Epizyme (EPZM) 10-Q 9 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Jul. 31, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	EPZM
Entity Registrant Name	Epizyme, Inc.
Entity Central Index Key	0001571498
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Current Reporting Status	Yes
Entity Common Stock Shares Outstanding		91,036,622
Entity Shell Company	false
Entity File Number	001-35945
Entity Tax Identification Number	261349956
Entity Address, Address Line One	400 Technology Square
Entity Address, City or Town	Cambridge
Entity Address, State or Province	Massachusetts
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	229-5872

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 45,932	$ 86,671
Marketable securities	285,048	153,633
Accounts receivable	13,273	20,067
Prepaid expenses and other current assets	14,106	12,164
Total current assets	358,359	272,535
Property and equipment, net	1,992	2,057
Operating lease assets	10,029
Restricted cash and other assets	1,032	909
Total assets	371,412	275,501
Current liabilities:
Accounts payable	7,069	4,780
Accrued expenses	17,713	19,700
Current portion of operating lease obligation	2,876
Current portion of deferred revenue	6,259	13,300
Other current liabilities	16	53
Total current liabilities	33,933	37,833
Operating lease obligation, net of current portion	7,950
Deferred revenue, net of current portion	3,806	3,806
Other long-term liabilities	38	853
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.0001 par value; 5,000 shares authorized; 350 shares issued and outstanding (equivalent to 3,500 shares of common stock upon conversion at a 10:1 ratio)	37,432
Common stock, $0.0001 par value; 125,000 shares authorized; 91,016 shares and 79,175 shares issued and outstanding, respectively	9	8
Additional paid-in capital	952,524	819,779
Accumulated other comprehensive income (loss)	248	(54)
Accumulated deficit	(664,528)	(586,724)
Total stockholders’ equity	325,685	233,009
Total liabilities and stockholders’ equity	$ 371,412	$ 275,501

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
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	350,000
Preferred stock, shares outstanding	350,000
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	91,016,000	79,175,000
Common stock, shares outstanding	91,016,000	79,175,000
Common stock conversion ratio	10.00%
Common stock upon conversion	3,500,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Collaboration revenue	$ 5,900	$ 12,000	$ 13,791	$ 12,000
Type of Revenue [Extensible List]	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember
Operating expenses:
Research and development	$ 40,907	$ 31,346	$ 67,803	$ 56,968
General and administrative	15,698	10,914	27,684	20,274
Total operating expenses	56,605	42,260	95,487	77,242
Operating loss	(50,705)	(30,260)	(81,696)	(65,242)
Other income, net:
Interest income, net	2,253	1,143	3,911	2,042
Other (expense) income, net	(13)	(11)	(19)	7
Other income, net	2,240	1,132	3,892	2,049
Net loss	(48,465)	(29,128)	(77,804)	(63,193)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities	216	46	302	23
Comprehensive loss	(48,249)	(29,082)	(77,502)	(63,170)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(48,465)	(29,128)	(77,804)	(63,193)
Accretion of convertible preferred stock			(2,940)
Net loss attributable to common stockholders	$ (48,465)	$ (29,128)	$ (80,744)	$ (63,193)
Net loss per share attributable to common stockholders - basic and diluted	$ (0.53)	$ (0.42)	$ (1.04)	$ (0.91)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders - basic and diluted	90,876	69,490	77,315	69,438

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (77,804)	$ (63,193)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	416	582
Stock-based compensation	7,939	6,311
Amortization of discount on investments	(1,759)	(565)
Changes in operating assets and liabilities:
Accounts receivable	6,794	382
Contract asset		(12,000)
Prepaid expenses and other current assets	(1,942)	469
Accounts payable	2,092	3,988
Accrued expenses	(1,972)	619
Deferred revenue	(7,041)
Operating lease assets	1,255
Operating lease liabilities	(1,290)
Other assets	(123)	(18)
Other liabilities	(18)	196
Net cash used in operating activities	(73,453)	(63,229)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of available-for-sale securities	(297,904)	(165,688)
Maturities of available-for-sale securities	168,548	68,334
Purchases of property and equipment	(190)	(109)
Net cash used in investing activities	(129,546)	(97,463)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payments under capital lease obligation		(129)
Proceeds from issuance of common stock, net of commissions	122,992
Proceeds from issuance of preferred stock, net of commissions	37,433
Proceeds from stock options exercised	1,739	1,604
Proceeds from issuance of shares under employee stock purchase plan	360	459
Payment of public offering costs	(264)
Net cash provided by financing activities	162,260	1,934
Net increase (decrease) in cash, cash equivalents and restricted cash	(40,739)	(158,758)
Cash, cash equivalents and restricted cash, beginning of period	87,133	227,126
Cash, cash equivalents and restricted cash, end of period	46,394	68,368
SUPPLEMENTAL CASH FLOW INFORMATION:
Cash paid for income taxes	45	38
Purchases of property and equipment unpaid at period end	$ 208	$ 88

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Statement Of Stockholders Equity [Abstract]
Issuance of common stock, commissions and offering costs	$ 284

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock [Member]	Common Stock [Member]	Preferred Stock [Member]	Preferred Stock [Member]Series A Convertible Preferred Stock [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member]Series A Convertible Preferred Stock [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance, Value at Dec. 31, 2017	$ 235,371		$ 7			$ 723,510		$ (488,097)	$ (49)
Beginning Balance, Shares at Dec. 31, 2017			69,301,691
Cumulative catch up related to the adoption of ASU | ASU 2016-09 [Member]	25,003							25,003
Exercise of stock options and vesting of restricted stock units, Value	1,500					1,500
Exercise of stock options and vesting of restricted stock units, Shares			152,734
Stock-based compensation	2,887					2,887
Issuance of shares under employee stock purchase plan, Value	459					459
Issuance of shares under employee stock purchase plan, Shares			31,116
Unrealized gain on available for sale securities	(23)								(23)
Net loss	(34,065)							(34,065)
Ending Balance, Value at Mar. 31, 2018	231,132		$ 7			728,356		(497,159)	(72)
Ending Balance, Shares at Mar. 31, 2018			69,485,541
Beginning Balance, Value at Dec. 31, 2017	235,371		$ 7			723,510		(488,097)	(49)
Beginning Balance, Shares at Dec. 31, 2017			69,301,691
Unrealized gain on available for sale securities	23
Net loss	(63,193)
Ending Balance, Value at Jun. 30, 2018	205,578		$ 7			731,884		(526,287)	(26)
Ending Balance, Shares at Jun. 30, 2018			69,495,001
Beginning Balance, Value at Mar. 31, 2018	231,132		$ 7			728,356		(497,159)	(72)
Beginning Balance, Shares at Mar. 31, 2018			69,485,541
Exercise of stock options and vesting of restricted stock units, Value	105					105
Exercise of stock options and vesting of restricted stock units, Shares			9,460
Stock-based compensation	3,423					3,423
Unrealized gain on available for sale securities	46								46
Net loss	(29,128)							(29,128)
Ending Balance, Value at Jun. 30, 2018	205,578		$ 7			731,884		(526,287)	(26)
Ending Balance, Shares at Jun. 30, 2018			69,495,001
Beginning Balance, Value at Dec. 31, 2018	233,009		$ 8			819,779		(586,724)	(54)
Beginning Balance, Shares at Dec. 31, 2018			79,175,380
Issuance of common stock (net of commissions and offering costs), Value	122,708		$ 1			122,707
Issuance of common stock (net of commissions and offering costs), Shares			11,500,000
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Value		$ 37,432			$ 34,492		$ 2,940
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares					350,000
Accretion of Series A Convertible Preferred Stock					$ 2,940		$ (2,940)
Exercise of stock options and vesting of restricted stock units, Value	886					886
Exercise of stock options and vesting of restricted stock units, Shares			89,726
Stock-based compensation	3,211					3,211
Issuance of shares under employee stock purchase plan, Value	360					360
Issuance of shares under employee stock purchase plan, Shares			37,972
Unrealized gain on available for sale securities	86								86
Net loss	(29,339)							(29,339)
Ending Balance, Value at Mar. 31, 2019	368,353		$ 9	$ 37,432		946,943		(616,063)	32
Ending Balance, Shares at Mar. 31, 2019			90,803,078	350,000
Beginning Balance, Value at Dec. 31, 2018	233,009		$ 8			819,779		(586,724)	(54)
Beginning Balance, Shares at Dec. 31, 2018			79,175,380
Accretion of Series A Convertible Preferred Stock	2,940
Unrealized gain on available for sale securities	302
Net loss	(77,804)
Ending Balance, Value at Jun. 30, 2019	325,685		$ 9	$ 37,432		952,524		(664,528)	248
Ending Balance, Shares at Jun. 30, 2019			91,015,639	350,000
Beginning Balance, Value at Mar. 31, 2019	368,353		$ 9	$ 37,432		946,943		(616,063)	32
Beginning Balance, Shares at Mar. 31, 2019			90,803,078	350,000
Exercise of stock options and vesting of restricted stock units, Value	853					853
Exercise of stock options and vesting of restricted stock units, Shares			208,749
Stock-based compensation	4,681					4,681
Issuance of shares of common stock in lieu of board fees	47					47
Issuance of shares of common stock in lieu of board fees, Shares			3,812
Unrealized gain on available for sale securities	216								216
Net loss	(48,465)							(48,465)
Ending Balance, Value at Jun. 30, 2019	$ 325,685		$ 9	$ 37,432		$ 952,524		$ (664,528)	$ 248
Ending Balance, Shares at Jun. 30, 2019			91,015,639	350,000

