Epizyme (EPZM) 10-Q 8 May 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	May 01, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Trading Symbol	EPZM
Entity Registrant Name	EPIZYME, INC.
Entity Central Index Key	1,571,498
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock Shares Outstanding		69,487,808

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 130,289	$ 226,664
Marketable securities	117,634	49,775
Accounts receivable	24	382
Prepaid expenses and other current assets	8,915	8,983
Total current assets	256,862	285,804
Property and equipment, net	2,275	2,527
Restricted cash and other assets	1,064	1,028
Total assets	260,201	289,359
Current liabilities:
Accounts payable	7,227	7,001
Accrued expenses	17,402	17,549
Current portion of capital lease obligation		110
Other current liabilities	4	4
Total current liabilities	24,633	24,664
Deferred revenue	3,806	28,809
Other long-term liabilities	630	515
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.0001 par value; 5,000 shares authorized; no shares issued and outstanding
Common stock, $0.0001 par value; 125,000 shares authorized; 69,486 shares and 69,302 shares issued and outstanding, respectively	7	7
Additional paid-in capital	728,356	723,510
Accumulated other comprehensive loss	(72)	(49)
Accumulated deficit	(497,159)	(488,097)
Total stockholders’ equity	231,132	235,371
Total liabilities and stockholders’ equity	$ 260,201	$ 289,359

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	69,486,000	69,302,000
Common stock, shares outstanding	69,486,000	69,302,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating expenses:
Research and development	$ 25,622	$ 24,695
General and administrative	9,360	8,269
Total operating expenses	34,982	32,964
Operating loss	(34,982)	(32,964)
Other income, net:
Interest income, net	899	438
Other income	18	4
Other income, net	917	442
Net loss	(34,065)	(32,522)	[1]
Other comprehensive income (loss):
Unrealized (loss) gain on available-for-sale securities	(23)	12
Comprehensive loss	$ (34,088)	$ (32,510)
Loss per share allocable to common stockholders:
Basic	$ (0.49)	$ (0.56)
Diluted	$ (0.49)	$ (0.56)
Weighted average shares outstanding:
Basic	69,386	58,219
Diluted	69,386	58,219
[1]	Revised as a result of the adoption of ASU 2016-18

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss		$ (34,065)	$ (32,522)	[1]
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization		361	411	[1]
Stock-based compensation		2,887	2,757	[1]
Amortization of discount on investments		(106)	(18)	[1]
Changes in operating assets and liabilities:
Accounts receivable		358	15	[1]
Prepaid expenses and other current assets		68	(2,230)	[1]
Accounts payable		226	1,913	[1]
Accrued expenses		(147)	(3,577)	[1]
Other assets		(36)	(4)	[1]
Other liabilities		113	20	[1]
Net cash used in operating activities		(30,341)	(33,235)	[1]
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of available-for-sale securities		(100,763)	(40,785)	[1]
Maturities of available-for-sale securities		32,988	58,200	[1]
Purchases of property and equipment		(108)	(118)	[1]
Net cash (used in) provided by investing activities		(67,883)	17,297	[1]
CASH FLOWS FROM FINANCING ACTIVITIES:
Payments under capital lease obligation		(110)	(150)	[1]
Proceeds from public offering, net of commissions	[1]		1,587
Proceeds from stock options exercised		1,500	641	[1]
Issuance of shares under employee stock purchase plan		459	346	[1]
Net cash provided by financing activities		1,849	2,424	[1]
Net (decrease) in cash, cash equivalents and restricted cash		(96,375)	(13,514)	[1]
Cash, cash equivalents and restricted cash, beginning of period		227,126	78,357	[1]
Cash, cash equivalents and restricted cash, end of period		130,751	64,843	[1]
SUPPLEMENTAL CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end		$ 44	438	[1]
ASU 2016-09 [Member]
SUPPLEMENTAL CASH FLOW INFORMATION:
Cumulative adjustment related to the adoption	[1]		$ 115
[1]	Revised as a result of the adoption of ASU 2016-18

