MyoKardia (MYOK) 10-Q 6 May 20 2020 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 01, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Trading Symbol	MYOK
Entity Registrant Name	MYOKARDIA, INC.
Entity Central Index Key	0001552451
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		46,680,808
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-37609
Entity Tax Identification Number	44-5500552
Entity Address, Address Line One	1000 Sierra Point Parkway
Entity Address, City or Town	Brisbane
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94005
City Area Code	650
Local Phone Number	741-0900
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 131,204	$ 101,436
Short-term investments	218,809	314,691
Prepaid expenses and other current assets	7,486	7,709
Total current assets	357,499	423,836
Property and equipment, net	19,801	15,743
Operating lease right-of-use assets	51,981	417
Long-term investments	10,077	14,153
Restricted cash and other	1,968	1,945
Total assets	441,326	456,094
Current liabilities
Accounts payable	5,573	6,237
Accrued liabilities	32,236	41,292
Operating lease liabilities - current	7,851	383
Total current liabilities	45,660	47,912
Operating lease liability	44,658
Other long-term liabilities	1,908	1,908
Total liabilities	92,226	49,820
Commitments and contingencies (Note 7)
Stockholders’ equity
Preferred stock, $0.0001 par value; 5,000,000 shares authorized; none issued and outstanding	0	0
Common stock, $0.0001 par value, 150,000,000 shares authorized at March 31, 2020 and December 31, 2019; 46,612,186 and 46,379,073 shares issued and outstanding at March 31, 2020 and December 31, 2019, respectively	5	5
Additional paid-in capital	897,058	884,486
Accumulated other comprehensive income	671	549
Accumulated deficit	(548,634)	(478,766)
Total stockholders’ equity	349,100	406,274
Total liabilities and stockholders’ equity	$ 441,326	$ 456,094

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	46,612,186	46,379,073
Common stock, shares outstanding	46,612,186	46,379,073

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating expenses:
Research and development	$ 51,878	$ 26,190
Selling, general and administrative	19,902	13,551
Total operating expenses	71,780	39,741
Loss from operations	(71,780)	(39,741)
Interest and other income, net	1,912	2,271
Net loss	(69,868)	(37,470)
Other comprehensive income	122	363
Comprehensive loss	$ (69,746)	$ (37,107)
Net loss per share, basic and diluted	$ (1.50)	$ (0.93)
Weighted average number of shares used to compute net loss per share, basic and diluted	46,566,995	40,506,313

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income/ (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2018	$ 370,567	$ 4	$ 573,183	$ (67)	$ (202,553)
Beginning balance, shares at Dec. 31, 2018		40,288,949
Issuance of common stock in connection with the2019 follow-on offering, net of issuance costs of $17,638	271,213	$ 1	271,212
Issuance of common stock in connection with the2019 follow-on offering, net of issuance costs of $17,638, shares		5,663,750
Issuance of common stock upon the exercise of options and release of stock awards	280		280
Issuance of common stock upon the exercise of options and release of stock awards, shares		49,076
Vesting of early exercised stock options	8		8
Stock-based compensation	6,981		6,981
Unrealized gains	363			363
Net loss	(37,470)				(37,470)
Ending balance at Mar. 31, 2019	611,942	$ 5	851,664	296	(240,023)
Ending balance, shares at Mar. 31, 2019		46,001,775
Beginning balance at Dec. 31, 2019	406,274	$ 5	884,486	549	(478,766)
Beginning balance, shares at Dec. 31, 2019		46,379,073
Issuance of common stock upon the exercise of options and release of stock awards	$ 1,820		1,820
Issuance of common stock upon the exercise of options and release of stock awards, shares	107,870	233,113
Stock-based compensation	$ 10,752		10,752
Unrealized gains	122			122
Net loss	(69,868)				(69,868)
Ending balance at Mar. 31, 2020	$ 349,100	$ 5	$ 897,058	$ 671	$ (548,634)
Ending balance, shares at Mar. 31, 2020		46,612,186

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Common Stock
Issuance costs	$ 17,638

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flow from operating activities:
Net loss	$ (69,868)	$ (37,470)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	10,752	6,981
Depreciation	779	461
Amortization of discount on investments	(418)	(292)
Loss on disposal of equipment	49
Change in operating assets and liabilities:
Prepaid expenses and other current assets	226	448
Operating lease right-of-use assets	1,066	635
Other long-term assets		(13)
Accounts payable	(467)	1,950
Accrued liabilities	(1,512)	(180)
Prepayment from collaboration partner		(9,874)
Operating lease liabilities	(504)
Other long-term liabilities		(692)
Net cash used in operating activities	(59,897)	(38,046)
Cash flow from investing activities:
Purchases of investments		(32,697)
Sales of investments	4,000	4,000
Maturities of investments	96,475	16,000
Purchases of property and equipment	(12,627)	(813)
Net cash provided by (used in) investing activities	87,848	(13,510)
Cash flow from financing activities:
Proceeds from issuance of common stock in follow-on offerings, net of issuance and financing costs		271,485
Proceeds from exercise of stock options and employee stock purchase plan	1,817	280
Net cash provided by financing activities	1,817	271,765
Net increase in cash, cash equivalents and restricted cash	29,768	220,209
Cash, cash equivalents and restricted cash, beginning of period	103,630	248,265
Cash, cash equivalents and restricted cash, end of period	133,398	468,474
Non-cash investing and financing activities:
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 2,015	570
Vesting of early exercised options and restricted stock		8
Unpaid financing-related costs		$ 262