Overview

Overview	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Overview	1. Overview Epizyme, Inc. (collectively referred to with its wholly owned, controlled subsidiary, Epizyme Securities Corporation, as “Epizyme” or the “Company”) is a late-stage biopharmaceutical company that is committed to rewriting treatment for cancer and other serious diseases through the discovery, development, and commercialization of novel epigenetic medicines. By focusing on the genetic drivers of disease, the Company’s science seeks to match targeted medicines with the patients who need them. The Company is developing its lead product candidate, tazemetostat, an oral, first-in-class selective inhibitor of the EZH2 histone methyltransferase, or HMT, in a broad range of cancer types and settings, and developing its novel G9a program, Through June 30, 2019, the Company has raised, including amounts received under collaboration agreements, an aggregate of $1,155.1 million to fund its operations, of which $239.6 million was non-equity funding through its collaboration agreements, $839.5 million was from the sale of common stock and series A Convertible Preferred Stock in the Company’s public offerings and $76.0 million was from the sale of redeemable convertible preferred stock in private financings prior to the Company’s initial public offering in May 2013. As of June 30, 2019, the Company had $331.0 million in cash, cash equivalents and marketable securities. The Company commenced active operations in early 2008. Since its inception, the Company has generated an accumulated deficit of $664.5 million through June 30, 2019, and will require substantial additional capital to fund its research and development and commercialization plans for tazemetostat, if approved. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, risks of failure of clinical trials and preclinical studies, the need to obtain additional financing to fund the future development and commercialization of tazemetostat and the rest of its pipeline, the need to obtain marketing approval for its product candidates, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development by competitors of technological innovations and ability to transition from clinical-stage manufacturing to commercial-stage production of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The condensed consolidated financial statements of the Company included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission, or the SEC. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018, or the Annual Report. The unaudited condensed consolidated financial statements include the accounts of Epizyme, Inc. and its wholly owned, controlled subsidiary, Epizyme Securities Corporation. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation. In the opinion of management, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair presentation of the results for the reported interim periods. The Company considers events or transactions that occur after the balance sheet date but before the condensed consolidated financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The three months ended June 30, 2019 and 2018 are referred to as the second quarter of 2019 and 2018, respectively. The results of operations for interim periods are not necessarily indicative of results to be expected for the full year or any other interim period. Certain reclassifications have been made to prior periods to conform to current period presentation. Reclassification of prior year amounts have been made to present capital lease liability in other current and other long-term liabilities in the consolidated Balance Sheets. There was no impact on total operating expenses or net income (loss) resulting from these reclassifications. Significant Accounting Policies During the quarter ended March 31, 2019, the Company adopted Accounting Standards Codification, Topic 842, Leases using the required Recently Adopted Accounting Pronouncements Summary of Significant Accounting Policies Going Concern At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to the current cash, cash equivalent and marketable securities balances. After considering the Company’s current research and development plans, the building of commercial infrastructure and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of June 30, 2019, the Company did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued. Pending Accounting Pronouncements In June 2016, the Financial Accounting Standards Board, or FASB, issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements, or ASC 808, Recently Adopted Accounting Pronouncements Leases In February 2016, the FASB issued ASU 2016-02, Leases, or ASC 842, which requires lessees to recognize a right-of-use asset and lease liability for most lease arrangements. The new standard is effective for annual reporting periods beginning after December 15, 2018. A modified retrospective transition approach is required to be applied to leases existing as of, or entered into after, the beginning of the earliest comparative period presented in the financial statements, with certain practical expedients available. Effective January 1, 2019, the Company adopted ASC 842 using the required modified retrospective approach and utilizing the effective date as its date of initial application, for which prior periods are presented in accordance with the previous guidance in ASC 840, Leases, or ASC 840. At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present in the arrangement. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets and short-term and long-term lease liabilities, as applicable. The Company has elected not to recognize on the balance sheet leases with terms of 12 months or less. The Company typically only includes an initial lease term in its assessment of a lease arrangement. Options to renew a lease are not included in the Company’s assessment unless there is reasonable certainty that the Company will renew. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate, which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. In transition to ASC 842, the Company utilized the remaining lease term of its leases in determining the appropriate incremental borrowing rates. In accordance with ASC 842, components of a lease should be split into three categories: lease components (e.g., land, building, etc.), non-lease components (e.g., common area maintenance, consumables, etc.), and non-components (e.g., property taxes, insurance, etc.). The fixed and in-substance fixed contract consideration (including any consideration related to non-components) must be allocated based on the respective relative fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, certain expedients are available. Entities may elect the practical expedient to not separate lease and non-lease components by class of underlying asset. Rather, entities would account for each lease component and the related non-lease component together as a single component. In adopting ASC 842, the Company elected to utilize the available package of practical expedients permitted under the transition guidance within the new standard, which does not require the reassessment of the following: i) whether existing or expired arrangements are or contain a lease, ii) the lease classification of existing or expired leases, and iii) whether previous initial direct costs would qualify for capitalization under the new lease standard. The adoption of this standard resulted in the recognition of operating lease liabilities and right-of-use assets of $11.5 million and $10.7 million, respectively, on the Company’s condensed consolidated balance sheet relating to its leases for its corporate headquarters in Cambridge, Massachusetts and other operating leases. The adoption of the standard did not have a material effect on the Company’s condensed consolidated statements of operation and comprehensive loss or condensed consolidated statements of cash flows. As of June 30, 2019, the Company recognized operating lease liabilities and right-of-use assets under the new guidance of $10.8 million and $10.0 million, respectively, on the Company’s balance sheet. Compensation In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation, or ASC 718: Improvements to Nonemployee Share-Based Payment Accounting,

Cash

Cash	6 Months Ended
	Jun. 30, 2019
Cash And Cash Equivalents [Abstract]
Cash	3. Cash A reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same such amounts shown in the condensed consolidated statements of cash flows, is as follows: As of June 30, 2019 2018 (In thousands) Cash and cash equivalents $ 45,932 $ 67,906 Restricted cash, as part of other assets 462 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 46,394 $ 68,368 The $0.5 million in restricted cash relates to a letter of credit as a security deposit for the office and laboratory lease at Technology Square in Cambridge, Massachusetts. The Company has recorded cash held to secure this letter of credit as restricted cash in restricted cash and other assets on the condensed consolidated balance sheet .

Marketable Securities

Marketable Securities	6 Months Ended
	Jun. 30, 2019
Investments Debt And Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities The following table summarizes the available-for-sale securities held at June 30, 2019 (in thousands): Description Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial paper $ 138,975 $ 109 $ — $ 139,084 Corporate notes 143,825 139 — 143,964 U.S. government agency securities and U.S. treasuries 2,000 — — 2,000 Total $ 284,800 $ 248 $ — $ 285,048 The following table summarizes the available-for-sale secu rities held at December 31, 201 8 (in thousands): Description Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial paper $ 73,110 $ — $ (22 ) $ 73,088 Corporate notes 80,575 — (30 ) 80,545 Total $ 153,685 $ — $ (52 ) $ 153,633 The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At June 30, 2019, the balance in the Company’s accumulated other comprehensive loss was composed solely of activity related to the Company’s available-for-sale marketable securities. There were no realized gains or losses recognized on the sale or maturity of available-for-sale securities during the three months ended June 30, 2019, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same period. The aggregate fair value of available-for-sale securities held by the Company in an unrealized loss position for less than twelve months as of June 30, 2019 was $9.4 million, which consisted of 1 commercial paper security and 2 corporate notes securities. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of June 30, 2019 was less than $0.1 million. The Company does not intend to sell and it is unlikely that the Company will be required to sell the above investments before recovery of their amortized cost bases, which may be maturity. The Company determined that there was no material change in the credit risk of any of its investments. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of June 30, 2019. The weighted-average maturity of the Company’s portfolio was approximately seven months at June 30, 2019.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	5. Fair Value Measurements The Company’s financial instruments as of June 30, 2019 and December 31, 2018 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of June 30, 2019 and December 31, 2018, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of June 30, 2019 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 37,071 $ 37,071 — $ — Marketable securities: Commercial paper 139,084 — 139,084 — Corporate notes 143,964 — 143,964 — U.S. government agency securities and U.S. treasuries 2,000 — 2,000 Total $ 322,119 $ 37,071 $ 285,048 $ — Fair Value as of December 31, 2018 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 79,225 $ 50,785 $ 28,440 $ — Marketable securities: Commercial paper 73,088 — 73,088 — Corporate notes 80,545 — 80,545 — Total $ 232,858 $ 50,785 $ 182,073 $ — Cash equivalents and marketable securities have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, utilizing third-party pricing services or other market observable data. The Company measures its cash equivalents at fair value on a recurring basis. The Company classifies some of its cash equivalents within Level 1 of the fair value hierarchy because they are valued using observable inputs that reflect quoted prices for identical assets in active markets. The Company measures its marketable securities at fair value on a recurring basis and classifies those instruments and some cash equivalents within Level 2 of the fair value hierarchy. The pricing services used by management utilize industry standard valuation models, including both income and market based approaches and observable market inputs to determine the fair value o f marketable securities and those cash equivalents classified within Level 2 of the fair value hierarchy .

Supplemental Balance Sheet Info

Supplemental Balance Sheet Information	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Supplemental Balance Sheet Information	6. Supplemental Balance Sheet Information Accrued expenses consisted of the following: June 30, 2019 December 31, 2018 (In thousands) Employee compensation and benefits $ 3,871 $ 5,509 Research and development expenses 9,801 11,272 Professional services and other 4,041 2,919 Accrued expenses $ 17,713 $ 19,700

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	7. Income Taxes The Company did not record a federal or state income tax provision or benefit for the three months ended June 30, 2019 and 2018 due to the expected and known loss before income taxes to be incurred, or incurred, as applicable, for the years ended December 31, 2019 and 2018, as well as the Company’s continued maintenance of a full valuation allowance against its net deferred tax assets, with the exception of the deferred tax asset related to alternative minimum tax credit.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies There have been no significant changes to the Company’s commitments and contingencies in the three and six months ended June 30, 2019, as compared to those disclosed in Note 7, Commitments and Contingencies

