Epizyme (EPZM) 10-K 13 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Mar. 01, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	EPZM
Entity Registrant Name	EPIZYME, INC.
Entity Central Index Key	1,571,498
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		69,440,282
Entity Public Float			$ 673.1

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 226,664	$ 77,895
Marketable securities	49,775	164,297
Accounts receivable	382	23
Prepaid expenses and other current assets (Note 4)	8,983	6,457
Total current assets	285,804	248,672
Property and equipment, net (Note 3)	2,527	3,124
Restricted cash and other assets	1,028	645
Total assets	289,359	252,441
Current liabilities:
Accounts payable	7,001	4,994
Accrued expenses (Note 5)	17,549	16,007
Current portion of capital lease obligation	110	620
Other current liabilities	4
Total current liabilities	24,664	21,621
Capital lease obligation, net of current portion		110
Deferred revenue, net of current portion	28,809	28,809
Other long-term liabilities	515	201
Commitments and contingencies (Note 7)
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; 69,302 shares and 58,050 shares issued and outstanding, respectively	7	6
Additional paid-in capital	723,510	555,473
Accumulated other comprehensive loss	(49)	(106)
Accumulated deficit	(488,097)	(353,673)
Total stockholders’ equity	235,371	201,700
Total liabilities and stockholders’ equity	$ 289,359	$ 252,441

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
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	69,302,000	58,050,000
Common stock, shares outstanding	69,302,000	58,050,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 10,000,000	$ 8,007,000	$ 2,560,000
Operating expenses:
Research and development	109,661,000	91,461,000	111,209,000
General and administrative	37,181,000	28,372,000	23,900,000
Total operating expenses	146,842,000	119,833,000	135,109,000
Operating loss	(136,842,000)	(111,826,000)	(132,549,000)
Other income, net:
Interest income, net	2,165,000	1,531,000	94,000
Other income, net	32,000	83,000	79,000
Other income, net	2,197,000	1,614,000	173,000
Loss before income taxes	(134,645,000)	(110,212,000)	(132,376,000)
Income tax benefit	336,000	0	0
Net loss	(134,309,000)	(110,212,000)	(132,376,000)
Other comprehensive income (loss):
Unrealized gain on available for sale securities	57,000	(106,000)
Comprehensive loss	$ (134,252,000)	$ (110,318,000)	$ (132,376,000)
Loss per share allocable to common stockholders:
Basic	$ (2.18)	$ (1.93)	$ (3.32)
Diluted	$ (2.18)	$ (1.93)	$ (3.32)
Weighted average shares outstanding:
Basic	61,471	57,126	39,839
Diluted	61,471	57,126	39,839

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (134,309,000)	$ (110,212,000)	$ (132,376,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Acquired in-process research and development			40,000,000
Depreciation and amortization	1,639,000	1,589,000	1,419,000
Stock-based compensation	11,431,000	10,568,000	9,849,000
Amortization of discount on investments	265,000	(51,000)
Loss on disposal of property and equipment			27,000
Deferred income taxes	(368,000)
Changes in operating assets and liabilities:
Accounts receivable	(359,000)	239,000	1,813,000
Prepaid expenses and other current assets	(2,525,000)	(1,542,000)	(1,638,000)
Accounts payable	1,967,000	341,000	(3,647,000)
Accrued expenses	1,523,000	4,673,000	4,292,000
Deferred revenue		(1,900,000)	7,558,000
Restricted cash and other assets	(17,000)	106,000	(178,000)
Other long-term liabilities	320,000	(182,000)	(44,000)
Net cash used in operating activities	(120,433,000)	(96,371,000)	(72,925,000)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of available for sale securities	(126,356,000)	(229,887,000)
Proceeds from sales/maturities of available for sale securities	240,670,000	65,097,000
Acquisition of in-process research and development			(40,000,000)
Purchases of property and equipment	(984,000)	(624,000)	(183,000)
Net cash provided by (used in) investing activities	113,330,000	(165,414,000)	(40,183,000)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from public offering of common stock, net of commissions	152,922,000	130,438,000	130,712,000
Payment of common stock offering costs	(388,000)	(483,000)	(367,000)
Payment under capital lease obligation	(620,000)	(561,000)	(441,000)
Proceeds from stock options exercised	3,281,000	1,589,000	996,000
Issuance of shares under employee stock purchase plan	677,000	374,000	436,000
Net cash provided by financing activities	155,872,000	131,357,000	131,336,000
Net (decrease) increase in cash and cash equivalents	148,769,000	(130,428,000)	18,228,000
Cash and cash equivalents, beginning of period	77,895,000	208,323,000	190,095,000
Cash and cash equivalents, end of period	226,664,000	$ 77,895,000	208,323,000
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Equipment acquired under capital lease			$ 1,732,000
Cumulative catch up related to the adoption of ASU 2016-09	115,000
Property and equipment included in accounts payable or accruals	58,000
Cash paid for income taxes	$ 33,000

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance, Value at Dec. 31, 2014	$ 160,282	$ 3	$ 271,364	$ (111,085)
Beginning Balance, Shares at Dec. 31, 2014		34,426,012
Issuance of common stock (net of commissions and offering costs), Value	130,345	$ 1	130,344
Issuance of common stock (net of commissions and offering costs), Shares		6,701,448
Exercise of stock options, Value	996		996
Exercise of stock options, Shares		634,760
Stock-based compensation	9,849		9,849
Issuance of shares under employee stock purchase plan, Value	436		436
Issuance of shares under employee stock purchase plan, Shares		23,554
Net loss	(132,376)			(132,376)
Ending Balance, Value at Dec. 31, 2015	169,532	$ 4	412,989	(243,461)
Ending Balance, Shares at Dec. 31, 2015		41,785,774
Issuance of common stock (net of commissions and offering costs), Value	129,955	$ 2	129,953
Issuance of common stock (net of commissions and offering costs), Shares		15,420,220
Exercise of stock options, Value	1,589		1,589
Exercise of stock options, Shares		788,097
Stock-based compensation	10,568		10,568
Issuance of shares under employee stock purchase plan, Value	374		374
Issuance of shares under employee stock purchase plan, Shares		56,189
Unrealized gain (loss) on available for sale securities	(106)				$ (106)
Net loss	(110,212)			(110,212)
Ending Balance, Value at Dec. 31, 2016	201,700	$ 6	555,473	(353,673)	(106)