Overview

Overview	3 Months Ended
	Mar. 31, 2018
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 clinical-stage biopharmaceutical company that is committed to rewriting treatment for cancer and other serious diseases through discovering, developing, and commercializing 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 broadly developing its lead product candidate, tazemetostat, an oral, first-in-class selective inhibitor of the EZH2 histone methyltransferase, or HMT, in a range of cancer types and settings, and developing the lead development candidate in the Company’s novel G9a program, EZM8266, for the treatment of sickle cell disease, or SCD. Through March 31, 2018, the Company has raised, including amounts received under collaboration agreements, an aggregate of $891.6 million to fund its operations, of which $217.8 million was non-equity funding through its collaboration agreements, $597.8 million was from the sale of common 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 March 31, 2018, the Company had $247.9 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 $497.2 million through March 31, 2018, and will require substantial additional capital to fund its research and development. 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	3 Months Ended
	Mar. 31, 2018
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, 2017, 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 March 31, 2018 and 2017 are referred to as the first quarter of 2018 and 2017, 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. Significant Accounting Policies During the quarter ended March 31, 2018, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers using the Recently Adopted Accounting Pronouncements Restricted Cash, 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 security balances. After considering the Company’s current research and development plans and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of March 31, 2018, 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. Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers modified retrospective transition method. Under this method, results for reporting periods beginning after January 1, 2018 are presented pursuant to ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance The Company has entered into collaboration and license agreements, which are within the scope of ASC 606, to discover, develop, manufacture and commercialize product candidates. The terms of these agreements typically contain multiple promises or obligations, which may include: (i) licenses, or options to obtain licenses, to compounds directed to specific HMT targets (referred to as “exclusive licenses”) and (ii) research and development activities to be performed on behalf of the collaboration partner related to the licensed HMT targets. Payments to the Company under these agreements may include non-refundable license fees, customer option exercise fees, payments for research activities, reimbursement of certain costs, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. The Company first evaluates license and/or collaboration arrangements to determine whether the arrangement (or part of the arrangement) represents a collaborative arrangement pursuant to ASC Topic 808, Collaborative Arrangements, arrangement) , which represent a collaborative relationship and not a customer relationship, outside the scope of ASC 606. The Company’s collaborations primarily represent revenue Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as contract assets. Exclusive Licenses – If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, which generally include research and development services, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a license is distinct from the other promises, the Company considers relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining promise, whether the value of the license is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Research and Development Services – The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. For performance obligations that include research and development services, the Company generally recognizes revenue allocated to such performance obligations based on an appropriate measure of progress. The Company utilizes judgment to determine the appropriate method of measuring progress for purposes of recognizing revenue, which is generally an input measure such as costs incurred. The Company evaluates the measure of progress each reporting period as described under above. Reimbursements from the partner that are the result of a collaborative relationship with the partner, instead of a customer relationship, such as co-development activities, are recorded as a reduction to research and development expense. Customer Options – The Company’s arrangements may provide a collaborator with the right to select a target for licensing either at the inception of the arrangement or within an initial pre-defined selection period, which may, in certain cases, include the right of the collaborator to extend the selection period. Under these agreements, fees may be due to the Company (i) at the inception of the arrangement as an upfront fee or payment, (ii) upon the exercise of an option to acquire a license or (iii) upon extending the selection period as an extension fee or payment. If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the inception of the arrangement. The Company allocates the transaction price to material rights based on the relative stand-alone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised or expires. Milestone Payments – At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant revenue reversal would not occur. Royalties – For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 8, Collaborations Pending Accounting Pronouncements In February 2016, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2016-02, Leases (Topic 842) Recently Adopted Accounting Pronouncements Revenue Recognition In May 2014, the FASB, issued ASU, 2014-09, Revenue From Contracts With Customers . ASU 2014-09 amends Accounting Standards