Organization

Organization	3 Months Ended
	Mar. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization MyoKardia, Inc. (the Company) is a clinical-stage biopharmaceutical company pioneering a precision medicine approach to discover, develop and commercialize targeted therapies for the treatment of serious and neglected rare cardiovascular diseases. The Company’s initial focus is on the treatment of cardiomyopathies, a group of diseases of the heart muscle. MyoKardia’s pipeline includes: mavacamten and MYK-224, which are being studied for the treatment of hypertrophic cardiomyopathy; LUS-1, being studied for the treatment of diseases of diastolic dysfunction; and danicamtiv (formerly MYK-491) and ACT-1, being studied for the treatment of diseases of systolic dysfunction. MyoKardia’s most advanced programs are: mavacamten, which is in four clinical trials including a Phase 3 study in patients with hypertrophic cardiomyopathy (HCM); danicamtiv, which recently completed a Phase 2a multiple-ascending dose study in patients with stable systolic heart failure and is being advanced to a Phase 2 study in patients with genetic dilated cardiomyopathy; and MYK-224, which is in a Phase 1 randomized, placebo-controlled study in healthy volunteers. The Company was incorporated on June 8, 2012 in Delaware and its corporate headquarters and operations are in Brisbane, California. Liquidity The Company has incurred significant operating losses since inception and has an accumulated deficit of $548.6 million as of March 31, 2020. The Company has relied on its ability to fund its operations through private and public equity financings and to a lesser extent, through a license and collaboration arrangement with a collaboration partner, Sanofi S.A. (Sanofi) via its subsidiary, Aventis Inc. The collaboration agreement ended on December 31, 2018 and the Company had no revenues relating to its Sanofi collaboration after December 31, 2018, nor has it received reimbursements of research and development expenses after June 30, 2019. The Company has not yet received regulatory approval to commercialize or sell any product and does not have customers. Management expects operating losses and negative operating cash flows to continue for the foreseeable future. As the Company continues to incur losses, a transition to profitability is dependent upon the successful development, approval, and commercialization of the Company’s products and product candidates and the achievement of a level of revenues adequate to support its cost structure. The Company’s ultimate success depends on the outcome of its research and development activities and anticipates the need to raise additional capital to fully implement its business plan. The Company intends to raise such capital through the issuance of additional equity, debt and/or strategic alliances with partner companies. As of March 31, 2020, the Company had $360.1 million of cash, cash equivalents and short and long-term investments, which management believes will be sufficient to meet the Company’s anticipated operating and capital expenditure requirements for the twelve months following the date of issuance of these financial statements. In addition, the Company is closely monitoring ongoing developments in connection with the COVID-19 pandemic, which may negatively impact its financial and operating results. The Company will continue to assess its operating expenses and cash and cash equivalents and, if circumstances warrant, the Company will make appropriate adjustments to its operating plan

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	2. Basis of Presentation and Summary of Significant Accounting Policies Basis of Presentation The accompanying condensed consolidated financial statements are unaudited, include the Company’s accounts and those of its wholly-owned subsidiaries MyoKardia Australia Pty Ltd and MyoKardia Netherlands B.V., and have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). The condensed consolidated balance sheet at December 31, 2019, has been derived from the audited consolidated financial statements as of that date but does not include all of the information and footnotes required by U.S. GAAP for complete financial statements. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full fiscal year or any interim period and should be read in conjunction with the audited financial statements for the year ended December 31, 2019 and the notes thereto, which are included in the Company’s Annual Report on Form 10-K as of and for the year ended December 31, 2019 . The significant accounting policies used in preparation of these condensed consolidated financial statements for the periods shown are consisten t with those discussed in notes to the consolidated financial statements in the Company’s 2019 Annual Report on Form 10-K and are updated below as necessary. The Company currently operates in one business segment, which is the identification, development and commercialization of therapies for the treatment of serious and neglected rare Reconciliation of Cash, Cash Equivalents, and Restricted Cash as Reported in Consolidated Statements of Cash Flows Cash as reported in the consolidated statements of cash flows includes the aggregate amounts of cash, cash equivalents and restricted cash as presented on the consolidated balance sheets. Restricted cash at March 31, 2020 and December 31, 2019 represents cash balances held as security in connection with the Company’s facility lease agreements. The following table provides a reconciliation of cash, cash equivalents and restricted cash within the consolidated balance sheets to the total shown in the consolidated statements of cash flows (in thousands) : March 31, 2020 December 31, 2019 Cash and cash equivalents $ 131,204 $ 101,436 Restricted cash included in prepaid expenses and other current assets 337 337 Restricted cash included in restricted cash and other 1,857 1,857 Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows $ 133,398 $ 103,630 Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities and the reported amounts of revenues and expenses in the consolidated financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trials accrued liabilities, income tax valuation allowance and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates. Recently Adopted Accounting Pronouncements In November 2018, the Financial Accounting Standards Board (FASB) issued ASU 2018-18 (Topic 808), Clarifying the Interaction Between Topic 808 and Topic 606 In August 2018, the FASB issued ASU 2018-13 (Topic 820), Fair Value Measurement In June 2016, the FASB issued ASU No. 2016-13 (Topic 326), Financial Instruments –Measurement of Credit Losses on Financial Instruments should be estimated when the fair value of the debt securities is below their associated amortized costs. The Company adopted this amendment in the first quarter of 2020 and the adoption did not have a material impact to the Company’s financial statements.