Ending Balance, Shares at Dec. 31, 2016		58,050,280
Cumulative catch up related to the adoption of ASU 2016-09 (Note 2)			115	(115)
Issuance of common stock (net of commissions and offering costs), Value	152,534	$ 1	152,533
Issuance of common stock (net of commissions and offering costs), Shares		10,689,253
Exercise of stock options and vesting of restricted stock units, Value	$ 3,281		3,281
Exercise of stock options and vesting of restricted stock units, Shares		478,471
Exercise of stock options, Shares	459,000
Stock-based compensation	$ 11,431		11,431
Issuance of shares under employee stock purchase plan, Value	677		677
Issuance of shares under employee stock purchase plan, Shares		83,687
Unrealized gain (loss) on available for sale securities	57				57
Net loss	(134,309)			(134,309)
Ending Balance, Value at Dec. 31, 2017	$ 235,371	$ 7	$ 723,510	$ (488,097)	$ (49)
Ending Balance, Shares at Dec. 31, 2017		69,301,691

Consolidated Statements of Sto7

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Stockholders Equity [Abstract]
Issuance of common stock, commissions and offering costs	$ 388	$ 483	$ 367

The Company

The Company	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
The Company	1. The Company 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 (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. Through December 31, 2017, 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 December 31, 2017, the Company had $276.4 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 $488.1 million through December 31, 2017 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	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned, controlled subsidiary, Epizyme Securities Corporation. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation. Use of Estimates The preparation of these consolidated financial statements in accordance with accounting principles generally accepted in the United States requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities, as of the date of the consolidated financial statements, and the reported amounts of collaboration revenue and expenses during the reporting period. Actual results and outcomes may differ materially from management’s estimates, judgments and assumptions. Subsequent Events The Company considers events or transactions that occur after the balance sheet date but before the consolidated financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company evaluated all events and transactions through the date these financial statements were filed with the Securities and Exchange Commission. Cash and cash equivalents The Company considers all highly liquid securities with original final maturities of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents are comprised of funds in money market accounts, commercial paper and corporate notes. Marketable securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. The Company considers all available-for-sale securities, including those with maturity dates beyond 12 months, as available to support current operational liquidity needs and therefore classifies all securities with maturity dates beyond 90 days at the date of purchase as current assets within the consolidated balance sheets. Available-for-sale securities are maintained by the Company’s investment managers and may consist of commercial paper, high-grade corporate notes, U.S. Treasury securities, and U.S. government agency securities. Available-for-sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income (expense). The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2017 was $42,783,000, which consisted of 4 commercial paper securities, 14 corporate notes securities and 1 U.S. government agency security. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2016 was $134,860,000, which consisted of 11 commercial paper securities and 23 corporate notes securities. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, mark the investment to market through a charge to the Company’s statement of operations and comprehensive loss. 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 there was no material change in the credit risk of the above investments, and as a result, the Company determined it did not hold any investments with an other-than-temporary impairment as of December 31, 2017 and 2016. 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 following table summarizes the available for sale securities held at December 31, 2016 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 68,407 $ — $ (32 ) $ 68,375 Corporate notes 70,489 — (81 ) 70,408 U.S. government agency securities and U.S. Treasuries 25,507 7 — 25,514 Total $ 164,403 $ 7 $ (113 ) $ 164,297 The Company did not hold any available-for-sale securities prior to the second quarter of 2016. Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents within the consolidated balance sheets and are not included in the tables above. All marketable securities held at December 31, 2017 and 2016 have maturities of less than one year. The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2017, 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 year ended December 31, 2017 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 December 31, 2017 was $42.8 million. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of December 31, 2017 was less than $0.1 million. 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 December 31, 2017. The weighted-average maturity of the Company’s portfolio was approximately one month at December 31, 2017. Fair Value Measurements The Financial Accounting Standards Board, or FASB, Codification Topic 820, Fair Value Measurements and Disclosures, Level 1 Quoted prices in active markets for identical assets or liabilities. Level 2 Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. The Company’s financial instruments as of December 31, 2017 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. The Company’s financial instruments as of December 31, 2016 consisted primarily of cash and cash equivalents, marketable securities, accounts receivable and accounts payable. As of December 31, 2017 and December 31, 2016, the Company’s financial assets recognized at fair value consisted of the following: 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 $ — Fair Value as of December 31, 2016 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 62,854 $ 59,862 $ 2,992 $ — Marketable securities: Commercial paper 68,375 — 68,375 — Corporate notes 70,408 — 70,408 — U.S. government agency securities and treasuries 25,514 — 25,514 — Total $ 227,151 $ 59,862 $ 167,289 $ — 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 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. 