Codification, or ASC, 605, Revenue Recognition (“ASC 605”), by outlining a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers. In addition, the FASB issued ASUs 2016-10 and 2016-12, which provide clarifying amendments to ASU 2014-09. ASU 2014-09 and its related amendments will be effective for the Company for interim and annual periods beginning after December 15, 2017. The new standards are codified under ASC 606, Revenue From Contracts with Customers (“ASC 606”). As a result of adopting ASC 606 on January 1, 2018, the Company recorded a cumulative-effect credit to opening accumulated deficit of $25.0 million as of January 1, 2018 and a corresponding decrease to deferred revenue, net of current portion. No revenue for the three months ended March 31, 2018 was recognized in accordance with ASC 606. There was no impact to revenue recognized for the three months ended March 31, 2018 as a result of the adoption of ASC 606. Deferred revenue as of March 31, 2018 was $3.8 million under ASC 606, as compared to a balance of $28.9 million, which would have resulted under ASC 605. The cumulative-effect change relates principally to the Company’s treatment of option rights under its agreement with Celgene Corporation, or Celgene, and the identification of more performance obligations under ASC 606 in comparison with identified units of accounting under ASC 605. The adoption did not impact the previous accounting for the Company’s agreements with Glaxo Group Limited, or GSK and Eisai Co. Ltd., or Eisai. Pursuant to ASC 605, the Company had deemed Celgene’s options to license the three small molecule HMT inhibitors targeting three predefined targets, or the Option Targets, as non-substantive and therefore included the services that it would be required to perform upon option exercise as deliverables. ASC 606 provides that only options that are deemed to be material rights are a performance obligation and that any goods or services required upon exercise of the option be excluded from the evaluation of performance obligations until the option is exercised. As a result of this change to the guidance, (1) the pre-IND research services performed by the Company for each of the three Option Targets were deemed to be distinct performance obligations whereas each had previously been combined into one unit of accounting with the respective license that is subject to the exercise of the option and (2) a lesser amount of transaction price was allocated to the options. For further discussion of the change and the adoption of this standard, see Note 8, Collaborations Cash As of January 1, 2018, the Company adopted ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments As of January 1, 2018, the Company adopted ASU 2016-18, 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 March 31, 2018 2017 (In thousands) Cash and cash equivalents $ 130,289 $ 64,381 Restricted cash, as part of other assets 462 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 130,751 $ 64,843 The $0.5 million 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 consolidated balance sheet. There were no other material changes to the Company’s consolidated financial statements or disclosures. Share-Based Payment As of January 1, 2018, the Company adopted ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2018
Investments Debt And Equity Securities [Abstract]
Marketable Securities	3. Marketable Securities The following table summarizes the available-for-sale securities held at March 31, 2018 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 57,949 $ — $ (26 ) $ 57,923 Corporate notes 59,757 — (46 ) 59,711 U.S. government agency securities and U.S. Treasuries — — — — Total $ 117,706 $ — $ (72 ) $ 117,634 The following table summarizes the available-for-sale securities held at December 31, 2017 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 16,964 $ — $ (6 ) $ 16,958 Corporate notes 31,610 — (43 ) 31,567 U.S. government agency securities and U.S. Treasuries 1,250 — — 1,250 Total $ 49,824 $ — $ (49 ) $ 49,775 The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At March 31, 2018, 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 March 31, 2018, 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 March 31, 2018 was $73.6 million, which consisted of 8 commercial paper securities and 14 corporate notes securities. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of March 31, 2018 was less than $0.1 million. The aggregate fair value of available-for-sale securities held by the Company in an unrealized gain position for less than twelve months as of March 31, 2018 was $44.0 million, which consisted of 3 commercial paper securities and 7 corporate notes securities. The aggregate unrealized gain for those securities in an unrealized gain position for less than twelve months as of March 31, 2018 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 March 31, 2018. The weighted-average maturity of the Company’s portfolio was approximately three months at March 31, 2018.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements The Company’s financial instruments as of March 31, 2018 and December 31, 2017 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of March 31, 2018 and December 31, 2017, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of March 31, 2018 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 114,926 $ 64,853 $ 50,073 $ — Marketable securities: Commercial paper 57,923 — 57,923 — Corporate notes 59,711 — 59,711 — U.S. government agency securities and treasuries — — — — Total $ 232,560 $ 64,853 $ 167,707 $ — Fair Value as of December 31, 2017 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 207,251 $ 207,251 $ — $ — Marketable securities: Commercial paper 16,958 — 16,958 — Corporate notes 31,567 — 31,567 — U.S. government agency securities and treasuries 1,250 — 1,250 — Total $ 257,026 $ 207,251 $ 49,775 $ — 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 the majority 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 of marketable securities and those cash equivalents classified within Level 2 of the fair value hierarchy.