Sanofi License and Collaboratio

Sanofi License and Collaboration Agreement	3 Months Ended
	Mar. 31, 2020
Collaboration And License Agreement Disclosure [Abstract]
Sanofi License and Collaboration Agreement	3. Sanofi License and Collaboration Agreement Sanofi (Aventis Inc.) Agreement Overview, Termination and Repurchase of Royalty Rights Until December 31, 2018 the Company had an exclusive license and collaboration agreement (Collaboration Agreement) with Aventis Inc., a wholly-owned subsidiary of Sanofi S.A. (Sanofi). On December 31, 2018, Sanofi notified the Company of its intent to terminate the collaboration, specifically, Sanofi elected not to continue with the mavacamten, MYK-224 and danicamtiv programs. As a result, cost sharing and Sanofi’s reimbursement of our research and development costs for mavacamten and MYK-224 ended in the first half of 2019. At that time Sanofi had continuing rights to royalties in the event of commercialization of the mavacamten and MYK-224 programs. In July 2019, the Company repurchased those rights from Sanofi for $80.0 million. Neither the Company nor Sanofi have any material continuing rights or obligations under the Collaboration Agreement.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements Fair value accounting is applied for all financial assets and liabilities, including short-term and long-term investments and non-financial assets and liabilities that are recognized or disclosed at fair value in the consolidated financial statements on a recurring basis (at least annually). The carrying amount of the Company’s financial instruments, including accounts payable and accrued liabilities and other current liabilities approximate fair value due to their short-term maturities. Marketable securities are stated at their estimated fair values. The counterparties to the agreements relating to the Company’s investment securities consist of the U.S. Treasury, governmental agencies, various major corporations and financial institutions with high credit standing. The carrying amounts for financial instruments consisting of cash and cash equivalents, receivable from collaboration partner, accounts payable and accrued liabilities approximate fair value due to their short maturities. The accounting guidance for fair value provides a framework for measuring fair value, clarifies the definition of fair value and expands disclosures regarding fair value measurements. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The accounting guidance establishes a three-tiered hierarchy, which prioritizes the inputs used in the valuation methodologies in measuring fair value as follows: Level 1—Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2—Inputs other than quoted market prices included in Level 1 are either directly or indirectly observable for the asset or liability through correlation with market data at the measurement date and for the duration of the instrument’s anticipated life. Level 3—Inputs reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model. The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): Fair Value Measurements at March 31, 2020 Total Level 1 Level 2 Level 3 Assets Money market funds $ 130,176 $ 130,176 $ — $ — U.S. government agency obligations 107,657 — 107,657 — Corporate securities 121,229 — 121,229 — Total $ 359,062 $ 130,176 $ 228,886 $ — Fair Value Measurements at December 31, 2019 Total Level 1 Level 2 Level 3 Assets Money market funds $ 100,441 $ 100,441 $ — $ — U.S. government agency obligations 134,055 — 134,055 — Corporate securities 194,789 — 194,789 — Total $ 429,285 $ 100,441 $ 328,844 $ — The following table is a summary of amortized cost, unrealized gain and loss, and fair value of the Company’s marketable securities by contractual maturities (in thousands): Fair Value Measurements at March 31, 2020 Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 130,153 $ 23 $ — $ 130,176 Short-term investments (due within one year) 218,129 827 (147 ) 218,809 Long-term investments (due between one and two years) 10,104 27 (54 ) 10,077 Total $ 358,386 $ 877 $ (201 ) $ 359,062 Fair Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 100,440 $ 1 $ — $ 100,441 Short-term investments (due within one year) 314,181 523 (13 ) 314,691 Long-term investments (due between one and two years) 14,110 47 (4 ) 14,153 Total $ 428,731 $ 571 $ (17 ) $ 429,285

Leases

Leases	3 Months Ended
	Mar. 31, 2020
Lessee Disclosure [Abstract]
Leases	5. Leases The Company determines if an arrangement is or contains a lease at inception. Operating lease right-of-use (ROU) assets and liabilities are presented separately on our consolidated balance sheets. The Company does not have any finance leases. Operating lease ROU assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term beginning at the commencement date. As the Company’s leases do not provide enough information to determine an implicit interest rate, the Company determines its incremental borrowing rate based on the rate of interest that the Company would have to pay to borrow on a collateralized basis over a similar term in a similar economic environment. The operating lease ROU assets also include any lease payments made and excludes lease incentives and initial direct costs incurred. Lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. In September 2018, the Company entered into a lease agreement (the “Lease”) for approximately 129,800 square feet of office and laboratory space in Brisbane, California, which is now the Company’s corporate headquarters. The Company performed an evaluation of the Lease and determined it is an operating lease. The lease commencement date was in January 2020 and on the commencement date, the Company recognized $52.6 million of ROU asset and lease liability on its consolidated balance sheet in accordance with ASC 842. The Lease grants the Company an option to extend the Lease for an additional 10-year period. This optional period was not included in the measurement of the ROU asset or lease liability as it was not reasonably certain that the Company would exercise the option. The Lease requires the Company to share in prorated operating expenses and property taxes based upon actual amounts incurred The Lease provides for annual base rent of approximately $5.5 million in the first year of the lease term. The annual base rent for the second twelve months will be approximately $8.5 million, which will increase on an annual basis beginning from the 25th month to approximately $11.1 million for the tenth year of the lease. As of March 31, 2020, the Company has capitalized $10.6 million of leasehold improvements within property and equipment and amortizes it over the lease term. Supplemental balance sheet information related to operating leases was as follows (in thousands, except lease term and discount rate). March 31, 2020 December 31, 2019 Assets Operating lease right-of-use assets $ 51,981 $ 417 Liabilities Operating lease liabilities - current $ 7,851 $ 383 Operating lease liabilities - noncurrent 44,658 — $ 52,509 $ 383 Weighted average remaining lease term (years) 9.8 0.3 Weighted average discount rate 13.5 % 6.0 % Information related to operating lease activity during the three months ended March 31, 2020 and 2019 was as follows (in thousands): Three Months Ended March 31, 2020 2019 Operating lease cost $ 2,738 $ 669 Variable lease cost 532 — Total lease cost $ 3,270 $ 669 Operating lease right-of-use assets obtained in exchange for lease obligations $ 52,630 $ 1,095 Operating lease payments $ 2,174 $ 727 Future annual payments of operating lease liabilities as of March 31, 2020 are as follows (in thousands): Year ending December 31: Amount 2020 (nine months remaining) $ 6,185 2021 8,449 2022 8,745 2023 9,051 2024 9,368 Thereafter 52,442 Total future lease payments 94,240 Less: imputed interest (41,731 ) Total operating lease liabilities $ 52,509 The operating leases require the Company to share in prorated operating expenses and property taxes based upon actual amounts incurred; those amounts are not fixed for future periods and, therefore, are not included in the future commitments listed above.