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 December 31, 2017, 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. Accounts Receivable Accounts receivable are amounts due from collaboration partners as a result of research and development services provided, reimbursements under equally co-funded global development arrangements or milestones achieved but not yet paid. The Company considered the need for an allowance for doubtful accounts and has concluded that no allowance was needed as of December 31, 2017 or 2016, as the estimated risk of loss on its accounts receivable was determined to be minimal. Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk include cash, cash equivalents, marketable securities and accounts receivable. The Company attempts to minimize the risks related to cash, cash equivalents and marketable securities by working with highly rated financial institutions that invest in a broad and diverse range of financial instruments as defined by the Company. The Company has established guidelines relative to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. The Company maintains its funds in accordance with its investment policy, which defines allowable investments, specifies credit quality standards and is designed to limit the Company’s credit exposure to any single issuer. Accounts receivable represent amounts due from collaboration partners. The Company monitors economic conditions to identify facts or circumstances that may indicate that any of its accounts receivable are at risk of collection. Acquired In-Process Research and Development The Company records upfront payments that relate to the acquisition of a development-stage product candidate as research and development expense in the period in which they are incurred, provided that the acquired development-stage product candidate did not also include processes or activities that would constitute a business, the product candidate has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use. In the first quarter of 2015, the Company accounted for the upfront payment paid to Eisai Co., Ltd., (‘Eisai”), in connection with the amended and restated collaboration and license agreement, pursuant to which the Company reacquired the worldwide rights, excluding Japan, to tazemetostat, as in-process research and development expense. Property and Equipment The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives: Asset Category Useful Lives Laboratory equipment 5 - 10 years Office furniture and equipment 3 - 10 years Leasehold improvements 3 - 10 years or term of respective lease, if shorter Amortization of capital lease assets is included in depreciation expense. The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation, from the related accounts, and the resulting gain or loss is reflected in the results of operations. Impairment of Long-Lived Assets The Company reviews long-lived assets to be held and used, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying amount of the assets or asset group may not be recoverable. Evaluation of recoverability is based on an estimate of undiscounted future cash flows resulting from the use of the asset or asset group and its eventual disposition. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the asset or asset group, the assets are written down to their estimated fair values. No such impairments were recorded during 2017, 2016 or 2015. Income Taxes The Company records deferred income taxes to recognize the effect of temporary differences between tax and financial statement reporting. The Company calculates the deferred taxes using enacted tax rates expected to be in place when the temporary differences are realized and records a valuation allowance to reduce deferred tax assets if it is determined that it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company considers many factors when assessing the likelihood of future realization of deferred tax assets, including recent earnings results, expectations of future taxable income, carryforward periods available and other relevant factors. The Company records changes in the required valuation allowance in the period that the determination is made. The Company assesses its income tax positions and records tax benefits for all years subject to examination based upon management’s evaluation of the facts, circumstances and information available as of the reporting date. For those tax positions where it is more likely than not that a tax benefit will be sustained, the Company records the largest amount of tax benefit with a greater than 50.0% likelihood of being realized upon ultimate settlement with a taxing authority having full knowledge of all relevant information. For those income tax positions where it is not more likely than not that a tax benefit will be sustained, the Company does not recognize a tax benefit in the financial statements. The Company records interest and penalties related to uncertain tax positions, if applicable, as a component of income tax expense. Refer to Note 6, Income Taxes On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“TCJA”). This legislation makes broad and complex changes to the U.S. tax code, including, but not limited to, (i) reducing the U.S. federal statutory tax rate from 35% to 21%; (ii) eliminating the corporate alternative minimum tax (AMT) and changing how existing AMT credits can be realized; (iii) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017, and (iv) modifying the officer’s compensation limitation. The Company recognizes the effects of changes in tax law, including the TCJA, in the period the law is enacted. Accordingly, the effects of the TCJA have been recognized in the financial statements for the year ended December 31, 2017. Revenue Recognition The Company recognizes revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; delivery has occurred or services have been rendered; the Company’s price to the customer is fixed or determinable and collectability is reasonably assured. The Company has entered into