Supplemental Balance Sheet Info

Supplemental Balance Sheet Information	3 Months Ended
	Mar. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Supplemental Balance Sheet Information	5. Supplemental Balance Sheet Information Accrued expenses consisted of the following: March 31, 2018 December 31, 2017 (In thousands) Employee compensation and benefits $ 2,179 $ 4,628 Research and development expenses 12,792 11,658 Professional services and other 2,431 1,263 Accrued expenses $ 17,402 $ 17,549

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	6. Income Taxes The Company did not record a federal or state income tax provision or benefit for the three months ended March 31, 2018 and 2017 due to the expected and known loss before income taxes to be incurred, or incurred, as applicable, for the years ended December 31, 2018 and 2017, 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. In accordance with SAB 118, the Company's preliminary estimate of the effects of the Tax Cuts and Jobs Act, or the

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies There have been no significant changes to the Company’s commitments and contingencies in the three months ended March 31, 2018, as compared to those disclosed in Note 7, Commitments and Contingencies

Collaborations

Collaborations	3 Months Ended
	Mar. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	8. Collaborations Celgene In April 2012, the Company entered into a collaboration and license agreement with 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 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 March 31, 2018. 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 (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 March 31, 2018, the Company had recognized revenue of $99.2 million under the agreement as collaboration revenue in the Company’s consolidated statements of operations and comprehensive loss and in accumulated deficit as a result of the cumulative-effect recognition upon adoption of ASC 606. The amounts received that have not yet been recognized as revenue, related to the material rights, are recorded in deferred revenue on the Company’s condensed consolidated balance sheet. Deferred revenue related to the agreement amounted to $3.8 million as of March 31, 2018, all of which is included in noncurrent liabilities. 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 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 $31.0 million of preclinical 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. As of March 31, 2018, for the two remaining targets, the Company is eligible to receive up to $70.0 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. The Company determined the next milestone that might be achieved under this agreement is for $8.0 million due at the first dosing of a patient for an undisclosed target under a Phase 1 study. 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 March 31, 2018, the Company has earned a total of $69.0 million under the GSK agreement, which the Company recognized as collaboration revenue in the condensed consolidated statements of operations and comprehensive loss, including $10.0 million of milestone revenue in the year ended December 31, 2017. The Company did not have any deferred revenue related to this agreement as of March 31, 2018 or December 31, 2017 and any future revenues will relate to any milestone payments and royalties received under the agreement with respect to the two remaining targets, if any. The future potential milestone payments were excluded from the transaction price for the GSK agreement, as all future milestone amounts were fully constrained. The Company will reevaluate the likelihood of achieving future milestones at the end of each reporting period. The remaining future milestone payments are related to performance obligations that have been satisfied. Therefore, if the risk of significant reversal is resolved, any future milestone revenue from the arrangement will be recognized as revenue in the period the risk is relieved. 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 Roche Molecular companion diagnostic agreement, 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 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. Companion Diagnostics 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. Before the additional amendment, the Company was responsible for the remaining development costs of $10.5 million due under the agreement. 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. The Company expects the remaining development costs under the amended agreement to be incurred and paid through 2019. 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.

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation Total stock-based compensation expense related to stock options, restricted stock units and shares issued under the employee stock purchase plan was $2.9 million and $2.8 million for the three months ended March 31, 2018 and 2017, respectively. Stock-based compensation expense is classified in the condensed consolidated statements of operations and comprehensive loss as follows: Three Months Ended March 31, 2018 2017 (In thousands) Research and development $ 1,125 $ 1,392 General and administrative 1,762 1,365 Total $ 2,887 $ 2,757 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 $10.39 and $8.20 per option for those options granted during the three months ended March 31, 2018 and 2017, respectively. Key assumptions used to apply this pricing model were as follows: Three Months Ended March 31 2018 2017 Risk-free interest rate 2.5 % 1.8 % Expected life of options 6.0 years 6.0 years Expected volatility of underlying stock 71.4 % 74.5 % Expected dividend yield 0.0 % 0.0 % The following is a summary of stock option activity for the three months ended March 31, 2018: 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, 2017 4,576 $ 14.57 Granted 1,625 16.06 Exercised (153 ) 9.82 Forfeited or expired (334 ) 19.42 Outstanding at March 31, 2018 5,714 $ 14.84 8.46 $ 22,348 Exercisable at March 31, 2018 1,761 $ 15.71 7.05 $ 8,488 As of March 31, 2018, there was $34.7 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 March 31, 2018, there were no restricted stock units outstanding.

Loss per Share

Loss per Share	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Loss per Share	10. Loss Per Share Basic and diluted loss per share allocable to common stockholders are computed as follows: Three Months Ended March 31, 2018 2017 (In thousands except per share data) Net loss $ (34,065 ) $ (32,522 ) Weighted average shares outstanding 69,386 58,219 Basic and diluted loss per share allocable to common stockholders $ (0.49 ) $ (0.56 ) 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 March 31, 2018 2017 (In thousands) Stock options 5,714 5,186 Unvested restricted stock units — 57 Shares issuable under employee stock purchase plan 7 16 5,721 5,259

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	11. Related Party Transactions Celgene has made a series of equity investments in the Company, owning 3,674,640 shares of common stock representing 5.3% of the Company’s outstanding common stock as of March 31, 2018. Refer to Note 8, Collaborations, The Company has received consulting and advisory services from a director. The Company paid this director approximately $190,000 and $0 for these services during the three months ended March 31, 2018 and March 31, 2017, respectively. Of these amounts, $72,000 and $48,000 of amounts due to the director were included in accrued expenses at March 31, 2018 and December 31, 2017, respectively.