Leases

Leases	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Leases	9. Leases The Company enters into lease arrangements for its facilities as well as certain equipment. A summary of the arrangements are as follows: Operating Leases The Company leases office and laboratory space at Technology Square in Cambridge, Massachusetts under a Lease Agreement, dated as of June 15, 2012, as amended, or the Lease, with ARE-TECH Square, LLC, a Delaware limited liability company, or Landlord, with a term that originally continued through May 31, 2018, and a Company option to extend the term of the lease at the then-current market rent, as defined in the Lease, through November 30, 2022. In May 2017, the Company entered into a Third Amendment to Lease, or the Third Amendment, with the Landlord, and a Fourth Amendment to Lease with the Landlord, or the Fourth Amendment, and, together with the Third Amendment, the Amendments. Under the Amendments, the Company extended the term of the lease to November 30, 2022 but retained the right to terminate the Lease, effective as of December 31, 2018, by giving written notice to the Landlord by December 31, 2017 and paying an early termination fee. The Company did not exercise this right. Under the Lease as amended, the Company has agreed to pay a monthly base rent of approximately $0.2 million for the period commencing December 1, 2017 through May 31, 2018, with an increase on June 1, 2018 of approximately $33,000 and annual increases of approximately $9,000 on December 1 of each subsequent year until December 1, 2021. The Company has a $0.5 million letter of credit as a security deposit for this lease and has recorded cash held to secure this letter of credit as restricted cash and other assets on the consolidated balance sheet. In applying the ASC 842 transition guidance, the Company determined the classification of this lease to be operating and recorded a lease liability and a right-of-use asset on the ASC 842 effective date. In addition, the Company has a capital lease related to computer hardware equipment, an operating lease for storage space in Colorado and an operating lease for office space in North Carolina. In applying the ASC 842 transition guidance, the Company determined the classification of the storage space and office space as operating leases and recorded lease liabilities and right-of-use assets on the ASC 842 effective date. The Company is required to pay certain variable costs to the L andlord in addition to fixed rent. These costs include common area maintenance, real estate taxes, and parking. The following table contains a summary of the lease costs recognized under Topic 842 and other information pertaining to the Company’s operating leases for the three and six months ended June 30, 2019: Three Months Ended June 30, Six Months Ended June 30, 2019 2019 (In thousands) Lease cost Operating lease cost $ 889 $ 1,777 Variable lease cost 305 628 Total lease cost $ 1,194 $ 2,405 Other information Operating cash flows used for operating leases $ 907 $ 1,812 Weighted average remaining lease term 3.4 years 3.4 years Weighted average discount rate 8.55 % 8.55 % Future minimum lease payments under the Company’s non-cancelable operating leases as of June 30, 2019, are as follows: 2019 (In thousands) 2019 $ 1,825 2020 3,729 2021 3,622 2022 3,340 Thereafter — Total lease payments $ 12,516 Less: imputed interest (1,690 ) Total operating lease liabilities at June 30, 2019 $ 10,826