Balance Sheet Components

Balance Sheet Components	3 Months Ended
	Mar. 31, 2020
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	6. Balance Sheet Components Property and Equipment Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets, ranging from two to five years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the related lease term. Upon retirement or sale, the cost and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statement of operations and comprehensive loss. Property and equipment consist of the following (in thousands): March 31, 2020 December 31, 2019 Scientific equipment $ 12,430 $ 10,642 Furniture and equipment 3,185 2,572 Capitalized software 389 389 Leasehold improvements 12,272 509 Construction in progress — 9,568 Total 28,276 23,680 Less: Accumulated depreciation (8,475 ) (7,937 ) Property and equipment, net $ 19,801 $ 15,743 Depreciation expense was $0.8 million and $0.5 million for the three months ended March 31, 2020 and 2019, respectively. Accrued Liabilities Accrued liabilities consist of the following (in thousands): March 31, 2020 December 31, 2019 Clinical research and development $ 12,946 $ 11,494 Outside services 8,164 6,592 Payroll-related liabilities 6,166 11,724 Construction in progress — 9,139 Other 4,960 2,343 Total accrued liabilities $ 32,236 $ 41,292

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Purchase Commitments The Company conducts product research and development programs through a combination of internal and collaborative programs that include, among others, arrangements with universities, contract research organizations and clinical research sites. The Company has contractual arrangements with these organizations; however, these contracts are generally cancelable on 30 days’ notice and the obligations under these contracts are largely based on services performed. Contingencies From time to time, the Company may have contingent liabilities that arise in the ordinary course of business activities. The Company accrues for such liabilities when it is probable that future expenditures will be made and such expenditures can be reasonably estimated. There were no contingent liabilities requiring accrual or disclosure as of March 31, 2020 or December 31, 2019. Guarantees and Indemnifications The Company enters into standard indemnification arrangements in the ordinary course of business. Pursuant to certain of these arrangements, the Company indemnifies, holds harmless and agrees to reimburse the indemnified parties for losses suffered or incurred by the indemnified party, in connection with any trade secret, copyright, patent or other intellectual property infringement claim by any third-party with respect to the Company’s technology. The term of these indemnification arrangements is generally perpetual. The maximum potential amount of future payments the Company could be required to make under these agreements is not determinable because it involves claims that may be made against the Company in the future but have not yet been made. The Company indemnifies its officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws and agreements providing for indemnification entered into with its officers and directors. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity. The maximum amount of potential future indemnification of directors and officers is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with its exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.

Stockholders' Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2020
Stockholders Equity Note [Abstract]
Stockholders' Equity	8. Stockholders’ Equity On January 3, 2020, the Company entered into a sales agreement with a sales agent to establish an at-the-market (ATM) offering program, under which the Company is permitted to offer and sell, from time to time, shares of common stock having a maximum aggregate offering price of up to $200 million. As of March 31, 2020, no securities have been issued pursuant to the ATM agreement. In March 2019, the Company completed a follow-on offering and issued 5,663,750 shares of common stock at a price of $51.00 per share, which included 738,750 shares sold directly to the underwriters upon exercise of their over-allotment option. During the three months ended March 31, 2019, the Company received proceeds totaling approximately $271.2 million from the offering, net of underwriting discounts and commissions and offering expenses. Common Stock Reserved for Issuance The Company has reserved shares of common stock for issuance as follows: March 31, 2020 December 31, 2019 Options and awards issued and outstanding 6,181,912 5,315,254 Shares available for issuance under 2015 Stock Option and Incentive Plan 1,440,826 685,435 Shares available for issuance under 2015 Employee Stock Purchase Plan 1,604,331 1,140,541 Total 9,227,069 7,141,230

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation The Company classifies stock-based compensation expense in the accompanying condensed consolidated statements of operations and comprehensive loss based on the department to which a recipient belongs. The following table sets forth stock-based compensation expense related to equity awards granted to employees and consultants for all periods presented (in thousands): Three 2020 2019 Research and development $ 5,192 $ 2,963 Selling, general and administrative 5,560 4,018 Total $ 10,752 $ 6,981 The following summarizes option and other equity award activity under the 2012 Equity Incentive Plan and 2015 Stock Option and Incentive Plan: Shares Subject to Weighted Average Outstanding Options Exercise Price per Share Balance at December 31, 2019 4,574,158 $ 31.81 Options granted 801,937 70.08 Options exercised (107,870 ) 16.88 Options canceled/forfeited (27,486 ) 44.50 Balance at March 31, 2020 5,240,739 37.91 Shares Subject to Weighted Average Outstanding Awards Grant Date Fair Value Balance at December 31, 2019 741,096 $ 46.59 RSUs awarded 333,324 70.09 RSUs released (125,243 ) 43.19 RSUs forfeited (8,004 ) 47.68 Balance at March 31, 2020 941,173 55.36 Restricted stock units (“RSUs”) settle into shares of common stock upon vesting and the fair value is the market price on the date of grant. In relation to stock options and awards that vest upon the achievement of performance criteria, there was $0.3 million and zero in stock-based compensation expense recorded for the three months ended March 31, 2020 and 2019, respectively .

Net Loss per Share

Net Loss per Share	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Net Loss per Share	10. Net Loss per Share The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share amounts): Three Months Ended March 31, 2020 2019 Numerator Net loss $ (69,868 ) $ (37,470 ) Denominator Weighted average shares outstanding 46,566,995 40,512,983 Less: weighted average shares subject to repurchase — (6,670 ) Weighted average shares used to compute basic and diluted net loss per share 46,566,995 40,506,313 Net loss per share, basic and diluted $ (1.50 ) $ (0.93 ) The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive: As of March 31, 2020 2019 Common stock subject to repurchase — 5,003 Options and awards issued and outstanding 6,181,912 5,257,140 As of March 31, 2020, the Company has contributions from plan participants of $1.0 million under the 2015 ESPP, which if converted, would be equivalent to approximately 20,000 shares based on 85% of the stock price at the beginning of the offering period. As of March 31, 2019, the Company had contributions from plan participants of $0.7 million under the 2015 ESPP, which if converted, would have been equivalent to approximately 15,000 shares based on 85% of the stock price at the beginning of the offering period.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes Deferred tax assets and deferred tax liabilities are determined based on temporary differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some portion or all of the deferred tax assets will not be realized. The Company does not recognize a tax benefit for uncertain tax positions unless it is more likely than not that the position will be sustained upon examination by tax authorities, including resolution of any related appeals or litigation processes, based on the technical merits of the position. The tax benefit potentially recorded for these positions is measured at the largest amount of cumulative benefit that has greater than a 50 percent likelihood of being realized upon ultimate settlement. Deferred tax assets that do not meet these recognition criteria are not recorded and the Company recognizes a liability for uncertain tax positions that may result in tax payments. If such unrecognized tax benefits were realized and not subject to valuation allowances, the entire amount would impact the tax provision. The Coronavirus Aid, Relief, and Economic Security Act (the “CARES Act”) was enacted by the United States Congress on March 27, 2020. The Company has not participated in any provision of the CARES Act to date and has decided not to apply for a loan under the Paycheck Protection Program (“PPP”) component of this legislation. The CARES Act did not impact the Company’s provision for income taxes or consolidated financial statements for the three months ended March 31, 2020.