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2018
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, 2017, 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 March 31, 2018 and 2017 are referred to as the first quarter of 2018 and 2017, 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.
Significant Accounting Policies	Significant Accounting Policies During the quarter ended March 31, 2018, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers using the Recently Adopted Accounting Pronouncements Restricted Cash, 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 security balances. After considering the Company’s current research and development plans and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of March 31, 2018, 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.
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers modified retrospective transition method. Under this method, results for reporting periods beginning after January 1, 2018 are presented pursuant to ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance The Company has entered into collaboration and license agreements, which are within the scope of ASC 606, to discover, develop, manufacture and commercialize product candidates. The terms of these agreements typically contain multiple promises or obligations, which may include: (i) licenses, or options to obtain licenses, to compounds directed to specific HMT targets (referred to as “exclusive licenses”) and (ii) research and development activities to be performed on behalf of the collaboration partner related to the licensed HMT targets. Payments to the Company under these agreements may include non-refundable license fees, customer option exercise fees, payments for research activities, reimbursement of certain costs, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. The Company first evaluates license and/or collaboration arrangements to determine whether the arrangement (or part of the arrangement) represents a collaborative arrangement pursuant to ASC Topic 808, Collaborative Arrangements, arrangement) , which represent a collaborative relationship and not a customer relationship, outside the scope of ASC 606. The Company’s collaborations primarily represent revenue Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as contract assets. Exclusive Licenses – If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, which generally include research and development services, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a license is distinct from the other promises, the Company considers relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining promise, whether the value of the license is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Research and Development Services – The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. For performance obligations that include research and development services, the Company generally recognizes revenue allocated to such performance obligations based on an appropriate measure of progress. The Company utilizes judgment to determine the appropriate method of measuring progress for purposes of recognizing revenue, which is generally an input measure such as costs incurred. The Company evaluates the measure of progress each reporting period as described under above. Reimbursements from the partner that are the result of a collaborative relationship with the partner, instead of a customer relationship, such as co-development activities, are recorded as a reduction to research and development expense. Customer Options – The Company’s arrangements may provide a collaborator with the right to select a target for licensing either at the inception of the arrangement or within an initial pre-defined selection period, which may, in certain cases, include the right of the collaborator to extend the selection period. Under these agreements, fees may be due to the Company (i) at the inception of the arrangement as an upfront fee or payment, (ii) upon the exercise of an option to acquire a license or (iii) upon extending the selection period as an extension fee or payment. If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the inception of the arrangement. The Company allocates the transaction price to material rights based on the relative stand-alone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised or expires. Milestone Payments – At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant revenue reversal would not occur. Royalties – For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 8, Collaborations
Pending Accounting Pronouncements	Pending Accounting Pronouncements In February 2016, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2016-02, Leases (Topic 842)
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements Revenue Recognition In May 2014, the FASB, issued ASU, 2014-09, Revenue From Contracts With Customers . ASU 2014-09 amends Accounting Standards Codification, or ASC, 605, Revenue Recognition (“ASC 605”), by outlining a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers. In addition, the FASB issued ASUs 2016-10 and 2016-12, which provide clarifying amendments to ASU 2014-09. ASU 2014-09 and its related amendments will be effective for the Company for interim and annual periods beginning after December 15, 2017. The new standards are codified under ASC 606, Revenue From Contracts with Customers (“ASC 606”). As a result of adopting ASC 606 on January 1, 2018, the Company recorded a cumulative-effect credit to opening accumulated deficit of $25.0 million as of January 1, 2018 and a corresponding decrease to deferred revenue, net of current portion. No revenue for the three months ended March 31, 2018 was recognized in accordance with ASC 606. There was no impact to revenue recognized for the three months ended March 31, 2018 as a result of the adoption of ASC 606. Deferred revenue as of March 31, 2018 was $3.8 million under ASC 606, as compared to a balance of $28.9 million, which would have resulted under ASC 605. The cumulative-effect change relates principally to the Company’s treatment of option rights under its agreement with Celgene Corporation, or Celgene, and the identification of more performance obligations under ASC 606 in comparison with identified units of accounting under ASC 605. The adoption did not impact the previous accounting for the Company’s agreements with Glaxo Group Limited, or GSK and Eisai Co. Ltd., or Eisai. Pursuant to ASC 605, the Company had deemed Celgene’s options to license the three small molecule HMT inhibitors targeting three predefined targets, or the Option Targets, as non-substantive and therefore included the services that it would be required to perform upon option exercise as deliverables. ASC 606 provides that only options that are deemed to be material rights are a performance obligation and that any goods or services required upon exercise of the option be excluded from the evaluation of performance obligations until the option is exercised. As a result of this change to the guidance, (1) the pre-IND research services performed by the Company for each of the three Option Targets were deemed to be distinct performance obligations whereas each had previously been combined into one unit of accounting with the respective license that is subject to the exercise of the option and (2) a lesser amount of transaction price was allocated to the options. For further discussion of the change and the adoption of this standard, see Note 8, Collaborations
Cash	Cash As of January 1, 2018, the Company adopted ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments As of January 1, 2018, the Company adopted ASU 2016-18, 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 March 31, 2018 2017 (In thousands) Cash and cash equivalents $ 130,289 $ 64,381 Restricted cash, as part of other assets 462 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 130,751 $ 64,843 The $0.5 million 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 consolidated balance sheet. There were no other material changes to the Company’s consolidated financial statements or disclosures.
Share-Based Payment	Share-Based Payment As of January 1, 2018, the Company adopted ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [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 March 31, 2018 2017 (In thousands) Cash and cash equivalents $ 130,289 $ 64,381 Restricted cash, as part of other assets 462 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 130,751 $ 64,843