Collaborations

Collaborations	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	10. Collaborations Celgene In April 2012, the Company entered into a collaboration and license agreement with Celgene Corporation, or Celgene. On July 8, 2015, the Company entered into an amendment and restatement of the collaboration and license agreement with Celgene. Original Agreement Structure Under the original agreement, the Company granted Celgene an exclusive license, for all countries other than the United States, to small molecule HMT inhibitors targeting the DOT1L HMT, including pinometostat, and an option, on a target-by-target basis, to exclusively license, for all countries other than the United States, rights to small molecule HMT inhibitors targeting any HMT targets, other than the EZH2 HMT, including tazemetostat, and targets covered by the Company’s collaboration and license agreement dated January 8, 2011 with GlaxoSmithKline, or GSK. Under the original agreement, Celgene’s option was exercisable during an option period that would have expired on July 9, 2015. Under the original agreement, the Company received a $65.0 million upfront payment and $25.0 million from the sale of its series C redeemable convertible preferred stock to an affiliate of Celgene, of which $3.0 million was considered a premium and included as collaboration arrangement consideration for a total upfront payment of $68.0 million. In addition, the Company has received a $25.0 million clinical development milestone payment and $7.0 million of global development co-funding through June 30, 2019. The Company was also eligible to receive $35.0 million in an additional clinical development milestone payment and up to $100.0 million in regulatory milestone payments related to DOT1L as well as up to $65.0 million in payments, including a combination of clinical development milestone payments and an option exercise fee for each available target to which Celgene had the right to exercise its option during an initial option period that would have ended in July 2015 but was extended pursuant to the amended and restated agreement as discussed below under “Amended and Restated Agreement Structure” (each a “selected target”), and up to $100.0 million in regulatory milestone payments for each selected target. As to DOT1L and each selected target, the Company retained all product rights in the United States and was eligible to receive royalties for each target at defined percentages ranging from the mid-single digits to the mid-teens on net product sales outside of the United States subject to reduction in specified circumstances. The Company was obligated to conduct and solely fund research and development costs of the Phase 1 clinical trials for pinometostat. For all remaining DOT1L program development costs, Celgene and the Company were to equally co-fund global development and each party was to solely fund territory-specific development costs for its territory. Amended and Restated Agreement Structure Under the amended and restated collaboration and license agreement: • Celgene retained its exclusive license to small molecule HMT inhibitors targeting DOT1L, including pinometostat, • Celgene’s other option rights were narrowed to small molecule HMT inhibitors targeting three predefined targets, or the Option Targets, • The exclusive licenses to HMT inhibitors targeting two of the Option Targets that Celgene may acquire were expanded to include the United States, with the exclusive license to HMT inhibitors targeting the third Option Target continuing to be for all countries other than the United States, • Celgene’s option period was extended for each of the Option Targets and Celgene’s option is exercisable at the time of the Company’s investigational new drug application, or IND, filing for an HMT inhibitor targeting the applicable Option Target, upon the payment by Celgene at such time of a pre-specified development milestone-based license payment, • Celgene’s license may be maintained beyond the end of Phase 1 clinical development for each of the Option Targets, upon payment by Celgene at such time of a pre-specified development milestone-based license payment, and • The Company’s research and development obligations with respect to each Option Target under the amended and restated agreement were extended for at least an additional three years, subject to Celgene exercising its option with respect to such Option Target at IND filing. Subject to the Company’s opt-out rights, the Company’s research and development obligations were expanded to include the completion of a Phase 1 clinical trial as to each Option Target following Celgene’s exercise of its option at IND filing. Under the amended and restated agreement, the Company received a $10.0 million upfront payment in exchange for the Company’s extension of Celgene’s option rights to the Option Targets and the Company’s research and development obligations. In addition, the Company is eligible to earn an aggregate of up to $75.0 million in development milestones and license payments, up to $365.0 million in regulatory milestone payments and up to $170.0 million in sales milestone payments related to the three Option Targets. The Company is also eligible to receive royalties on each of the Option Targets as specified in the amended and restated agreement. The Company is also eligible to earn $35.0 million in an additional clinical development milestone payment and up to $100.0 million in regulatory milestone payments related to DOT1L. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone payments or royalty payments from Celgene. Due to the varying stages of development of each target, the Company is not able to determine the next milestone that might be earned, if any. The amended and restated agreement eliminated the right of first negotiation that the Company had granted to Celgene under the original agreement with respect to business combination transactions that the Company may desire to pursue with third parties. The Company is primarily responsible for the research strategy under the collaboration. During each applicable option period the Company is required to use commercially reasonable efforts to carry out a mutually agreed-upon research plan for each Option Target. Subject to the Company’s opt-out right for the DOT1L target and each of the Option Targets, the Company is required to conduct and solely fund development costs of the Phase 1 clinical trials for HMT inhibitors directed to such targets, including for pinometostat. After the completion of Phase 1 development, as to DOT1L and the Option Target for which the Company retains U.S. rights, Celgene and the Company will equally co-fund global development and each party will solely fund territory-specific development costs for its respective territory; and, as to the other two Option Targets, after the completion of Phase 1 development, Celgene will solely fund all development costs on a worldwide basis . Accounting Considerations of the Amended and Restated Agreement The Company assessed the amended arrangement in accordance with ASC 606 and concluded that the contract counterparty, Celgene, is a customer based on the arrangement structure, through the satisfaction of each target’s performance obligations. As of the amendment, the Company identified the following performance obligations under the arrangement, whether satisfied or not: • an exclusive license to small molecule HMT inhibitors targeting DOT1L, including pinometostat, combined with pre-IND research services for DOT1L; • post-IND research and development services for DOT1L through a Phase 1 clinical trial; • pre-IND research services for each Option Target; and • material rights related to each of Celgene’s options at the time of an IND filing to license HMT inhibitors targeting each Option Target. The Company determined that the DOT1L license and pre-IND research and development activities for DOT1L were not distinct from one another, due to the limited economic benefit that Celgene would derive from the DOT1L license if it did not obtain the research services After IND effectiveness, the Company concluded that the DOT1L license would be distinct apart from any remaining research and development services because Celgene, or other market participants, would have the ability to execute human clinical trials on the identified compound. Accordingly, the DOT1L license and pre-IND research services for DOT1L were accounted for as a combined performance obligation. The post-IND research and development services for DOT1L have been accounted for as a separate performance obligation. The pre-IND research services for each Option Target were the only performance obligations not subject to the exercise of a customer option at the time of the amendment for each Option Target and therefore represent three separate performance obligations (one for each Option Target). The Company evaluated the option rights at the time of an IND filing to determine whether they provide Celgene with material rights. The Company concluded that the options were issued at a discount, and therefore provide material rights. As such, the option rights at the time of an IND filing for each Option Target represent three separate performance obligations (one for each Option Target) as of the amendment of the arrangement. The license to each HMT inhibitor targeting each respective Option Target, the Company’s research and development obligations through the completion of a Phase 1 clinical trial for each Option Target, and the option to maintain the license beyond the end of Phase 1 clinical development for each Option Target are all subject to Celgene’s exercise of the option rights at the time of an IND filing and, therefore, are not considered performance obligations as of the amendment. Under the agreement, the Company determined that the total transaction price was $103.0 million as of the amendment of the arrangement, comprised the following: • $68.0 million total upfront payment received under the original agreement, as described above; • $25.0 million clinical development milestone payment for DOT1L; and • $10.0 million upfront payment under the amended and restated agreement. The option exercise fees of $75.0 million in the aggregate, for the options at the time of IND and completion of Phase 1, that may be received are excluded from the transaction price until each customer option is exercised. The future potential milestone payments were excluded from the transaction price, as all milestone amounts were fully constrained. The Company will reevaluate the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjust its estimate of the transaction price. The transaction price was allocated to the performance obligations based on the estimated stand-alone selling prices at the time of the amendment. For the DOT1L performance obligation that includes the license and pre-IND research services, the stand-alone selling price was determined considering the stage and status of the program and the technology involved and the level of development expected, as well as the expected cost and margin for the research services. For the post-IND research and development services for DOT1L and the pre-IND research services for each Option Target, the stand-alone selling price was determined considering the expected cost and a reasonable margin for the respective services. The material rights from the option rights at the time of an IND filing for each Option Target were valued based on the estimated discount at which the option is priced and the Company’s estimated probability of the options’ exercise as of the time of the amendment. The Company believes that a change in the assumptions used to determine its stand-alone selling price for the performance obligations most likely would not have a significant effect on the allocation of consideration received (or receivable) to the performance obligations that were not satisfied as of the adoption of ASC 606. The Company allocated the following amounts of the total transaction price to the performance obligations as of the amendment date: • $65.1 million, including the $25.0 million clinical development milestone payment for DOT1L, to the two DOT1L performance obligations, which were satisfied prior to the ASC 606 adoption date; • $34.1 million to the three Pre-IND research services performance obligations related to the Option Targets, which were substantially satisfied as of the ASC 606 adoption date; and • $3.8 million to the three material rights related to Celgene’s option rights at the time of an IND filing for each Option Target, which shall not be satisfied until the option is exercised or one of the parties opts out of the arrangement. All performance obligations, except for the three material rights were substantially satisfied as of the adoption of ASC 606 and therefore all of the transaction price allocated to those performance obligations has been recognized as revenue under ASC 606. Through June 30, 2019 GSK In January 2011, the Company entered into a collaboration and license agreement with GSK, to discover, develop and commercialize novel small molecule HMT inhibitors directed to available targets from the Company’s platform. Under the terms of the agreement, the Company granted GSK exclusive worldwide license rights to HMT inhibitors directed to three targets. Additionally, as part of the research collaboration, the Company agreed to provide research and development services related to the licensed targets pursuant to agreed upon research plans during a research term that ended January 8, 2015. In March 2014, the Company and GSK amended certain terms of this agreement for the third licensed target, revising the license terms with respect to candidate compounds and amending the corresponding financial terms, including reallocating milestone payments and increasing royalty rates as to the third target. Subsequent to a GSK strategic portfolio prioritization, the Company received notice in October 2017 that GSK terminated the agreement with respect to the third target, effective December 31, 2017, which returned all rights to that target to the Company. The two other targets, PRMT5 and PRMT1, continue to be subject to the agreement and were not impacted by the termination with respect to the third target. The Company substantially completed all research obligations under this agreement by the end of the first quarter of 2015 and completed the transfer of the remaining data and materials for these programs to GSK in the second quarter of 2015. Agreement Structure Under the agreement, the Company has received and recognized as collaboration revenue a $20.0 million upfront payment, a $3.0 million payment upon the execution of the March 2014 agreement amendment, $6.0 million of fixed research funding, $9.0 million for research and development services and $51.0 million of preclinical and research and development milestone payments. The preclinical and research and development milestone payments total includes a $10.0 million milestone payment earned in May 2017 related to the second target in the collaboration, upon GSK’s initiation of good laboratory practices toxicology studies, as well as a $6.0 million clinical milestone following GSK’s initiation of patient dosing in a Phase 1 clinical trial of a PRMT5 inhibitor that the Company discovered and licensed to GSK. In 2018, the Company recognized a $12.0 million milestone based on the Company’s determination that it was earned in 2018 relating to the first dosing of a patient in a Phase 2 clinical trial of GSK3326595, a PRMT5 inhibitor discovered by us and licensed to GSK under the collaboration agreement, as well as a $8.0 million milestone payment earned in 2018 relating to the initiation of a patient dosing in a Phase 1 clinical trial of GSK3368715, a PRMT1 inhibitor discovered by us and licensed to GSK under the collaboration agreement. As of June 30, 2019, for the two remaining targets, the Company is eligible to receive up to $50.0 million in clinical development milestone payments, up to $197.0 million in regulatory milestone payments and up to $ 128.0 million in sales-based milestone payments. As a result of the termination of the agreement as it relates to the third target, the Company will receive no additional payments related to that target. In addition, GSK is required to pay the Company royalties, at percentages from the mid-single digits to the low double-digits, on a licensed product-by-licensed product basis, on worldwide net product sales, subject to reduction in specified circumstances. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone payments or royalty payments from GSK. GSK became solely responsible for development and commercialization for each licensed target in the collaboration when the research term ended on January 8, 2015. Collaboration Revenue Through June 30, 2019, the Company has earned a total of $89.0 million under the GSK agreement, which the Company recognized as collaboration revenue in the condensed consolidated statements of operations and comprehensive loss. Eisai In April 2011, the Company entered into a collaboration and license agreement with Eisai Co. Ltd., or Eisai, under which the Company granted Eisai an exclusive worldwide license to its small molecule HMT inhibitors directed to the EZH2 HMT, including the Company’s product candidate tazemetostat, while retaining an opt-in right to co-develop, co-commercialize and share profits with Eisai as to licensed products in the United States. As of December 31, 2014, the Company had completed its performance obligations under the original agreement. In March 2015, the Company entered into an amended and restated collaboration and license agreement with Eisai, under which the Company reacquired worldwide rights, excluding Japan, to its EZH2 program, including tazemetostat. Under the amended and restated agreement, the Company is responsible for global development, manufacturing and commercialization outside of Japan of tazemetostat and any other EZH2 product candidates, with Eisai retaining development and commercialization rights in Japan, as well as a right to elect to manufacture tazemetostat and any other EZH2 product candidates in Japan and waived the right of first negotiation for the rest of Asia. Under the original agreement, Eisai was solely responsible for funding all research, development and commercialization costs for EZH2 compounds. Under the amended and restated agreement, the Company is solely responsible for funding global development, manufacturing and commercialization costs for EZH2 compounds outside of Japan, including the remaining development costs due under a companion diagnostic agreement with Roche Molecular Systems Inc., or Roche Molecular, and Eisai is solely responsible for funding Japan-specific development and commercialization costs for EZH2 compounds. The Company recorded the reacquisition of worldwide rights, excluding Japan, to the EZH2 program, including tazemetostat, under the amended and restated agreement with Eisai as an acquisition of an in-process research and development asset. As this asset was acquired without corresponding processes or activities that would constitute a business, had not achieved regulatory approval for marketing and, absent obtaining such approval, had no alternative future use, the Company recorded the $40.0 million upfront payment made to Eisai in March 2015 as research and development expense in the consolidated statements of operations and comprehensive loss. The Company has also agreed to pay Eisai up to $20.0 million in clinical development milestone payments, including a $10.0 million milestone upon the earlier of initiation of a first phase 3 clinical trial of any EZH2 product or the first submission of an NDA or Market Authorization Application, or MAA, up to $50.0 million in regulatory milestone payments, including a $25.0 million milestone payment upon regulatory approval of the first NDA or MAA, and royalties at a percentage in the mid-teens on worldwide net sales of any EZH2 product, excluding net sales in Japan. The Company is eligible to receive from Eisai royalties at a percentage in the mid-teens on net sales of any EZH2 product in Japan. In the second quarter of 2019, the Company submitted its first NDA to the U.S. Food and Drug Administration, or FDA, for the treatment of patients with epithelioid sarcoma, triggering the payment of the $10 million clinical development milestone to Eisai and the recording of this amount to research and development expense. The Company paid the $10.0 million clinical development milestone to Eisai in June 2019. During the three and six months ended June 30, 2019 LYSA In May 2016, the Company entered into a collaboration agreement with the Lymphoma Academic Research Organisation, or LYSARC, for the first planned combination trial of tazemetostat. LYSARC is the operational arm of the Lymphoma Study Association, or LYSA, a premier cooperative group in France dedicated to clinical and translational research for lymphoma. This Phase 1b/2 study is evaluating tazemetostat in combination with R-CHOP, the standard of care first line combination treatment for diffuse large B-cell lymphoma, or DLBCL, as a first line treatment in elderly, high-risk patients with DLBCL and is being sponsored by LYSARC. LYSA is managing the study operations for the trial, and the Company is recognizing its share of the related expenses as those costs are incurred over the duration of the trial. In addition, the Company is planning an expansion of this trial to include a cohort of patients with high-risk front-line FL . Genentech In June 2016, the Company entered into a collaboration agreement with Genentech Inc., or Genentech, a member of the Roche Group, to conduct a Phase 1b clinical trial to investigate the anti-cancer effects of the Company’s EZH2 inhibitor, tazemetostat, and Genentech’s anti-PD-L1 cancer immunotherapy, atezolizumab, when used in combination. The trial is evaluating this combination regimen for the treatment of patients with relapsed or refractory DLBCL. Under the agreement, each company is supplying its respective anti-cancer agent to support the trial and sharing equally in the trial costs. Genentech is managing the study operations for the trial, and the Company is recognizing its share of the related expenses as those costs are incurred over the duration of the trial. In June 2017, the Company announced an expansion of the clinical collaboration with Genentech to investigate the combination of tazemetostat with atezolizumab in a Phase 1b/2 clinical trial for the treatment of patients with relapsed or refractory metastatic non-small cell lung cancer, or NSCLC. The trial will be part of MORPHEUS, Genentech’s open-label, multi-center, randomized umbrella trial evaluating the efficacy and safety of multiple immunotherapy-based treatment combinations for metastatic NSCLC. This trial was initiated at the end of 2017, but before patients had been enrolled in the study, recruitment was halted due to the partial hold placed on tazemetostat studies by the FDA in April 2018 following a safety report from one patient in the dose-ranging portion of the Phase 1 study who developed a secondary case of T-cell lymphoblastic lymphoma, or T-LBL. Due to the hold and strategic reprioritizations, in early 2019 the companies announced that they jointly opted not to move forward with the NSCLC combination study. Roche Molecular In December 2012, Eisai and the Company entered into an agreement with Roche Molecular under which Eisai and the Company engaged Roche Molecular to develop a companion diagnostic to identify patients who possess certain activating mutations of EZH2. In October 2013, this agreement was amended to include additional mutations in EZH2. The development costs due under the amended agreement with Roche Molecular were the responsibility of Eisai until the execution of the amended and restated collaboration and license agreement with Eisai in March 2015, at which time the Company assumed responsibility for the remaining development costs due under the agreement. In December 2015, the Company entered into a second amendment to the companion diagnostic agreement with Roche Molecular. The agreement was further amended in March 2018. Under the amended agreement, the Company is responsible for remaining development costs of $10.4 million due under the agreement and Eisai has agreed to reimburse the Company $0.9 million of this amount related to a regulatory milestone for Japan. As of June 30, 2019, the Company is responsible for the remaining development costs of $4.9 million due under the agreement. The Company expects the remaining development costs under the amended agreement to be incurred and paid through 2020. Under the agreement with Roche Molecular, Roche Molecular is obligated to use commercially reasonable efforts to develop and to make commercially available the companion diagnostic. Roche Molecular has exclusive rights to commercialize the companion diagnostic. The agreement with Roche Molecular will expire when the Company is no longer developing or commercializing tazemetostat. The Company may terminate the agreement by giving Roche Molecular 90 days’ written notice if the Company discontinues development and commercialization of tazemetostat or determines, in conjunction with Roche Molecular, that the companion diagnostic is not needed for use with tazemetostat. Either the Company or Roche Molecular may also terminate the agreement in the event of a material breach by the other party, in the event of material changes in circumstances that are contrary to key assumptions specified in the agreement or in the event of specified bankruptcy or similar circumstances. Under specified termination circumstances, Roche Molecular may become entitled to specified termination fees. Boehringer Ingelheim In November 2018, the Company entered into a collaboration and license agreement with Boehringer Ingelheim International GmbH, or Boehringer Ingelheim, to discover, research, develop and commercialize small molecule compounds that are inhibitors of an undisclosed histone acetyl transferase, or HAT, target and an undisclosed helicase target, along with associated predictive biomarkers, or the Target Projects. Under the terms of the agreement, the Company granted to Boehringer Ingelheim an exclusive, world-wide license to the undisclosed target inhibitors technology. The agreement also includes reciprocal licenses to utilize each other’s know-how, patents and technologies for activities under the agreement. Further, each party is granted the license to develop, manufacture, commercialize and otherwise exploit any compound or product that successfully achieves start of lead optimization, or SoLO. The Company is also obligated to provide research and development services through SoLO approval for both Target Projects, and to serve on the Joint Steering Committee throughout Agreement Structure Under the terms of the agreement, the Company received a $15.0 million upfront payment and $1.3 million in research funding for the costs to be incurred by the Company in connection with its research activities. As of June 30, 2019, $3.7 million in research funding will be received in the remainder of 2019, payable quarterly in three equal installments. At its discretion, Boehringer Ingelheim has the option to extend the research period by up to one year, subject the Company’s agreement to the specified research activities and additional research funding. The Company is eligible to receive up to $80.5 million in clinical development milestone payments, up to $106.5 million in regulatory milestone payments and up to $93.5 million in sales-based milestone payments. In addition, Boehringer Ingelheim is required to pay the Company tiered royalties, on a product by product, and country by country basis, at percentages ranging from the mid-single digits to low-double digits. Royalties will be payable on net product sales for therapies directed at the second target both in the United States and the rest of the world and net product sales outside of the United States for therapies directed at the first target. During the six months ended June 30, 2019 The next potential milestone payment that the Company might be entitled to receive under this agreement is a $5.5 million milestone, for the second Target Project, for the SoLO Approval for a compound, as defined in the agreement. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone or royalty payments from Boehringer Ingelheim. Accounting Considerations of the Agreement The Company assessed the arrangement in accordance with ASC 606 and concluded that the contract counterparty, Boehringer Ingelheim, is a customer based on the arrangement structure, through the satisfaction of each target’s performance obligations. The Company identified the following performance obligations under the arrangement: • the combination of the Epizyme license to the first undisclosed target inhibitor technology, and associated research and development services through the research period; and, • the combination of the Epizyme license to the second undisclosed target inhibitor technology, associated research and development services through the research period. The Company determined that each Epizyme license was not distinct from the associated research and development services due to the limited economic benefit that Boehringer Ingelheim would derive from the Epizyme license if the research services were not provided by the Company. Accordingly, the Epizyme licenses and associated research and development services, for each Target Project, are each accounted for as a combined performance obligation. Under the agreement, the Company determined as of June 30, 2019 • $15.0 million total upfront payment received under the agreement; • $5.0 million research funding payment to be received in 2019; and • $ 5.5 million development milestone for selection of a lead optimization candidate for the shared program targeting enzymes within helicase families. The future potential milestone payments are excluded from the transaction price, as the achievement of the milestone events are highly uncertain. As such, all milestone payments are fully constrained. The Company will reevaluate the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjust its estimate of the transaction price. The Company determined that a 50/50 allocation of transaction price between the two performance obligations is appropriate considering the following factors: (i) the standalone selling price of research and development components, estimated using the cost plus margin approach; and based on cost plus 10%; (ii) the license rights granted for each program (world-wide or ex-US only) and their potential market opportunities; (iii) the total potential milestone payments for each program; and (iv) the expected revenue recognition pattern for each program, which is expected to be relatively consistent. Therefore, $10.0 million is allocated to the first undisclosed target license and associated research services and $10.0 million is allocated to the second undisclosed target license and associated research services and will be recognized through December 31, 2019. The development milestones, will be allocated to each performance obligation as described in the contract. The milestone payments are defined by program and are directly attributable to distinct achievements in each program. The recognition of revenue for each milestone will be based on progress to date in satisfying the applicable performance obligation. During the six months ended June 30, 2019, the Company allocated the $5.5 million development milestone for selection of a lead optimization candidate for the shared program targeting enzymes within helicase families to the first Target Project. Collaboration Revenue Through June 30, 2019, the Company has recognized $15.5 million in total collaboration revenue under its agreeme