collaboration and license agreements to discover, develop, manufacture and commercialize compounds directed to specific HMT targets. The terms of these agreements typically contain multiple deliverables, 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, option fees, exercise fees, payments for research activities, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. Multiple-Element Revenue Arrangements. The Company’s collaborations primarily represent multiple-element revenue arrangements. To account for these transactions, the Company determines the elements, or deliverables, included in the arrangement and allocates arrangement consideration to the various elements based on each element’s relative selling price. The identification of individual elements in a multiple-element arrangement and the estimation of the selling price of each element involves significant judgment, including consideration as to whether each delivered element has standalone value to the collaborator. The Company determines the estimated selling price for deliverables within each agreement using vendor-specific objective evidence (“VSOE”) of selling price, if available, or third party evidence of selling price if VSOE is not available, or the Company’s best estimate of selling price, if neither VSOE nor third party evidence is available. Determining the best estimate of selling price for a deliverable requires significant judgment. The Company typically uses its best estimate of a selling price to estimate the selling price for licenses to its proprietary technology, since it often does not have VSOE or third party evidence of selling price for these deliverables. In those circumstances where the Company applies its best estimate of selling price to determine the estimated selling price of a license to its proprietary technology, it considers market conditions as well as entity-specific factors, including those factors contemplated in negotiating the agreements as well as internally developed estimates that include assumptions related to the market opportunity, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating its best estimate of selling price, the Company evaluates whether changes in the key assumptions used to determine its best estimate of selling price will have a significant effect on the allocation of arrangement consideration between deliverables. The Company recognizes consideration allocated to an individual element when all other revenue recognition criteria are met for that element. The Company’s multiple-element revenue arrangements generally include the following: • Exclusive Licenses —The deliverables under the Company’s collaboration agreements generally include exclusive licenses to discover, develop, manufacture and commercialize compounds with respect to one or more specified HMT targets. To account for this element of the arrangement, management evaluates whether the exclusive license has standalone value from the undelivered elements to the collaboration partner based on the consideration of the relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner. The Company may recognize arrangement consideration allocated to licenses upon delivery of the license if facts and circumstances indicate that the license has standalone value from the undelivered elements, which generally include research and development services. The Company defers arrangement consideration allocated to licenses if facts and circumstances indicate that the delivered license does not have standalone value from the undelivered elements. The Company has determined that certain of its exclusive licenses lack standalone value apart from the related research and development services and is therefore recognizing collaboration revenue from non-refundable exclusive license fees on a straight-line basis over the contracted or estimated period of performance, which is generally the period over which the research and development services are to be provided. • Research and Development Services —The deliverables under the Company’s collaboration and license agreements generally include deliverables related to research and development services to be performed by the Company on behalf of the collaboration partner. As the provision of research and development services is a part of the Company’s central operations, when the Company is principally responsible for the performance of these services under the agreements, the Company recognizes revenue on a gross basis for research and development services as those services are performed. • Option Arrangements —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. The accounting for option arrangements is dependent on the nature of the options granted to the collaboration partner. Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the options to secure exclusive licenses. Factors that the Company considers in evaluating whether options are substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the options, the cost to exercise the options relative to the total upfront consideration and the additional financial commitments or economic penalties imposed on the collaborator as a result of exercising the options. For arrangements under which the option to secure licenses is considered substantive, the Company does not consider the licenses to be deliverables at the inception of the arrangement. For arrangements under which the option to secure licenses is not considered substantive, the Company considers the license to be a deliverable at the inception of the arrangement and, upon delivery of the license, would apply the multiple-element revenue arrangement criteria to the license and any other deliverables to determine the appropriate revenue recognition. None of the options to secure exclusive licenses included in the Company’s collaborative arrangements have been determined to be substantive. Milestone Revenue. The Company’s collaboration and license agreements generally include contingent milestone payments related to specified preclinical research and development milestones, clinical development milestones, regulatory milestones and sales-based milestones. Preclinical research and development milestones are typically payable upon the selection of a compound candidate for the next stage of research and development. Clinical development milestones are typically payable when a product candidate