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2018
Investments Debt And Equity Securities [Abstract]
Summary of Available-for-Sale Securities Held	The following table summarizes the available-for-sale securities held at March 31, 2018 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 57,949 $ — $ (26 ) $ 57,923 Corporate notes 59,757 — (46 ) 59,711 U.S. government agency securities and U.S. Treasuries — — — — Total $ 117,706 $ — $ (72 ) $ 117,634 The following table summarizes the available-for-sale securities held at December 31, 2017 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 16,964 $ — $ (6 ) $ 16,958 Corporate notes 31,610 — (43 ) 31,567 U.S. government agency securities and U.S. Treasuries 1,250 — — 1,250 Total $ 49,824 $ — $ (49 ) $ 49,775

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of March 31, 2018 and December 31, 2017, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of March 31, 2018 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 114,926 $ 64,853 $ 50,073 $ — Marketable securities: Commercial paper 57,923 — 57,923 — Corporate notes 59,711 — 59,711 — U.S. government agency securities and treasuries — — — — Total $ 232,560 $ 64,853 $ 167,707 $ — Fair Value as of December 31, 2017 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 207,251 $ 207,251 $ — $ — Marketable securities: Commercial paper 16,958 — 16,958 — Corporate notes 31,567 — 31,567 — U.S. government agency securities and treasuries 1,250 — 1,250 — Total $ 257,026 $ 207,251 $ 49,775 $ —

Supplemental Balance Sheet In21

Supplemental Balance Sheet Information (Tables)	3 Months Ended
	Mar. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: March 31, 2018 December 31, 2017 (In thousands) Employee compensation and benefits $ 2,179 $ 4,628 Research and development expenses 12,792 11,658 Professional services and other 2,431 1,263 Accrued expenses $ 17,402 $ 17,549

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2018
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 March 31, 2018 2017 (In thousands) Research and development $ 1,125 $ 1,392 General and administrative 1,762 1,365 Total $ 2,887 $ 2,757
Assumptions Used in Applying Pricing Model	Key assumptions used to apply this pricing model were as follows: Three Months Ended March 31 2018 2017 Risk-free interest rate 2.5 % 1.8 % Expected life of options 6.0 years 6.0 years Expected volatility of underlying stock 71.4 % 74.5 % Expected dividend yield 0.0 % 0.0 %
Summary of Stock Option Activity	The following is a summary of stock option activity for the three months ended March 31, 2018: 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, 2017 4,576 $ 14.57 Granted 1,625 16.06 Exercised (153 ) 9.82 Forfeited or expired (334 ) 19.42 Outstanding at March 31, 2018 5,714 $ 14.84 8.46 $ 22,348 Exercisable at March 31, 2018 1,761 $ 15.71 7.05 $ 8,488

Loss per Share (Tables)

Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2018
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 March 31, 2018 2017 (In thousands except per share data) Net loss $ (34,065 ) $ (32,522 ) Weighted average shares outstanding 69,386 58,219 Basic and diluted loss per share allocable to common stockholders $ (0.49 ) $ (0.56 )
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 March 31, 2018 2017 (In thousands) Stock options 5,714 5,186 Unvested restricted stock units — 57 Shares issuable under employee stock purchase plan 7 16 5,721 5,259

Overview - Additional Informati

Overview - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Basis Of Presentation [Line Items]
Proceeds from sale of redeemable convertible preferred stock	$ 76,000
Initial public offering completion date	2013-05
Cash, cash equivalents, and marketable securities	$ 247,900
Accumulated deficit	(497,159)	$ (488,097)
Collaborative Arrangement [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount	891,600
Non-equity funding through collaboration agreement	217,800
IPO [Member]
Basis Of Presentation [Line Items]
Sale of common stock in public offering	$ 597,800