Convertible Preferred Stock

Convertible Preferred Stock	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Convertible Preferred Stock	11. On March 6, 2019, the Company entered into an Underwriting Agreement, or the Preferred Stock Agreement, that related to the public offering of 350,000 shares of Series A Convertible Preferred Stock, par value of $0.001 per share, or Series A Preferred Stock, for a purchase price to the public of $115.00 per share. All of the Series A Preferred Stock was sold by the Company for net proceeds of $37.4 million. Upon issuance, each share of Series A Preferred Stock included an embedded beneficial conversion feature because the market price of the Company’s common stock on the date of issuance of the Series A Preferred Stock was $12.34 per share. As a result, the Company recorded the intrinsic value of the beneficial conversion feature of $2.9 million as a discount on the Series A Preferred at issuance. Because the Series A Preferred Stock is immediately convertible upon issuance and does not include mandatory redemption provisions, the discount on the Series A Preferred Stock was immediately accreted. The Company evaluated the Series A Preferred Stock for liability or equity classification in accordance with the provisions of ASC 480, Distinguishing Liabilities from Equity, and determined that equity treatment was appropriate because the Series A Preferred Stock did not meet the definition of the liability instruments defined thereunder for convertible instruments. Specifically, the Series A Preferred Stock is not mandatorily redeemable and does not embody an obligation to buy back the shares outside of the Company’s control in a manner that could require the transfer of assets. Additionally, the Company determined that the Series A Preferred Stock would be recorded as permanent equity, not temporary equity, based on the guidance of ASC 480 given that the holders of equally and more subordinated equity would be entitled to also receive the same form of consideration upon the occurrence of the event that gives rise to the redemption or events of redemption are within the control of the company. Voting Rights Shares of Series A Preferred Stock will generally have no voting rights except as required by law and except that the consent of the holders of a majority of our outstanding shares of Series A Preferred Stock will be required to amend the terms of the Series A Preferred Stock or take certain other actions with respect to the Series A Preferred Stock. Dividends Shares of Series A Preferred Stock will be entitled to receive dividends equal to (on an as-if-converted-to-common stock basis), and in the same form and manner as, dividends actually paid on shares of the Company’s common stock. Liquidation Rights Subject to the prior and superior rights of the holders of any senior securities of the Company, upon liquidation, dissolution or winding up of the Company, whether voluntary or involuntary, each holder of shares of Series A Preferred Stock shall be entitled to receive, in preference to any distributions of any of the assets or surplus funds of the Company to the holders of common stock, an amount equal to $0.001 per share of Series A Preferred Stock, plus an additional amount equal to any dividends declared but unpaid on such shares, before any payments shall be made or any assets distributed to holders of any class of common stock. If, upon any such liquidation, dissolution or winding up of the Company, the assets of the Company shall be insufficient to pay the holders of shares of the Series A Preferred Stock the amount required under the preceding sentence, then all remaining assets of the Company shall be distributed ratably to holders of the shares of the Series A Preferred Stock in proportion to the respective amounts which would otherwise be payable in respect of the shares held by them upon such distribution if all amounts payable on or with respect to such shares were paid in full. Conversion Each share of Series A Preferred Stock shall be convertible, at any time and from time to time from and after the issuance date, at the option of the holder thereof, into a number of shares of common stock equal to 10 shares of common stock, provided that the holder will be prohibited from converting Series A Preferred Stock into shares of the Company’s common stock if, as a result of such conversion, the holder, together with its affiliates and attribution parties, would own more than 9.99% of the total number of shares of common stock then issued and outstanding. The holder can change this requirement to a higher or lower percentage, not to exceed 9.99% of the number of shares of common stock outstanding, upon 61 days’ notice to the Company. Redemption The Company is not obligated to redeem or repurchase any shares of Series A Preferred Stock. Shares of Series A Preferred Stock are not entitled to any redemption rights or mandatory sinking fund or analogous fund provisions.