initiates or advances in clinical trial phases or achieves defined clinical events such as proof-of-concept. Regulatory milestones are typically payable upon submission for marketing approval with regulatory authorities or upon receipt of actual marketing approvals for a compound, approvals for additional indications, or upon the first commercial sale. Sales-based milestones are typically payable when annual sales reach specified levels. At the inception of each arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether (a) the consideration is commensurate with either (i) the entity’s performance to achieve the milestone or (ii) the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the entity’s performance to achieve the milestone; (b) the consideration relates solely to past performance; and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. The Company generally considers non-refundable preclinical research and development, clinical development and regulatory milestones that the Company expects to be achieved as a result of the Company’s efforts during the period of the Company’s performance obligations under the collaboration and license agreements to be substantive and recognizes them as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met. If not considered to be substantive, the Company initially defers milestones and recognizes them over the remaining term of the Company’s performance obligations. Milestones that are not considered substantive because the Company does not contribute effort to the achievement of such milestones are generally achieved after the period of the Company’s performance obligations and are recognized as revenue upon achievement, assuming all other revenue recognition criteria are met, as there are no undelivered elements remaining and no continuing performance obligations. The Company is required to adopt new revenue accounting guidance in the first quarter of 2018, which it expects will impact this policy, the amount and timing of future revenue recognition, and the amount of revenue previously recognized in our published financial statements. For further discussion, see Recent Accounting Pronouncements Research and Development Expenses Research and development expenses are expensed as incurred. Research and development expenses are comprised of costs incurred in providing research and development activities, including salaries and benefits, facilities costs, overhead costs, contract research and development services, and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. External research and development expenses associated with the Company’s programs include clinical trial site costs, clinical manufacturing costs, costs incurred for consultants and other outside services, such as data management and statistical analysis support, and materials and supplies used in support of the clinical and preclinical programs. Internal costs of the Company’s clinical programs include salaries, stock-based compensation, and the portion of the Company’s facility costs allocated to research and development expense. When third-party service providers’ billing terms do not coincide with the Company’s period-end, the Company is required to make estimates of its obligations to those third parties, including clinical trial and pharmaceutical development costs, contractual services costs and costs for supply of its product candidates incurred in a given accounting period and record accruals at the end of the period. The Company bases its estimates on its knowledge of the research and development programs, services performed for the period, past history for related activities and the expected duration of the third-party service contract, where applicable. The Company generally accrues expenses related to research and development activities based on the services received and efforts expended pursuant to contracts with multiple contract research organizations that conduct and manage clinical trials, as well as other vendors that provide research and development services. The financial terms of these agreements are subject to negotiation, vary from contract to contract and may result in uneven payment flows. There may be instances in which payments made to vendors will exceed the level of services provided and result in a prepayment of the clinical expense. Payments under some of these contracts depend on factors such as the successful enrollment of subjects and the completion of clinical trial milestones. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from estimates, the Company would adjust the accrual or prepaid accordingly in future periods. Stock-Based Compensation The Company measures employee stock-based compensation based on the grant date fair value of the stock-based compensation award. The Company grants stock options at exercise prices equal to the fair value of the Company’s common stock on the date of grant, based on observable market prices. The Company recognizes employee stock-based compensation expense on a straight-line basis over the requisite service period of the awards. The Company recognizes forfeitures for employee and non-employee grants at the time they occur. The actual expense recognized over the vesting period will only represent those options that vest. Refer to Note 10, Employee Benefit Plans Earnings (Loss) per Share The Company computes basic earnings (loss) per share by dividing income (loss) allocable to common stockholders by the weighted average number of shares of common stock outstanding. During periods of income, the Company allocates participating securities a proportional share of income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities (the “two-class method”). The Company’s restricted stock and, prior to its automatic conversion, redeemable convertible preferred stock participate in any dividends declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, the Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The Company computes diluted earnings (loss) per share after giving consideration to the dilutive effect of stock options that are outstanding during the period, except where such non-participating securities would be anti-dilutive. Refer to