Summary of Significant Accoun25

Summary of Significant Accounting Policies - Additional Information (Detail)	Jul. 08, 2015OptionTarget	Mar. 31, 2018USD ($)OptionTargetAccountingUnitshares	Jan. 01, 2018USD ($)	Dec. 31, 2017USD ($)
Accounting Policies [Line Items]
Deferred revenue				$ 28,900,000
Letter of Credit [Member]
Accounting Policies [Line Items]
Security deposit		$ 500,000
Celgene [Member]
Accounting Policies [Line Items]
Deferred revenue		$ 3,800,000
Number of option targets | OptionTarget	3	3
Number of accounting units | AccountingUnit		1
ASC 606 [Member]
Accounting Policies [Line Items]
Cumulative-effect credit to opening accumulated deficit			$ 25,000,000
Revenue recognized		$ 0
Impact on revenue recognized		0
Deferred revenue		$ 3,800,000
ASU 2017-09 [Member]
Accounting Policies [Line Items]
Stock-based compensation expense awards modified | shares		0

Summary of Significant Accoun26

Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017		Dec. 31, 2016	[1]
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 130,289	$ 226,664	$ 64,381
Restricted cash, as part of other assets	462		462
Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows	$ 130,751	$ 227,126	$ 64,843	[1]	$ 78,357
[1]	Revised as a result of the adoption of ASU 2016-18

Marketable Securities - Summary

Marketable Securities - Summary of Available-for-Sale Securities Held (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 117,706	$ 49,824
Available-For-Sale Securities Held, Unrealized Losses	(72)	(49)
Available-For-Sale Securities Held, Fair Value	117,634	49,775
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	57,949	16,964
Available-For-Sale Securities Held, Unrealized Losses	(26)	(6)
Available-For-Sale Securities Held, Fair Value	57,923	16,958
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	59,757	31,610
Available-For-Sale Securities Held, Unrealized Losses	(46)	(43)
Available-For-Sale Securities Held, Fair Value	$ 59,711	31,567
U.S. Government Agency Securities and Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost		1,250
Available-For-Sale Securities Held, Fair Value		$ 1,250

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018USD ($)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	73,600,000
Available-for-sale securities, continuous unrealized gain position, less than twelve months, fair value	$ 44,000,000
Weighted average maturity period	3 months
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss	$ 100,000
Available-for-sale securities, continuous unrealized gain position, 12 months or longer, aggregate loss	$ 100,000
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in unrealized loss position | Security	8
Number of securities in unrealized gain position | Security	3
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in unrealized loss position | Security	14
Number of securities in unrealized gain position | Security	7

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 114,926	$ 207,251
Total	232,560	257,026
Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	57,923	16,958
Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	59,711	31,567
U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities		1,250
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	64,853	207,251
Total	64,853	207,251
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	50,073
Total	167,707	49,775
Level 2 [Member] | Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	57,923	16,958
Level 2 [Member] | Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	$ 59,711	31,567
Level 2 [Member] | U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities		$ 1,250

Supplemental Balance Sheet In30

Supplemental Balance Sheet Information - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Employee compensation and benefits	$ 2,179	$ 4,628
Research and development expenses	12,792	11,658
Professional services and other	2,431	1,263
Accrued expenses	$ 17,402	$ 17,549

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Federal income tax provision or benefit	$ 0	$ 0
State income tax provision or benefit	0	$ 0
Tax Cuts and Jobs Act of 2017, adjustments to provisional amounts	$ 0