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements are unaudited, include the Company’s accounts and those of its wholly-owned subsidiaries MyoKardia Australia Pty Ltd and MyoKardia Netherlands B.V., and have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). The condensed consolidated balance sheet at December 31, 2019, has been derived from the audited consolidated financial statements as of that date but does not include all of the information and footnotes required by U.S. GAAP for complete financial statements. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full fiscal year or any interim period and should be read in conjunction with the audited financial statements for the year ended December 31, 2019 and the notes thereto, which are included in the Company’s Annual Report on Form 10-K as of and for the year ended December 31, 2019 . The significant accounting policies used in preparation of these condensed consolidated financial statements for the periods shown are consisten t with those discussed in notes to the consolidated financial statements in the Company’s 2019 Annual Report on Form 10-K and are updated below as necessary. The Company currently operates in one business segment, which is the identification, development and commercialization of therapies for the treatment of serious and neglected rare
Reconciliation of Cash, Cash Equivalents, and Restricted Cash as Reported in Consolidated Statements of Cash Flows	Reconciliation of Cash, Cash Equivalents, and Restricted Cash as Reported in Consolidated Statements of Cash Flows Cash as reported in the consolidated statements of cash flows includes the aggregate amounts of cash, cash equivalents and restricted cash as presented on the consolidated balance sheets. Restricted cash at March 31, 2020 and December 31, 2019 represents cash balances held as security in connection with the Company’s facility lease agreements. The following table provides a reconciliation of cash, cash equivalents and restricted cash within the consolidated balance sheets to the total shown in the consolidated statements of cash flows (in thousands) : March 31, 2020 December 31, 2019 Cash and cash equivalents $ 131,204 $ 101,436 Restricted cash included in prepaid expenses and other current assets 337 337 Restricted cash included in restricted cash and other 1,857 1,857 Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows $ 133,398 $ 103,630
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities and the reported amounts of revenues and expenses in the consolidated financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trials accrued liabilities, income tax valuation allowance and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In November 2018, the Financial Accounting Standards Board (FASB) issued ASU 2018-18 (Topic 808), Clarifying the Interaction Between Topic 808 and Topic 606 In August 2018, the FASB issued ASU 2018-13 (Topic 820), Fair Value Measurement In June 2016, the FASB issued ASU No. 2016-13 (Topic 326), Financial Instruments –Measurement of Credit Losses on Financial Instruments should be estimated when the fair value of the debt securities is below their associated amortized costs. The Company adopted this amendment in the first quarter of 2020 and the adoption did not have a material impact to the Company’s financial statements.
Fair Value Measurements	Fair Value Measurements Fair value accounting is applied for all financial assets and liabilities, including short-term and long-term investments and non-financial assets and liabilities that are recognized or disclosed at fair value in the consolidated financial statements on a recurring basis (at least annually). The carrying amount of the Company’s financial instruments, including accounts payable and accrued liabilities and other current liabilities approximate fair value due to their short-term maturities. Marketable securities are stated at their estimated fair values. The counterparties to the agreements relating to the Company’s investment securities consist of the U.S. Treasury, governmental agencies, various major corporations and financial institutions with high credit standing. The carrying amounts for financial instruments consisting of cash and cash equivalents, receivable from collaboration partner, accounts payable and accrued liabilities approximate fair value due to their short maturities. The accounting guidance for fair value provides a framework for measuring fair value, clarifies the definition of fair value and expands disclosures regarding fair value measurements. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The accounting guidance establishes a three-tiered hierarchy, which prioritizes the inputs used in the valuation methodologies in measuring fair value as follows: Level 1—Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2—Inputs other than quoted market prices included in Level 1 are either directly or indirectly observable for the asset or liability through correlation with market data at the measurement date and for the duration of the instrument’s anticipated life. Level 3—Inputs reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model.
Leases	Leases The Company determines if an arrangement is or contains a lease at inception. Operating lease right-of-use (ROU) assets and liabilities are presented separately on our consolidated balance sheets. The Company does not have any finance leases. Operating lease ROU assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term beginning at the commencement date. As the Company’s leases do not provide enough information to determine an implicit interest rate, the Company determines its incremental borrowing rate based on the rate of interest that the Company would have to pay to borrow on a collateralized basis over a similar term in a similar economic environment. The operating lease ROU assets also include any lease payments made and excludes lease incentives and initial direct costs incurred. Lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term.
Property and Equipment	Property and Equipment Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets, ranging from two to five years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the related lease term. Upon retirement or sale, the cost and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statement of operations and comprehensive loss.
Contingencies	Contingencies From time to time, the Company may have contingent liabilities that arise in the ordinary course of business activities. The Company accrues for such liabilities when it is probable that future expenditures will be made and such expenditures can be reasonably estimated. There were no contingent liabilities requiring accrual or disclosure as of March 31, 2020 or December 31, 2019.
Guarantees and Indemnifications	Guarantees and Indemnifications The Company enters into standard indemnification arrangements in the ordinary course of business. Pursuant to certain of these arrangements, the Company indemnifies, holds harmless and agrees to reimburse the indemnified parties for losses suffered or incurred by the indemnified party, in connection with any trade secret, copyright, patent or other intellectual property infringement claim by any third-party with respect to the Company’s technology. The term of these indemnification arrangements is generally perpetual. The maximum potential amount of future payments the Company could be required to make under these agreements is not determinable because it involves claims that may be made against the Company in the future but have not yet been made. The Company indemnifies its officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws and agreements providing for indemnification entered into with its officers and directors. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity. The maximum amount of potential future indemnification of directors and officers is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with its exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.
Income Taxes	Income Taxes Deferred tax assets and deferred tax liabilities are determined based on temporary differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some portion or all of the deferred tax assets will not be realized. The Company does not recognize a tax benefit for uncertain tax positions unless it is more likely than not that the position will be sustained upon examination by tax authorities, including resolution of any related appeals or litigation processes, based on the technical merits of the position. The tax benefit potentially recorded for these positions is measured at the largest amount of cumulative benefit that has greater than a 50 percent likelihood of being realized upon ultimate settlement. Deferred tax assets that do not meet these recognition criteria are not recorded and the Company recognizes a liability for uncertain tax positions that may result in tax payments. If such unrecognized tax benefits were realized and not subject to valuation allowances, the entire amount would impact the tax provision.
Stock-Based Compensation	Stock-Based Compensation The Company classifies stock-based compensation expense in the accompanying condensed consolidated statements of operations and comprehensive loss based on the department to which a recipient belongs. The following table sets forth stock-based compensation expense related to equity awards granted to employees and consultants for all periods presented (in thousands): In relation to stock options and awards that vest upon the achievement of performance criteria, there was $0.3 million and zero in stock-based compensation expense recorded for the three months ended March 31, 2020 and 2019, respectively .