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, net	3. Property and Equipment, net Property and equipment, net consists of the following: December 31, 2017 2016 (In thousands) Laboratory equipment $ 4,138 $ 3,691 Computer and office equipment, furniture (1) 4,807 4,495 Leasehold improvements 354 464 Construction in progress 71 — Property and equipment 9,370 8,650 Less: accumulated depreciation and amortization (6,843 ) (5,526 ) Property and equipment, net $ 2,527 $ 3,124 (1) In 2015, the Company acquired $1.7 million of computer hardware and equipment, pursuant to a capital lease, the term of which expires in February 2018. Accumulated depreciation related to these assets totaled $1.6 million and $1.1 million as of December 31, 2017 and 2016, respectively. Depreciation and amortization expense was $1.6 million, $1.6 million and $1.4 million for the years ended December 31, 2017, 2016, and 2015, respectively.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Prepaid Expenses and Other Current Assets	4. Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following: December 31, 2017 2016 (In thousands) Prepaid clinical and manufacturing costs $ 5,724 $ 3,666 Interest receivable on available for sale securities 249 295 Other prepaid expenses and other receivables 3,010 2,496 Total prepaid expenses and other current assets $ 8,983 $ 6,457

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses consisted of the following: December 31, 2017 2016 (In thousands) Employee compensation and benefits $ 4,628 $ 4,100 Research and development expenses 11,658 10,925 Professional services and other 1,263 982 Accrued expenses $ 17,549 $ 16,007

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	6. Income Taxes The Company’s losses before income taxes consist solely of domestic losses. The provision for (benefit from) income taxes for the years ended December 31, 2017, 2016, and 2015 is as follows: 2017 2016 2015 (In thousands) Current $ 32.0 $ — $ — Deferred (368.0 ) — — Total (336.0 ) — — Income tax provision (benefit) $ (336.0 ) $ — $ — A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows: Year Ended December 31, 2017 2016 2015 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % State income taxes 4.8 4.5 5.2 Research and development and other tax credits 4.2 3.3 1.8 Permanent items (2.0 ) (1.9 ) (1.2 ) Change in valuation allowance 5.7 (40.1 ) (40.0 ) Return-to-provision adjustments (0.7 ) — — Change in deferred taxes — 0.2 0.2 Rate Change (45.7 ) — — Other — 0.0 — Effective income tax rate 0.3 % 0.0 % 0.0 % Deferred Tax Assets (Liabilities) The Company’s deferred tax assets (liabilities) consist of the following: December 31, 2017 2016 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 116,869 $ 94,390 Research and development and other credit carryforwards 24,257 18,055 Capitalized start-up costs 1,175 1,899 Capitalized research and development costs 36 154 Deferred revenue 7,766 11,231 Accruals and allowances 1,250 1,435 Eisai license payment 8,985 14,035 Other 4,702 5,607 Gross deferred tax assets 165,040 146,806 Deferred tax asset valuation allowance (164,607 ) (146,548 ) Total deferred tax assets 433 258 Deferred tax liabilities: Depreciation and other (65 ) (258 ) Total deferred tax liabilities (65 ) (258 ) Net deferred tax asset (liability) $ 368 $ — The Company evaluated the expected recoverability of our net deferred tax assets as of December 31, 2017 and determined that, with the exception of the deferred tax asset related to alternative minimum tax, or AMT, credits, there was insufficient positive evidence to support the recoverability of these net deferred tax assets, concluding it is more likely than not that our net deferred tax assets would not be realized in the future; therefore, the Company provided a full valuation allowance against its net deferred tax asset balance as of December 31, 2017, with the exception of the deferred tax asset related to the AMT credit. The AMT credit becomes refundable beginning in 2018 through no later than 2022 under the Tax Cuts and Jobs Act, or TCJA, tax reform legislation, and as such, the related deferred tax asset will be able to be realized and the corresponding valuation allowance of $368,000 has been reversed as of December 31, 2017 and recognized as tax benefit. The Company evaluated the expected recoverability of its net deferred tax assets as of December 31, 2016 and 2015 and determined that there was insufficient positive evidence to support the recoverability of these net deferred tax assets, concluding it is more likely than not that the Company’s net deferred tax assets would not be realized in the future; therefore, the Company provided a full valuation allowance against its net deferred tax asset balance as of December 31, 2016 and 2015 and did not recognize any tax benefit for the years ended December 31, 2016 and 2015. On December 22, 2017, the President of the United States signed into law the TCJA. The TCJA makes significant changes in U.S. tax law including a reduction in the corporate tax rates, changes to net operating loss carryforwards and carrybacks, and a repeal of the corporate alternative minimum tax. The TCJA reduced the U.S. corporate tax rate from the current rate of 34 percent down to 21 percent starting on January 1, 2018. As a result of the enacted law, the Company was required to revalue deferred tax assets and liabilities at 21 percent. This revaluation resulted in a provision of $61.6 million to income tax expense in continuing operations related to the reduction in the carrying value of the Company’s deferred tax assets, offset by a corresponding reduction in the valuation allowance. The Company’s preliminary estimate of the effects of the TCJA, including the remeasurement of deferred tax assets and liabilities and the evaluation of the recoverability of the deferred tax assets related to AMT credits, is subject to the finalization of management’s analysis related to certain matters, such as developing interpretations of the provisions of the TCJA, changes to certain estimates and the filing of the Company’s tax returns. U.S. Treasury regulations, administrative interpretations or court decisions interpreting the TCJA may require further adjustments and changes in the Company’s estimates. The final determination of the TCJA and the remeasurement of deferred assets and liabilities will be completed as additional information becomes available, but no later than one year from the enactment of the TCJA. In all cases, the Company will continue to make and refine its calculations as