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Jul. 08, 2015OptionTarget	Jan. 01, 2011OptionTarget	May 31, 2017USD ($)	Mar. 31, 2015USD ($)	Mar. 31, 2014USD ($)	Mar. 31, 2018USD ($)OptionTargetSeparatePerformanceObligationPerformanceObligationMaterialRight	Dec. 31, 2017USD ($)	Dec. 31, 2013USD ($)	Mar. 31, 2018USD ($)	Mar. 31, 2018USD ($)
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Deferred revenue							$ 28,900,000
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Notice period in days						90 days
DOT1L [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Clinical development milestone payment						$ 35,000,000			$ 35,000,000	$ 35,000,000
DOT1L [Member] | Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments						100,000,000			100,000,000	100,000,000
DOT1l Performance Obligations [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Total transaction price						$ 65,100,000
Number of performance obligations | PerformanceObligation						2
Pre-IND Research Services Performance Obligations [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Total transaction price						$ 34,100,000
Number of performance obligations | PerformanceObligation						3
Material Rights [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Total transaction price						$ 3,800,000
Number of material rights | MaterialRight						3
Celgene [Member]
Revenue Recognition, Multiple-deliverable Arrangements [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					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
Celgene [Member] | Minimum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Option targets under amended and restated agreement extended period	3 years
Celgene [Member] | United States [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets | OptionTarget	2
Celgene [Member] | Non-US [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets | OptionTarget	1
Celgene [Member] | DOT1L [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Clinical development milestone achieved						25,000,000
Clinical development milestone payment						35,000,000			35,000,000	35,000,000
Additional milestone payments						100,000,000			100,000,000	100,000,000
Celgene [Member] | Available Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments						100,000,000			100,000,000	100,000,000
Additional payments						65,000,000			65,000,000	65,000,000
Celgene [Member] | Option Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Company received upfront payment						10,000,000
Celgene [Member] | Option Targets [Member] | Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments						365,000,000			365,000,000	365,000,000
Development milestone and license payments associated with the Option Targets						75,000,000
Sales-based milestone payments						170,000,000			170,000,000	170,000,000
GSK [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment received					$ 3,000,000
Clinical development milestone achieved			$ 6,000,000
Clinical development milestone payment						70,000,000			70,000,000	70,000,000
Additional milestone payments						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
Collaboration revenue							10,000,000
Deferred revenue						0	0		0	0
Fixed research funding received										6,000,000
Milestone payments received										31,000,000
Research and development services										9,000,000
Preclinical and research and development milestone payments received			$ 10,000,000
Additional payments received						0
Additional milestone payments might be acheived						8,000,000			8,000,000	8,000,000
Cash and accounts receivable										69,000,000
GSK [Member] | Collaboration Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment received								$ 20,000,000
Eisai [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment made				$ 40,000,000
Clinical development milestone payments obligation						20,000,000			20,000,000	20,000,000
Regulatory milestone payments obligation						50,000,000			50,000,000	50,000,000
Eisai [Member] | First Submission of NDA or MAA [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Clinical development milestone payments obligation						10,000,000			10,000,000	10,000,000
Regulatory milestone payments obligation						25,000,000			25,000,000	25,000,000
Collaborative Arrangement [Member] | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments						10,400,000	$ 10,500,000		10,400,000	10,400,000
Reimbursements receivable of development costs						$ 900,000			$ 900,000	$ 900,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options, restricted stock and shares issued	$ 2,887	$ 2,757	[1]
Weighted-average fair value of options granted	$ 10.39	$ 8.20
Unrecognized compensation cost related to stock options	$ 34,700
Expected weighted average period for recognition of compensation cost	2 years 11 months 19 days
Unvested Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding	0
[1]	Revised as a result of the adoption of ASU 2016-18

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 2,887	$ 2,757	[1]
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	1,125	1,392
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 1,762	$ 1,365
[1]	Revised as a result of the adoption of ASU 2016-18

Stock-Based Compensation - Assu

Stock-Based Compensation - Assumptions Used in Applying Pricing Model (Detail) - Employee Stock Option [Member]	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.50%	1.80%
Expected life of options	6 years	6 years
Expected volatility of underlying stock	71.40%	74.50%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018USD ($)$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding, Beginning balance | shares	4,576,000
Number of Options, Granted | shares	1,625,000
Number of Options, Exercised | shares	(153,000)
Number of Options, Forfeited or expired | shares	(334,000)
Number of Options, Outstanding, Ending balance | shares	5,714,000
Number of Options, Exercisable | shares	1,761,000
Weighted Average Exercise Price per Share, Outstanding, Beginning balance | $ / shares	$ 14.57
Weighted Average Exercise Price per Share, Granted | $ / shares	16.06
Weighted Average Exercise Price per Share, Exercised | $ / shares	9.82
Weighted Average Exercise Price per Share, Forfeited or expired | $ / shares	19.42
Weighted Average Exercise Price per Share, Outstanding, Ending balance | $ / shares	14.84
Weighted Average Exercise Price per Share, Exercisable | $ / shares	$ 15.71
Weighted Average Remaining Contractual Term (In Years), Outstanding	8 years 5 months 15 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	7 years 18 days
Aggregate Intrinsic Value, Outstanding | $	$ 22,348
Aggregate Intrinsic Value, Exercisable | $	$ 8,488

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
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net loss	$ (34,065)	$ (32,522)	[1]
Weighted average shares outstanding	69,386	58,219
Basic and diluted loss per share allocable to common stockholders	$ (0.49)	$ (0.56)
[1]	Revised as a result of the adoption of ASU 2016-18

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
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	5,721	5,259
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	5,714	5,186
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		57
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	7	16

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2013
Related Party Transaction [Line Items]
Equity investments in common stock	69,486,000		69,302,000
Director [Member]
Related Party Transaction [Line Items]
Amounts paid to director for consulting and advisory services	$ 190,000	$ 0
Director [Member] | Accrued Expenses [Member]
Related Party Transaction [Line Items]
Amounts due to director	$ 72,000	$ 48,000
Beneficial Owner [Member]
Related Party Transaction [Line Items]
Equity investments in common stock				3,674,640
Equity investments in outstanding common stock percentage	5.30%