Stock-Based Compensation

Stock-Based Compensation	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	12. Stock-Based Compensation Total stock-based compensation expense related to stock options, restricted stock units, shares issued under the employee stock purchase plan, and shares granted to non-employee directors in lieu of board fees was $4.7 million and $3.4 million for the three months ended June 30, 2019 and 2018, respectively, and $7.9 million and $6.3 million for the six months ended June 30, 2019 and 2018, respectively. Stock-based compensation expense is classified in the condensed consolidated statements of operations and comprehensive loss as follows: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 (In thousands) (In thousands) Research and development $ 1,732 $ 1,222 $ 2,897 $ 2,346 General and administrative 2,996 2,202 5,042 3,965 Total $ 4,728 $ 3,424 $ 7,939 $ 6,311 Stock Options The weighted-average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $8.56 and $10.08 per option for those options granted during the three months ended June 30, 2019 and 2018, respectively, and $6.57 and $10.34 per option for those options granted during the six months ended June 30, 2019 and 2018, respectively. Key assumptions used to apply this pricing model were as follows: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Risk-free interest rate 2.0 % 2.8 % 2.4 % 2.6 % Expected life of options 6.0 years 6.0 years 6.0 years 6.0 years Expected volatility of underlying stock 71.3 % 71.9 % 71.8 % 71.5 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % The following is a summary of stock option activity for the six months ended June 30, 2019: Number of Options Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (In thousands) (In years) (In thousands) Outstanding at December 31, 2018 5,153 $ 14.48 Granted 3,011 10.16 Exercised (203 ) 8.57 Forfeited or expired (666 ) 14.42 Outstanding at June 30, 2019 7,295 $ 12.87 8.32 $ 11,818 Exercisable at June 30, 2019 2,446 $ 15.53 6.74 $ 2,852 As of June 30, 2019, there was $33.3 million of unrecognized compensation cost related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.97 years. Restricted Stock Units As of June 30, 2019, 238,166 restricted stock units, or RSUs, were granted to executives. The awards granted to executives are service-based. Assuming all service conditions are achieved, 25% of the RSUs would vest annually for four years. Number of Service Based RSU Shares (in thousands) Weighted Average Grant Date Fair Value Outstanding at December 31, 2018 — $ — Granted 238 9.47 Vested — — Forfeited — — Outstanding at June 30, 2019 238 $ 9.47 Compensation expense totaling $0.2 million was recognized for the service-based RSUs for the six months ended June 30, 2019. As of June 30, 2019, there was $1.8 million of unrecognized compensation cost related to service-based RSUs that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 3.6 years. As of June 30, 2019, the Company granted 483,400 RSUs to executives and employees. The awards granted are performance-based. Assuming all performance conditions are achieved, 20% of the RSUs would vest on June 30, 2019, 30% would vest on December 31, 2019, 20% would vest on March 31, 2020, and the remaining 30% of the RSUs would vest on September 30, 2020. Number of Performance Based RSU Shares (in thousands) Weighted Average Grant Date Fair Value Outstanding at December 31, 2018 — $ — Granted 483 12.14 Vested (95 ) 12.14 Forfeited (10 ) 11.94 Outstanding at June 30, 2019 378 $ 12.15 Compensation expense totaling There was $4.7 million of unrecognized compensation cost related to performance-based RSUs that are expected to vest as of June 30, 2019. As of June 30, 2019, there were 616,486 RSUs outstanding.

Loss per Share

Loss per Share	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Loss per Share	13. Loss Per Share Basic and diluted loss per share allocable to common stockholders are computed as follows: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 (In thousands except per share data) (In thousands except per share data) Net loss $ (48,465 ) $ (29,128 ) $ (77,804 ) $ (63,193 ) Accretion of convertible preferred stock — — (2,940 ) — Net loss attributable to common stockholders $ (48,465 ) $ (29,128 ) $ (80,744 ) $ (63,193 ) Weighted average shares outstanding 90,876 69,490 77,315 69,438 Basic and diluted loss per share allocable to common stockholders $ (0.53 ) $ (0.42 ) $ (1.04 ) $ (0.91 ) The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 (In thousands) (In thousands) Stock options 7,295 5,818 7,295 5,818 Restricted stock units 616 — 616 — Shares issuable under employee stock purchase plan 26 20 26 20 Preferred stock (if converted) 3,500 — 3,500 — 11,437 5,838 11,437 5,838

Related Party Transactions

Related Party Transactions	6 Months Ended
	Jun. 30, 2019
Related Party Transactions [Abstract]
Related Party Transactions	14. Related Party Transactions Celgene has made a series of equity investments in the Company, owning 3,674,640 shares of common stock representing 4.0% of the Company’s outstanding common stock as of June 30, 2019. Refer to Note 10, Collaborations,

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The condensed consolidated financial statements of the Company included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission, or the SEC. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018, or the Annual Report. The unaudited condensed consolidated financial statements include the accounts of Epizyme, Inc. and its wholly owned, controlled subsidiary, Epizyme Securities Corporation. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation. In the opinion of management, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair presentation of the results for the reported interim periods. The Company considers events or transactions that occur after the balance sheet date but before the condensed consolidated financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The three months ended June 30, 2019 and 2018 are referred to as the second quarter of 2019 and 2018, respectively. The results of operations for interim periods are not necessarily indicative of results to be expected for the full year or any other interim period. Certain reclassifications have been made to prior periods to conform to current period presentation. Reclassification of prior year amounts have been made to present capital lease liability in other current and other long-term liabilities in the consolidated Balance Sheets. There was no impact on total operating expenses or net income (loss) resulting from these reclassifications.
Significant Accounting Policies	Significant Accounting Policies During the quarter ended March 31, 2019, the Company adopted Accounting Standards Codification, Topic 842, Leases using the required Recently Adopted Accounting Pronouncements Summary of Significant Accounting Policies
Going Concern	Going Concern At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to the current cash, cash equivalent and marketable securities balances. After considering the Company’s current research and development plans, the building of commercial infrastructure and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of June 30, 2019, the Company did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued.
Pending Accounting Pronouncements	Pending Accounting Pronouncements In June 2016, the Financial Accounting Standards Board, or FASB, issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements, or ASC 808,
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements
Leases	Leases In February 2016, the FASB issued ASU 2016-02, Leases, or ASC 842, which requires lessees to recognize a right-of-use asset and lease liability for most lease arrangements. The new standard is effective for annual reporting periods beginning after December 15, 2018. A modified retrospective transition approach is required to be applied to leases existing as of, or entered into after, the beginning of the earliest comparative period presented in the financial statements, with certain practical expedients available. Effective January 1, 2019, the Company adopted ASC 842 using the required modified retrospective approach and utilizing the effective date as its date of initial application, for which prior periods are presented in accordance with the previous guidance in ASC 840, Leases, or ASC 840. At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present in the arrangement. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets and short-term and long-term lease liabilities, as applicable. The Company has elected not to recognize on the balance sheet leases with terms of 12 months or less. The Company typically only includes an initial lease term in its assessment of a lease arrangement. Options to renew a lease are not included in the Company’s assessment unless there is reasonable certainty that the Company will renew. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate, which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. In transition to ASC 842, the Company utilized the remaining lease term of its leases in determining the appropriate incremental borrowing rates. In accordance with ASC 842, components of a lease should be split into three categories: lease components (e.g., land, building, etc.), non-lease components (e.g., common area maintenance, consumables, etc.), and non-components (e.g., property taxes, insurance, etc.). The fixed and in-substance fixed contract consideration (including any consideration related to non-components) must be allocated based on the respective relative fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, certain expedients are available. Entities may elect the practical expedient to not separate lease and non-lease components by class of underlying asset. Rather, entities would account for each lease component and the related non-lease component together as a single component. In adopting ASC 842, the Company elected to utilize the available package of practical expedients permitted under the transition guidance within the new standard, which does not require the reassessment of the following: i) whether existing or expired arrangements are or contain a lease, ii) the lease classification of existing or expired leases, and iii) whether previous initial direct costs would qualify for capitalization under the new lease standard. The adoption of this standard resulted in the recognition of operating lease liabilities and right-of-use assets of $11.5 million and $10.7 million, respectively, on the Company’s condensed consolidated balance sheet relating to its leases for its corporate headquarters in Cambridge, Massachusetts and other operating leases. The adoption of the standard did not have a material effect on the Company’s condensed consolidated statements of operation and comprehensive loss or condensed consolidated statements of cash flows. As of June 30, 2019, the Company recognized operating lease liabilities and right-of-use assets under the new guidance of $10.8 million and $10.0 million, respectively, on the Company’s balance sheet.
Compensation	Compensation In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation, or ASC 718: Improvements to Nonemployee Share-Based Payment Accounting,

Cash (Tables)

Cash (Tables)	6 Months Ended
	Jun. 30, 2019
Cash And Cash Equivalents [Abstract]
Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash	A reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same such amounts shown in the condensed consolidated statements of cash flows, is as follows: As of June 30, 2019 2018 (In thousands) Cash and cash equivalents $ 45,932 $ 67,906 Restricted cash, as part of other assets 462 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 46,394 $ 68,368

Marketable Securities (Tables)

Marketable Securities (Tables)	6 Months Ended
	Jun. 30, 2019
Investments Debt And Equity Securities [Abstract]
Summary of Available-for-Sale Securities Held	The following table summarizes the available-for-sale securities held at June 30, 2019 (in thousands): Description Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial paper $ 138,975 $ 109 $ — $ 139,084 Corporate notes 143,825 139 — 143,964 U.S. government agency securities and U.S. treasuries 2,000 — — 2,000 Total $ 284,800 $ 248 $ — $ 285,048 The following table summarizes the available-for-sale secu rities held at December 31, 201 8 (in thousands): Description Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial paper $ 73,110 $ — $ (22 ) $ 73,088 Corporate notes 80,575 — (30 ) 80,545 Total $ 153,685 $ — $ (52 ) $ 153,633

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of June 30, 2019 and December 31, 2018, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of June 30, 2019 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 37,071 $ 37,071 — $ — Marketable securities: Commercial paper 139,084 — 139,084 — Corporate notes 143,964 — 143,964 — U.S. government agency securities and U.S. treasuries 2,000 — 2,000 Total $ 322,119 $ 37,071 $ 285,048 $ — Fair Value as of December 31, 2018 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 79,225 $ 50,785 $ 28,440 $ — Marketable securities: Commercial paper 73,088 — 73,088 — Corporate notes 80,545 — 80,545 — Total $ 232,858 $ 50,785 $ 182,073 $ —

Supplemental Balance Sheet In_2

Supplemental Balance Sheet Information (Tables)	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: June 30, 2019 December 31, 2018 (In thousands) Employee compensation and benefits $ 3,871 $ 5,509 Research and development expenses 9,801 11,272 Professional services and other 4,041 2,919 Accrued expenses $ 17,713 $ 19,700

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Summary of Lease Costs and Company's Operating Leases	The following table contains a summary of the lease costs recognized under Topic 842 and other information pertaining to the Company’s operating leases for the three and six months ended June 30, 2019: Three Months Ended June 30, Six Months Ended June 30, 2019 2019 (In thousands) Lease cost Operating lease cost $ 889 $ 1,777 Variable lease cost 305 628 Total lease cost $ 1,194 $ 2,405 Other information Operating cash flows used for operating leases $ 907 $ 1,812 Weighted average remaining lease term 3.4 years 3.4 years Weighted average discount rate 8.55 % 8.55 %
Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases	Future minimum lease payments under the Company’s non-cancelable operating leases as of June 30, 2019, are as follows: 2019 (In thousands) 2019 $ 1,825 2020 3,729 2021 3,622 2022 3,340 Thereafter — Total lease payments $ 12,516 Less: imputed interest (1,690 ) Total operating lease liabilities at June 30, 2019 $ 10,826