additional analysis is completed. In addition, the Company’s estimates may also be affected as it gains a more thorough understanding of the tax law. As of December 31, 2017, the Company had operating loss carryforwards of approximately $428.5 million and $428.7 million available to offset future taxable income for United States federal and state income tax purposes, respectively. The U.S. federal tax operating loss carryforwards expire commencing in 2029. The state tax operating loss carryforwards expire commencing in 2030. Additionally, as of December 31, 2017, the Company had research and development tax credit carryforwards of approximately $7.2 million and $2.5 million available to be used as a reduction of federal income taxes and state income taxes, respectively, which expire at various dates from 2028 through 2037, as well as federal orphan drug tax credit carryforwards of $14.7 million, which would expire at various dates from 2033 through 2037, and a $0.4 million federal alternative minimum tax credit carryforward. The Company’s ability to use its operating loss carryforwards and tax credits to offset future taxable income is subject to restrictions under Section 382 of the U.S. Internal Revenue Code, (“the Internal Revenue Code”). These restrictions may limit the future use of the operating loss carryforwards and tax credits if certain ownership changes described in the Internal Revenue Code occur. Future changes in stock ownership may occur that would create further limitations on the Company’s use of the operating loss carryforwards and tax credits. In such a situation, the Company may be required to pay income taxes, even though significant operating loss carryforwards and tax credits exist. Uncertain Tax Positions The following is a rollforward of the Company’s unrecognized tax benefits: December 31, 2017 2016 (In thousands) Unrecognized tax benefits - as of beginning of year $ 4,206 $ 3,480 Gross increases - tax positions of prior periods — — Gross increases - current period tax positions 1,017 726 Unrecognized tax benefits - as of end of year $ 5,223 $ 4,206 None of the Company’s unrecognized tax benefits would result in income tax expense or impact the Company’s effective tax rate if recognized. The Company had no accrued tax-related interest or penalties as of December 31, 2017 or 2016. The Company files income tax returns in the U.S. federal tax jurisdiction and Colorado, Indiana, Massachusetts, and North Carolina state tax jurisdictions. Since the Company is in a loss carryforward position, the Company is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Commitments The Company leases office and laboratory space at Technology Square in Cambridge, Massachusetts under a Lease Agreement, dated as of June 15, 2012, as amended (the “Lease”) with ARE-TECH Square, LLC, a Delaware limited liability company (the “Landlord”), with a term that originally continued through May 31, 2018, and a Company option to extend the term of the lease at the then-current market rent, as defined in the Lease, through November 30, 2022. In May 2017, the Company entered into a Third Amendment to Lease (the “Third Amendment”) with the Landlord, and a Fourth Amendment to Lease with the Landlord (the “Fourth Amendment,” and, together with the Third Amendment, the “Amendments”). Under the Amendments, the Company extended the term of the lease to November 30, 2022 but retained the right to terminate the Lease effective as of December 31, 2018, by giving written notice to the Landlord by December 31, 2017 and paying an early termination fee. The Company did not exercise this right. Under the Lease as amended, the Company has agreed to pay a monthly base rent of approximately $0.2 million for the period commencing December 1, 2017 through May 31, 2018, with an increase on June 1, 2018 and annual increases December 1 of each subsequent year until December 1, 2021. The Company has a $0.5 million letter of credit as a security deposit for this lease and has recorded cash held to secure this letter of credit as restricted cash in restricted cash and other assets on the consolidated balance sheet. The Company recognizes rent expense, including escalation charges, on a straight-line basis over the initial term of the lease agreement. In addition, the Company has a capital lease related to computer hardware equipment, an operating lease for storage space in Colorado and an operating lease for office space in North Carolina. Rent expense was $3.3 million, $2.8 million and $2.7 million for the years ended December 31, 2017, 2016, and 2015, respectively. The Company’s contractual commitments under these leases, excluding common area maintenance charges and real estate taxes, as of December 31, 2017 are as follows: Total 2018 2019 2020 2021 2022 (In thousands) Operating leases $ 17,262 $ 3,270 $ 3,516 $ 3,605 $ 3,538 $ 3,333 Capital lease, including amounts representing interest 111 111 — — — — Total commitments $ 17,373 $ 3,381 $ 3,516 $ 3,605 $ 3,538 $ 3,333 In addition to commitments under leasing arrangements, as of December 31, 2017, the Company has committed to $10.4 million of remaining development costs payable to Roche Molecular, expected to be paid through 2019 upon certain development and regulatory milestones, under the amended companion diagnostic agreement. Developmental costs of $1.5 million were paid in 2017 upon the achievement of a milestone under the companion diagnostic agreement with Roche Molecular. In addition, the contractual commitments table above does not include potential future milestones or royalties that the Company may be required to make under license and collaboration agreements, including potential future milestones or royalties payable to Eisai under the amended collaboration and license agreement, due to the uncertainty of the occurrence of the events requiring payment under these agreements. Refer to Note 9, Collaborations Additionally, the Company enters into contracts in the normal course of business with clinical research organizations for clinical and preclinical research studies, external manufacturers for product for use in clinical trials, and other research supplies and other services as part of the Company’s operations. These contracts generally provide for termination on notice, and therefore are cancelable contracts and not included in the contractual commitments table above.