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Summary of Cash Reported in Condensed Consolidated Statements of Cash Flows	Cash as reported in the consolidated statements of cash flows includes the aggregate amounts of cash, cash equivalents and restricted cash as presented on the consolidated balance sheets. Restricted cash at March 31, 2020 and December 31, 2019 represents cash balances held as security in connection with the Company’s facility lease agreements. The following table provides a reconciliation of cash, cash equivalents and restricted cash within the consolidated balance sheets to the total shown in the consolidated statements of cash flows (in thousands) : March 31, 2020 December 31, 2019 Cash and cash equivalents $ 131,204 $ 101,436 Restricted cash included in prepaid expenses and other current assets 337 337 Restricted cash included in restricted cash and other 1,857 1,857 Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows $ 133,398 $ 103,630

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Summary of Fair Value Assets Measured on Recurring Basis	The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): Fair Value Measurements at March 31, 2020 Total Level 1 Level 2 Level 3 Assets Money market funds $ 130,176 $ 130,176 $ — $ — U.S. government agency obligations 107,657 — 107,657 — Corporate securities 121,229 — 121,229 — Total $ 359,062 $ 130,176 $ 228,886 $ — Fair Value Measurements at December 31, 2019 Total Level 1 Level 2 Level 3 Assets Money market funds $ 100,441 $ 100,441 $ — $ — U.S. government agency obligations 134,055 — 134,055 — Corporate securities 194,789 — 194,789 — Total $ 429,285 $ 100,441 $ 328,844 $ —
Summary of Fair Value Measurement of Available-for-sale Securities	The following table is a summary of amortized cost, unrealized gain and loss, and fair value of the Company’s marketable securities by contractual maturities (in thousands): Fair Value Measurements at March 31, 2020 Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 130,153 $ 23 $ — $ 130,176 Short-term investments (due within one year) 218,129 827 (147 ) 218,809 Long-term investments (due between one and two years) 10,104 27 (54 ) 10,077 Total $ 358,386 $ 877 $ (201 ) $ 359,062 Fair Amortized Cost Unrealized Gain Unrealized Loss Fair Value Cash equivalents (due within 90 days) $ 100,440 $ 1 $ — $ 100,441 Short-term investments (due within one year) 314,181 523 (13 ) 314,691 Long-term investments (due between one and two years) 14,110 47 (4 ) 14,153 Total $ 428,731 $ 571 $ (17 ) $ 429,285

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2020
Lessee Disclosure [Abstract]
Supplemental Balance Sheet Information Related to Operating Leases	Supplemental balance sheet information related to operating leases was as follows (in thousands, except lease term and discount rate). March 31, 2020 December 31, 2019 Assets Operating lease right-of-use assets $ 51,981 $ 417 Liabilities Operating lease liabilities - current $ 7,851 $ 383 Operating lease liabilities - noncurrent 44,658 — $ 52,509 $ 383 Weighted average remaining lease term (years) 9.8 0.3 Weighted average discount rate 13.5 % 6.0 %
Schedule of Information Related to Operating Lease Activity	Information related to operating lease activity during the three months ended March 31, 2020 and 2019 was as follows (in thousands): Three Months Ended March 31, 2020 2019 Operating lease cost $ 2,738 $ 669 Variable lease cost 532 — Total lease cost $ 3,270 $ 669 Operating lease right-of-use assets obtained in exchange for lease obligations $ 52,630 $ 1,095 Operating lease payments $ 2,174 $ 727
Schedule of Future Annual Payment of Operating Lease Liabilities	Future annual payments of operating lease liabilities as of March 31, 2020 are as follows (in thousands): Year ending December 31: Amount 2020 (nine months remaining) $ 6,185 2021 8,449 2022 8,745 2023 9,051 2024 9,368 Thereafter 52,442 Total future lease payments 94,240 Less: imputed interest (41,731 ) Total operating lease liabilities $ 52,509

Balance Sheet Components (Table

Balance Sheet Components (Tables)	3 Months Ended
	Mar. 31, 2020
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment	Property and equipment consist of the following (in thousands): March 31, 2020 December 31, 2019 Scientific equipment $ 12,430 $ 10,642 Furniture and equipment 3,185 2,572 Capitalized software 389 389 Leasehold improvements 12,272 509 Construction in progress — 9,568 Total 28,276 23,680 Less: Accumulated depreciation (8,475 ) (7,937 ) Property and equipment, net $ 19,801 $ 15,743
Summary of Accrued Liabilities	Accrued liabilities consist of the following (in thousands): March 31, 2020 December 31, 2019 Clinical research and development $ 12,946 $ 11,494 Outside services 8,164 6,592 Payroll-related liabilities 6,166 11,724 Construction in progress — 9,139 Other 4,960 2,343 Total accrued liabilities $ 32,236 $ 41,292

