

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax at federal statutory tax rate	21.00%	34.00%
Tax credits (net)	1.00%	8.00%
Federal statutory rate reduction	0.00%	(51.00%)
Change in valuation allowance	(17.00%)	10.00%
Stock-based compensation	(1.00%)	(0.00%)
Other	(3.00%)	(1.00%)
Total	0.00%	0.00%

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Income (Loss) Before Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States	$ (106,289)	$ (127,235)
Foreign	0	(555)
Total	$ (106,289)	$ (127,790)

Income Taxes - Summary of Defer

Income Taxes - Summary of Deferred Tax Assets, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 121,748	$ 98,630
Tax credits	64,797	64,185
Liability related to sale of future royalties	26,294	24,593
Reserves and accruals	5,772	10,524
Capitalized R&D	4,614	6,432
Depreciation and amortization	586	546
Total noncurrent deferred tax assets	223,811	204,910
Deferred tax liabilities:
Accounting method change	(2,682)	0
Other	(20)	0
Total noncurrent deferred tax liabilities	(2,702)	0
Less: Valuation allowance	(221,109)	(204,910)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Schedule of Acti

Income Taxes - Schedule of Activity Related to our Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Balance at the beginning of the year	$ 9,365	$ 7,565
Decrease related to prior year tax positions	0	0
Increase related to current year tax positions	110	1,800
Balance at the end of the year	$ 9,475	$ 9,365