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense	Stock-based compensation expense is classified in the condensed consolidated statements of operations and comprehensive loss as follows: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 (In thousands) (In thousands) Research and development $ 1,732 $ 1,222 $ 2,897 $ 2,346 General and administrative 2,996 2,202 5,042 3,965 Total $ 4,728 $ 3,424 $ 7,939 $ 6,311
Assumptions Used in Applying Pricing Model	Key assumptions used to apply this pricing model were as follows: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Risk-free interest rate 2.0 % 2.8 % 2.4 % 2.6 % Expected life of options 6.0 years 6.0 years 6.0 years 6.0 years Expected volatility of underlying stock 71.3 % 71.9 % 71.8 % 71.5 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 %
Summary of Stock Option Activity	The following is a summary of stock option activity for the six months ended June 30, 2019: Number of Options Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (In thousands) (In years) (In thousands) Outstanding at December 31, 2018 5,153 $ 14.48 Granted 3,011 10.16 Exercised (203 ) 8.57 Forfeited or expired (666 ) 14.42 Outstanding at June 30, 2019 7,295 $ 12.87 8.32 $ 11,818 Exercisable at June 30, 2019 2,446 $ 15.53 6.74 $ 2,852
Summary of Service Based Restricted Stock Units	As of June 30, 2019, 238,166 restricted stock units, or RSUs, were granted to executives. The awards granted to executives are service-based. Assuming all service conditions are achieved, 25% of the RSUs would vest annually for four years. Number of Service Based RSU Shares (in thousands) Weighted Average Grant Date Fair Value Outstanding at December 31, 2018 — $ — Granted 238 9.47 Vested — — Forfeited — — Outstanding at June 30, 2019 238 $ 9.47
Summary of Performance Based Restricted Stock Units	As of June 30, 2019, the Company granted 483,400 RSUs to executives and employees. The awards granted are performance-based. Assuming all performance conditions are achieved, 20% of the RSUs would vest on June 30, 2019, 30% would vest on December 31, 2019, 20% would vest on March 31, 2020, and the remaining 30% of the RSUs would vest on September 30, 2020. Number of Performance Based RSU Shares (in thousands) Weighted Average Grant Date Fair Value Outstanding at December 31, 2018 — $ — Granted 483 12.14 Vested (95 ) 12.14 Forfeited (10 ) 11.94 Outstanding at June 30, 2019 378 $ 12.15

Loss per Share (Tables)

Loss per Share (Tables)	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Basic and Diluted Loss per Share	Basic and diluted loss per share allocable to common stockholders are computed as follows: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 (In thousands except per share data) (In thousands except per share data) Net loss $ (48,465 ) $ (29,128 ) $ (77,804 ) $ (63,193 ) Accretion of convertible preferred stock — — (2,940 ) — Net loss attributable to common stockholders $ (48,465 ) $ (29,128 ) $ (80,744 ) $ (63,193 ) Weighted average shares outstanding 90,876 69,490 77,315 69,438 Basic and diluted loss per share allocable to common stockholders $ (0.53 ) $ (0.42 ) $ (1.04 ) $ (0.91 )
Common Stock Equivalents Excluded from Calculation of Diluted Loss per Share Attributable to Common Stockholders	The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 (In thousands) (In thousands) Stock options 7,295 5,818 7,295 5,818 Restricted stock units 616 — 616 — Shares issuable under employee stock purchase plan 26 20 26 20 Preferred stock (if converted) 3,500 — 3,500 — 11,437 5,838 11,437 5,838

Overview - Additional Informati

Overview - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Basis Of Presentation [Line Items]
Sale of common stock and series A preferred stock in public offering	$ 122,708
Proceeds from sale of redeemable convertible preferred stock		$ 76,000
Initial public offering completion date		May 2013
Cash, cash equivalents, and marketable securities		$ 331,000
Accumulated deficit		(664,528)	$ (586,724)
Collaborative Arrangement [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount		1,155,100
Non-equity funding through collaboration agreement		239,600
IPO [Member] | Common Stock and Series A Preferred Stock [Member]
Basis Of Presentation [Line Items]
Sale of common stock and series A preferred stock in public offering		$ 839,500

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Accounting Policies [Line Items]
Operating lease, liabilities	$ 10,826
Operating lease, right-of-use assets	$ 10,029
ASU 2016-02 [Member]
Accounting Policies [Line Items]
Operating lease, liabilities		$ 11,500
Operating lease, right-of-use assets		$ 10,700

Cash - Summary of Reconciliatio

Cash - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 45,932	$ 86,671	$ 67,906
Restricted cash, as part of other assets	462		462
Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows	$ 46,394		$ 68,368

Cash - Additional Information (

Cash - Additional Information (Detail) $ in Millions	Jun. 30, 2019USD ($)
Cash And Cash Equivalents [Line Items]
Security deposit	$ 0.5
Letter of Credit [Member]
Cash And Cash Equivalents [Line Items]
Security deposit	$ 0.5

Marketable Securities - Summary

Marketable Securities - Summary of Available-for-Sale Securities Held (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 284,800	$ 153,685
Available-For-Sale Securities Held, Gross Unrealized Gains	248
Available-For-Sale Securities Held, Gross Unrealized Losses		(52)
Available-For-Sale Securities Held, Fair Value	285,048	153,633
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	138,975	73,110
Available-For-Sale Securities Held, Gross Unrealized Gains	109
Available-For-Sale Securities Held, Gross Unrealized Losses		(22)
Available-For-Sale Securities Held, Fair Value	139,084	73,088
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	143,825	80,575
Available-For-Sale Securities Held, Gross Unrealized Gains	139
Available-For-Sale Securities Held, Gross Unrealized Losses		(30)
Available-For-Sale Securities Held, Fair Value	143,964	$ 80,545
U.S. Government Agency Securities and U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	2,000
Available-For-Sale Securities Held, Fair Value	$ 2,000

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2019USD ($)	Jun. 30, 2019USD ($)Security
Schedule of Available-for-sale Securities [Line Items]
Realized gains (losses) recognized on sale or maturity of marketable equity securities	$ 0
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	9,400,000	$ 9,400,000
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss	$ 100,000	$ 100,000
Weighted average maturity period		7 months
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in unrealized loss position | Security		1
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in unrealized loss position | Security		2

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	$ 37,071	$ 79,225
Total	322,119	232,858
Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	139,084	73,088
Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	143,964	80,545
U.S. Government Agency Securities and U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	2,000
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	37,071	50,785
Total	37,071	50,785
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		28,440
Total	285,048	182,073
Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	139,084	73,088
Level 2 [Member] | Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	143,964	$ 80,545
Level 2 [Member] | U.S. Government Agency Securities and U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 2,000

Supplemental Balance Sheet In_3

Supplemental Balance Sheet Information - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Employee compensation and benefits	$ 3,871	$ 5,509
Research and development expenses	9,801	11,272
Professional services and other	4,041	2,919
Accrued expenses	$ 17,713	$ 19,700

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Federal income tax provision or benefit	$ 0	$ 0
State income tax provision or benefit	$ 0	$ 0

Leases - Additional Information

Leases - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2019USD ($)
Lessee Lease Description [Line Items]
Operating Lease, existence of option to terminate	true
Monthly base rent	$ 200,000
Increase in base rent	33,000
Annual increase of base rent	9,000
Letter of credit as a security deposit	$ 500,000
Initial Term [Member]
Lessee Lease Description [Line Items]
Lease expiration date	May 31, 2018
Extended Term [Member]
Lessee Lease Description [Line Items]
Lease expiration date	Nov. 30, 2022

Leases - Summary of Lease Costs

Leases - Summary of Lease Costs and Company's Operating Leases (Detail) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019USD ($)	Jun. 30, 2019USD ($)
Leases [Abstract]
Operating lease cost	$ 889	$ 1,777
Variable lease cost	305	628
Total lease cost	1,194	2,405
Operating cash flows used for operating leases	$ 907	$ 1,812
Weighted average remaining lease term	3 years 4 months 24 days	3 years 4 months 24 days
Weighted average discount rate	8.55%	8.55%

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Jun. 30, 2019USD ($)
Leases [Abstract]
2019	$ 1,825
2020	3,729
2021	3,622
2022	3,340
Total lease payments	12,516
Total lease payments	12,516
Less: imputed interest	(1,690)
Total operating lease liabilities at June 30, 2019	$ 10,826