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	3 Months Ended
	Mar. 31, 2020
Equity [Abstract]
Schedule of Common Stock Reserved for Issuance	The Company has reserved shares of common stock for issuance as follows: March 31, 2020 December 31, 2019 Options and awards issued and outstanding 6,181,912 5,315,254 Shares available for issuance under 2015 Stock Option and Incentive Plan 1,440,826 685,435 Shares available for issuance under 2015 Employee Stock Purchase Plan 1,604,331 1,140,541 Total 9,227,069 7,141,230

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense Related to Equity Awards Granted to Employees and Consultants	The following table sets forth stock-based compensation expense related to equity awards granted to employees and consultants for all periods presented (in thousands): Three 2020 2019 Research and development $ 5,192 $ 2,963 Selling, general and administrative 5,560 4,018 Total $ 10,752 $ 6,981
Summary of Stock Option and Other Equity Award Activity Under Plans	The following summarizes option and other equity award activity under the 2012 Equity Incentive Plan and 2015 Stock Option and Incentive Plan: Shares Subject to Weighted Average Outstanding Options Exercise Price per Share Balance at December 31, 2019 4,574,158 $ 31.81 Options granted 801,937 70.08 Options exercised (107,870 ) 16.88 Options canceled/forfeited (27,486 ) 44.50 Balance at March 31, 2020 5,240,739 37.91 Shares Subject to Weighted Average Outstanding Awards Grant Date Fair Value Balance at December 31, 2019 741,096 $ 46.59 RSUs awarded 333,324 70.09 RSUs released (125,243 ) 43.19 RSUs forfeited (8,004 ) 47.68 Balance at March 31, 2020 941,173 55.36

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss per Share	The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share amounts): Three Months Ended March 31, 2020 2019 Numerator Net loss $ (69,868 ) $ (37,470 ) Denominator Weighted average shares outstanding 46,566,995 40,512,983 Less: weighted average shares subject to repurchase — (6,670 ) Weighted average shares used to compute basic and diluted net loss per share 46,566,995 40,506,313 Net loss per share, basic and diluted $ (1.50 ) $ (0.93 )
Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss per Share	The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive: As of March 31, 2020 2019 Common stock subject to repurchase — 5,003 Options and awards issued and outstanding 6,181,912 5,257,140

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	Jun. 8, 2012
Accumulated deficit	$ 548,634	$ 478,766
Cash, cash equivalents and short and long-term investments	$ 360,100

Basis of Presentation and Sum_4

Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended
	Mar. 31, 2020Segment
Basis Of Presentation And Summary Of Significant Accounting Policies [Abstract]
Number of reportable and operating segment	1

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Summary of Cash Reported in Condensed Consolidated Statements of Cash Flows (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash and cash equivalents	$ 131,204	$ 101,436
Restricted cash included in prepaid expenses and other current assets	337	337
Restricted cash included in restricted cash and other	1,857	1,857
Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows	$ 133,398	$ 103,630	$ 468,474	$ 248,265

Sanofi License and Collaborat_2

Sanofi License and Collaboration Agreement - Additional Information (Details) $ in Millions	Jul. 31, 2019USD ($)
Termination Agreement | Royalty Rights | Sanofi (Aventis Inc.)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount Payable to reacquire research and development asset	$ 80

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Fair Value Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets
Assets fair value	$ 359,062	$ 429,285
Money market funds
Assets
Assets fair value	130,176	100,441
Fair Value Measurements on Recurring Basis | Level 1
Assets
Assets fair value	130,176	100,441
Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	228,886	328,844
Fair Value Measurements on Recurring Basis | Money market funds | Level 1
Assets
Assets fair value	130,176	100,441
U.S. government agency obligations
Assets
Assets fair value	107,657	134,055
U.S. government agency obligations | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	107,657	134,055
Corporate securities
Assets
Assets fair value	121,229	194,789
Corporate securities | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	$ 121,229	$ 194,789

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Fair Value Measurement of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 358,386	$ 428,731
Unrealized Gain	877	571
Unrealized Loss	(201)	(17)
Fair Value	359,062	429,285
Cash equivalents (due within 90 days)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	130,153	100,440
Unrealized Gain	23	1
Fair Value	130,176	100,441
Short-term investments (due within one year)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	218,129	314,181
Unrealized Gain	827	523
Unrealized Loss	(147)	(13)
Fair Value	218,809	314,691
Long-term investments (due between one and two years)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	10,104	14,110
Unrealized Gain	27	47
Unrealized Loss	(54)	(4)
Fair Value	$ 10,077	$ 14,153

Leases - Additional Information

Leases - Additional Information (Details) $ in Thousands	1 Months Ended
	Jan. 31, 2020USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2018ft²
Lessee Lease Description [Line Items]
Operating lease right-of-use assets		$ 51,981	$ 417
Annual base rent, First year		5,500
Annual base rent, Second year		8,500
Annual base rent, Thereafter		11,100
Leasehold improvements within property and equipment		19,801	$ 15,743
Leasehold Improvements
Lessee Lease Description [Line Items]
Leasehold improvements within property and equipment		$ 10,600
New Facility
Lessee Lease Description [Line Items]
Area of leased property | ft²				129,800
Rent commencement date	2020-01
Additional period of extension in lease contract				10 years
Operating lease right-of-use assets	$ 52,600

Leases - Supplemental Balance S

Leases - Supplemental Balance Sheet Information Related to Operating Leases (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Operating Leases [Abstract]
Operating lease right-of-use assets	$ 51,981	$ 417
Operating lease liabilities - current	7,851	383
Operating lease liabilities - noncurrent	44,658
Operating lease, liability	$ 52,509	$ 383
Weighted average remaining lease term (years)	9 years 9 months 18 days	3 months 18 days
Weighted average discount rate	13.50%	6.00%