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Jul. 08, 2015USD ($)OptionTarget	Jan. 01, 2011OptionTarget	Nov. 30, 2018USD ($)Installment	May 31, 2017USD ($)	Mar. 31, 2015USD ($)	Mar. 31, 2014USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)SeparatePerformanceObligationPerformanceObligationMaterialRight	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2013USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2018USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue							$ 5,900,000	$ 12,000,000	$ 13,791,000	$ 12,000,000
Accounts receivable							13,273,000		$ 13,273,000		$ 20,067,000		$ 13,273,000	$ 13,273,000
Roche [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Notice period in days									90 days
DOT1L [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical development milestone payment	$ 35,000,000
DOT1L [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payments	$ 100,000,000
DOT1l Performance Obligations [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price									$ 65,100,000
Number of performance obligations | PerformanceObligation									2
Pre-IND Research Services Performance Obligations [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price									$ 34,100,000
Number of performance obligations | PerformanceObligation									3
Material Rights [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price									$ 3,800,000
Number of material rights | MaterialRight									3
Celgene [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Company received upfront payment													65,000,000
Proceeds from redeemable convertible preferred stock													25,000,000
Upfront payment received									$ 68,000,000				68,000,000
Clinical development milestone achieved													25,000,000
Global development co-funding													7,000,000
Number of option targets | OptionTarget	3
Number of separate performance obligations | SeparatePerformanceObligation									3
Total transaction price									$ 103,000,000
Option exercise fees									75,000,000
Collaboration revenue													99,200,000
Deferred revenue							3,800,000		3,800,000				3,800,000	3,800,000
Celgene [Member] | Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option targets under amended and restated agreement extended period	3 years
Celgene [Member] | United States [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of option targets | OptionTarget	2
Celgene [Member] | Non-US [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of option targets | OptionTarget	1
Celgene [Member] | DOT1L [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical development milestone achieved									25,000,000
Clinical development milestone payment							35,000,000		35,000,000				35,000,000	35,000,000
Additional milestone payments							100,000,000		100,000,000				100,000,000	100,000,000
Celgene [Member] | Available Targets [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payments							100,000,000		100,000,000				100,000,000	100,000,000
Additional payments							65,000,000		65,000,000				65,000,000	65,000,000
Celgene [Member] | Option Targets [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Company received upfront payment	$ 10,000,000								10,000,000
Celgene [Member] | Option Targets [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payments	365,000,000
Development milestone and license payments associated with the Option Targets	75,000,000
Sales-based milestone payments	$ 170,000,000
GSK [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received						$ 3,000,000
Clinical development milestone payment							50,000,000		50,000,000				50,000,000	50,000,000
Additional milestone payments							197,000,000		197,000,000				197,000,000	197,000,000
Number of option targets | OptionTarget		3
Sales-based milestone payments							128,000,000		128,000,000				128,000,000	128,000,000
Deferred revenue							0	$ 0	0	$ 0			0	0
Fixed research funding received														6,000,000
Milestone payments received														51,000,000
Research and development services														9,000,000
Preclinical and research and development milestone payments received				$ 10,000,000
Additional payments received									0
Cash and accounts receivable														89,000,000
Accounts receivable							12,000,000		12,000,000				12,000,000	12,000,000
GSK [Member] | Collaboration Revenue [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received												$ 20,000,000
GSK [Member] | Phase 1 Clinical Trial [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical development milestone achieved				$ 6,000,000							8,000,000
GSK [Member] | Phase 2 Clinical Trial [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical development milestone achieved											$ 12,000,000
Eisai [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment made					$ 40,000,000
Clinical development milestone payments obligation							20,000,000		20,000,000				20,000,000	20,000,000
Regulatory milestone payments obligation							50,000,000		50,000,000				50,000,000	50,000,000
Research and development reduction							1,300,000		2,300,000
Eisai [Member] | First Submission of NDA or MAA [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Regulatory milestone payments obligation							25,000,000		25,000,000				25,000,000	25,000,000
Eisai [Member] | Research And Development Expense [Member] | First Submission of NDA or MAA [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical development milestone payments obligation							10,000,000
Collaborative Arrangement [Member] | Roche [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining unpaid milestone payments							4,900,000		4,900,000				4,900,000	4,900,000	$ 10,400,000
Reimbursements receivable of development costs							900,000		900,000				900,000	900,000
Boehringer Ingelheim [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Company received upfront payment			$ 15,000,000
Upfront payment received									15,000,000
Total transaction price									25,500,000
Collaboration revenue									15,500,000
Deferred revenue							6,300,000		6,300,000				6,300,000	6,300,000
Research funding for costs to be incurred			1,300,000				5,000,000		5,000,000				$ 5,000,000	$ 5,000,000
Research funding for cost to be incurred Remainder			$ 3,700,000
Research funding costs, payment frequency			quarterly
Research funding costs, payable installments | Installment			3
Maximum extension term of research period			1 year
Potential milestone payment			$ 5,500,000
Potential milestone received			5,500,000
Amount of significant reversal of cumulative revenue of development milestone that would not occur									$ 5,500,000
Revenue, information used to determine transaction price									The Company determined that a 50/50 allocation of transaction price between the two performance obligations
Revenue, information used to allocate transaction price									(i) the standalone selling price of research and development components, estimated using the cost plus margin approach; and based on cost plus 10%; (ii) the license rights granted for each program (world-wide or ex-US only) and their potential market opportunities; (iii) the total potential milestone payments for each program; and (iv) the expected revenue recognition pattern for each program, which is expected to be relatively consistent.
Research and development cost margin approach based on cost plus percentage									10.00%
Amount allocated to first undisclosed target license and associated research services									$ 10,000,000
Amount allocated to second undisclosed target license and associated research services									10,000,000
Collaboration revenue under agreement							$ 5,900,000		$ 13,800,000
Boehringer Ingelheim [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical development milestone payment			80,500,000
Regulatory milestone payments			106,500,000
Sales-based milestone payments			$ 93,500,000

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2019	Mar. 06, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Preferred stock, shares issued	350,000
Preferred stock, par value	$ 0.0001		$ 0.0001
Net proceeds from sale of shares	$ 37,433
Series A Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Preferred stock, shares issued		350,000
Preferred stock, par value		$ 0.001
Purchase price per share		$ 115
Net proceeds from sale of shares	$ 37,400
Common stock price per share	$ 12.34
Intrinsic value of beneficial conversion feature, discount on share	$ 2,900
Preferred stock, voting rights	Shares of Series A Preferred Stock will generally have no voting rights except as required by law and except that the consent of the holders of a majority of our outstanding shares of Series A Preferred Stock will be required to amend the terms of the Series A Preferred Stock or take certain other actions with respect to the Series A Preferred Stock.
Liquidation preference per share	$ 0.001
Number of common shares convertible for each share of convertible preferred stock	10
Percentage of conversion restriction upon holder of convertible preferred stock shares	9.99%
Conversion of stock notice period	61 days

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options, restricted stock, shares issued and shares granted to non-employee directors	$ 4,728,000	$ 3,424,000	$ 7,939,000	$ 6,311,000
Weighted-average fair value of options granted	$ 8.56	$ 10.08	$ 6.57	$ 10.34
Unrecognized compensation cost	$ 33,300,000		$ 33,300,000
Expected weighted average period for recognition of compensation cost			2 years 11 months 19 days
Service Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected weighted average period for recognition of compensation cost			3 years 7 months 6 days
Restricted stock units granted			238,166
Vesting percentage			25.00%
Vesting period			4 years
Compensation expense was recognized			$ 200,000
Unrecognized compensation cost	$ 1,800,000		$ 1,800,000
Restricted stock units outstanding	238,000		238,000
Performance Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted			483,400
Compensation expense was recognized			$ 1,200,000
Unrecognized compensation cost	$ 4,700,000		$ 4,700,000
Restricted stock units outstanding	378,000		378,000
Performance Based RSU [Member] | Vest on June 30, 2019 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			20.00%
Performance Based RSU [Member] | Vest on December 31, 2019 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			30.00%
Performance Based RSU [Member] | Vest on March 31, 2020 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			20.00%
Performance Based RSU [Member] | Vest on September 30, 2020 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			30.00%
Unvested Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding	616,486		616,486

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 4,728	$ 3,424	$ 7,939	$ 6,311
Research And Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	1,732	1,222	2,897	2,346
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 2,996	$ 2,202	$ 5,042	$ 3,965

Stock-Based Compensation - Assu

Stock-Based Compensation - Assumptions Used in Applying Pricing Model (Detail) - Employee Stock Option [Member]	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.00%	2.80%	2.40%	2.60%
Expected life of options	6 years	6 years	6 years	6 years
Expected volatility of underlying stock	71.30%	71.90%	71.80%	71.50%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding, Beginning balance | shares	5,153,000
Number of Options, Granted | shares	3,011,000
Number of Options, Exercised | shares	(203,000)
Number of Options, Forfeited or expired | shares	(666,000)
Number of Options, Outstanding, Ending balance | shares	7,295,000
Number of Options, Exercisable | shares	2,446,000
Weighted Average Exercise Price per Share, Outstanding, Beginning balance | $ / shares	$ 14.48
Weighted Average Exercise Price per Share, Granted | $ / shares	10.16
Weighted Average Exercise Price per Share, Exercised | $ / shares	8.57
Weighted Average Exercise Price per Share, Forfeited or expired | $ / shares	14.42
Weighted Average Exercise Price per Share, Outstanding, Ending balance | $ / shares	12.87
Weighted Average Exercise Price per Share, Exercisable | $ / shares	$ 15.53
Weighted Average Remaining Contractual Term (In Years), Outstanding	8 years 3 months 25 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	6 years 8 months 26 days
Aggregate Intrinsic Value, Outstanding | $	$ 11,818
Aggregate Intrinsic Value, Exercisable | $	$ 2,852

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Service Based Restricted Stock Units (Detail) - Service Based RSU [Member]	6 Months Ended
	Jun. 30, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Granted | shares	238,166
Number of Outstanding Shares, Ending Balance | shares	238,000
Weighted Average Grant Date Fair Value, Granted | $ / shares	$ 9.47
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 9.47

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Performance Based Restricted Stock Units (Detail) - Performance Based RSU [Member]	6 Months Ended
	Jun. 30, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Granted | shares	483,400
Number of Shares, Vested | shares	(95,000)
Number of Shares, Forfeited | shares	(10,000)
Number of Outstanding Shares, Ending Balance | shares	378,000
Weighted Average Grant Date Fair Value, Granted | $ / shares	$ 12.14
Weighted Average Grant Date Fair Value, Vested | $ / shares	12.14
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	11.94
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 12.15

Loss per Share - Schedule of Ba

Loss per Share - Schedule of Basic and Diluted Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net loss	$ (48,465)	$ (29,339)	$ (29,128)	$ (34,065)	$ (77,804)	$ (63,193)
Accretion of convertible preferred stock					(2,940)
Net loss attributable to common stockholders	$ (48,465)		$ (29,128)		$ (80,744)	$ (63,193)
Weighted average shares outstanding	90,876		69,490		77,315	69,438
Basic and diluted loss per share allocable to common stockholders	$ (0.53)		$ (0.42)		$ (1.04)	$ (0.91)

Loss per Share - Common Stock E

Loss per Share - Common Stock Equivalents from Calculation of Diluted Loss per Share Attributable to Common Stockholders (Detail) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	11,437	5,838	11,437	5,838
Employee Stock Option [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	7,295	5,818	7,295	5,818
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	616		616
Shares Issuable Under Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	26	20	26	20
Preferred Stock (if converted) [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	3,500		3,500

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - shares	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2013
Related Party Transaction [Line Items]
Equity investments in common stock	91,016,000	79,175,000
Beneficial Owner [Member]
Related Party Transaction [Line Items]
Equity investments in common stock			3,674,640
Equity investments in outstanding common stock percentage	4.00%