Leases - Schedule of Informatio

Leases - Schedule of Information Related to Operating Lease Activity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Lease Cost [Abstract]
Operating lease cost	$ 2,738	$ 669
Variable lease cost	532
Total lease cost	3,270	669
Operating lease right-of-use assets obtained in exchange for lease obligations	52,630	1,095
Operating lease payments	$ 2,174	$ 727

Leases - Future Annual Payments

Leases - Future Annual Payments of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Operating Lease Liabilities Payments Due [Abstract]
2020 (nine months remaining)	$ 6,185
2021	8,449
2022	8,745
2023	9,051
2024	9,368
Thereafter	52,442
Total future lease payments	94,240
Less: imputed interest	(41,731)
Total operating lease liabilities	$ 52,509	$ 383

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Property Plant And Equipment [Line Items]
Property plant and equipment basis of valuation	cost
Property plant and equipment depreciation methods	the straight-line method over the estimated useful lives of the assets
Depreciation expense	$ 779	$ 461
Minimum
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	2 years
Maximum
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	5 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property plant and equipment estimated useful lives	shorter of their estimated useful lives or the related lease term

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment	$ 28,276	$ 23,680
Less: Accumulated depreciation	(8,475)	(7,937)
Property and equipment, net	19,801	15,743
Scientific Equipment
Property Plant And Equipment [Line Items]
Property and equipment	12,430	10,642
Furniture and Equipment
Property Plant And Equipment [Line Items]
Property and equipment	3,185	2,572
Capitalized Software
Property Plant And Equipment [Line Items]
Property and equipment	389	389
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	12,272	509
Property and equipment, net	$ 10,600
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment		$ 9,568

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Clinical research and development	$ 12,946	$ 11,494
Outside services	8,164	6,592
Payroll-related liabilities	6,166	11,724
Construction in progress		9,139
Other	4,960	2,343
Total accrued liabilities	$ 32,236	$ 41,292

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Purchase commitment cancellation notice period	30 days
Contingent liability for accrual	$ 0	$ 0

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Jan. 03, 2020
Class Of Stock [Line Items]
Proceeds from offering, net of underwriting discounts and commissions and offering expenses		$ 271,485
Follow-On-Offering
Class Of Stock [Line Items]
Common stock shares issued	5,663,750
Common stock shares issued, price per share	$ 51	$ 51
Proceeds from offering, net of underwriting discounts and commissions and offering expenses		$ 271,200
Over Allotment Option
Class Of Stock [Line Items]
Common stock shares issued	738,750
Maximum
Class Of Stock [Line Items]
Market offering aggregate offering price			$ 200,000

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Issuance (Details) - shares	Mar. 31, 2020	Dec. 31, 2019
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	9,227,069	7,141,230
Options and Awards Issued and Outstanding
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	6,181,912	5,315,254
2015 Stock Option and Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	1,440,826	685,435
2015 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance, shares	1,604,331	1,140,541

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense Related to Equity Awards Granted to Employees and Consultants (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 10,752	$ 6,981
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	5,192	2,963
Selling, general and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 5,560	$ 4,018

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option and Other Equity Award Activity Under Plans (Details)	3 Months Ended
	Mar. 31, 2020$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Subject to Outstanding Options, Beginning Balance | shares	4,574,158
Shares Subject to Outstanding Options, granted | shares	801,937
Shares Subject to Outstanding Options, exercised | shares	(107,870)
Shares Subject to Outstanding Options, canceled/forfeited | shares	(27,486)
Shares Subject to Outstanding Options, Ending Balance | shares	5,240,739
Weighted Average Exercise Price per Share, Beginning Balance | $ / shares	$ 31.81
Weighted Average Exercise Price Per Share, granted | $ / shares	70.08
Weighted Average Exercise Price per Share, exercised | $ / shares	16.88
Weighted Average Exercise Price Per Share, canceled/forfeited | $ / shares	44.50
Weighted Average Exercise Price Per Share, Ending Balance | $ / shares	$ 37.91
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Subject to Outstanding Awards, Beginning Balance | shares	741,096
Shares Subject to Outstanding Awards, RSUs awarded | shares	333,324
Shares Subject to Outstanding Awards, RSUs released | shares	(125,243)
Shares Subject to Outstanding Awards, RSUs forfeited | shares	(8,004)
Shares Subject to Outstanding Awards, Ending Balance | shares	941,173
Weighted Average Grant Date Fair Value. Beginning Balance | $ / shares	$ 46.59
Weighted Average Grant Date Fair Value. RSUs awarded | $ / shares	70.09
Weighted Average Grant Date Fair Value. RSUs released | $ / shares	43.19
Weighted Average Grant Date Fair Value. RSUs forfeited | $ / shares	47.68
Weighted Average Grant Date Fair Value. Ending Balance | $ / shares	$ 55.36

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	Mar. 31, 2020	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Unrecognized share based compensation expense	$ 300,000	$ 0

Net Loss per Share - Computatio

Net Loss per Share - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator
Net loss	$ (69,868)	$ (37,470)
Denominator
Weighted average shares outstanding	46,566,995	40,512,983
Less: weighted average shares subject to repurchase		(6,670)
Weighted average shares used to compute basic and diluted net loss per share	46,566,995	40,506,313
Net loss per share, basic and diluted	$ (1.50)	$ (0.93)

Net Loss per Share - Summary of

Net Loss per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss per Share (Details) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Common stock subject to repurchase
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share		5,003
Options and Awards Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	6,181,912	5,257,140

Net Loss per Share - Additional

Net Loss per Share - Additional Information (Details) - 2015 Employee Stock Purchase Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Line Items]
ESPP contributions from plan participants	$ 1	$ 0.7
Contributions converted shares under benefit plan	20,000	15,000
Percentage of stock price at the beginning of offering period	85.00%	85.00%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details)	3 Months Ended
	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Minimum percentage of likelihood of realization of tax benefits	50.00%