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Stockholders' (Deficit) Equity	8. Stockholders’ (Deficit) Equity Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to dividends when and if declared by the board of directors. In September 2017 and January 2016, the Company issued 10,557,000 and 15,333,334 shares of Common Stock, respectively, in connection with public offerings. The issuance of these shares contributed to significant increases in the Company’s shares outstanding as of December 31, 2017 and 2016 and in the weighted average shares outstanding for the years ended December 31, 2017 and 2016 when compared to the comparable prior year periods. The Company sold 132,253 shares and 23,581 shares, of Common Stock during the years ended December 31, 2017 and December 31, 2016, respectively, under an “at the market” program (“ATM Facility”) with Cowen and Company, LLC (“Cowen”) acting as sales agent under a sales agreement that the Company and Cowen entered into in April 2016. Cowen was compensated at a fixed commission rate of 3.0%. Transactions under the ATM Facility resulted in net proceeds of $1.6 million and $0.3 million in the years ended December 31, 2017 and 2016, respectively. The Company also incurred other issuance related costs of $0.1 million associated with the ATM Facility in the fourth quarter of 2016, which have been accounted for as an offset to additional paid in capital. Through March 10, 2017, including sales in the year ended December 31, 2016, the Company sold 155,834 shares of Common Stock under the sales agreement with Cowen. The Company terminated the sales agreement with Cowen, effective March 10, 2017. As of December 31, 2017, a total of 8,890,201 shares of common stock were reserved for issuance upon (i) the exercise of outstanding stock options and vesting of restricted stock units and (ii) the issuance of stock awards under the Company’s 2013 Stock Incentive Plan and 2013 Employee Stock Purchase Plan.

Collaborations

Collaborations	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	9. 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 GlaxoSmithKline (“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 December 31, 2017. 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 determined that the amended and restated agreement represented a modification of the original agreement. Accordingly, the Company determined that the remaining deferred revenue of $21.6 million attributable to the original agreement should be combined with the $10.0 million upfront payment received from Celgene upon execution of the amended and restated agreement. This combined amount, along with any development milestone-based license fees associated with those Option Targets where Celgene’s option rights are determined to be non-substantive, will be recognized on a prospective basis after (i) identifying the deliverables and units of accounting included in the amended and restated agreement, (ii) allocating the total arrangement fee among the units of accounting and (iii) determining the revenue recognition for each unit of accounting. The Company evaluated the deliverables in the amended and restated agreement and determined that the deliverables included its obligation to provide research and development services for DOT1L and the three Option Targets. In addition, the Company concluded that Celgene’s option rights are not substantive and the licenses to HMT inhibitors targeting each Option Target were therefore considered to be deliverables. As a result, the Company has determined the deliverables in the amended and restated arrangement to be: • Research and development services for DOT1L and the three Option Targets • Licenses to HMT inhibitors targeting the Option Targets The Company evaluated the nature of the deliverables to determine the units of accounting. At a program level (that is, for DOT1L and each Option Target), the Company believes there is standalone value. Therefore, the identified deliverables were evaluated on a program by program basis to determine the units of accounting. The Company determined the research services related to DOT1L have stand-alone value as the services are delivered and therefore represent a single unit of accounting. There are no other deliverables related to the DOT1L program. For two of the Option Targets, the Company concluded the licenses and related research services should be accounted for as a combined unit of accounting up until an IND filing. Prior to an IND filing, the delivered research services do not have standalone value from the remaining undelivered elements because Celgene could not resell the research services and no other vendor has the specialized skills and know-how related to HMT inhibitors necessary to provide the research services. After an IND filing, for these two Option Targets, the Company concluded the delivered pre-IND research services and licenses, once delivered, would have standalone value from the remaining research services because Celgene, or other market participants, would have the ability to execute human clinical trials on the identified compound. After an IND filing, the remaining research services represent a separate unit of accounting which has standalone value upon delivery. With respect to the third Option Target where the Company retains U.S. rights, because the license terms restrict Celgene’s access to information through the completion of a Phase 1 trial that would otherwise be necessary for Celgene, or other market participants, to execute human clinical trials on the identified compound, the Company determined the licenses did not have standalone value apart from the research services Accordingly, for the third Option Target, the licenses and research services will be accounted for as a combined unit of accounting. Because Celgene’s option rights with respect to the Option Targets are not considered substantive, any development milestone-based license fees associated with the Option Targets would be considered to be part of the total consideration for purposes of allocating the arrangement consideration. Accordingly, the Company has identified the total allocable arrangement consideration to be $106.6 million, as follows: (i) the remaining $21.6 million of deferred revenue under the original agreement, (ii) the $10.0 million upfront payment made in connection with the amended and restated agreement, and (iii) the $75.0 million of aggregate development milestone-based license fees associated with the Option Targets. The allocable arrangement consideration has been allocated to the identified deliverables using the relative selling price method. The Company estimated the selling price of the license deliverable for each Option Target using management’s best estimate of selling price after considering potential future cash flows under each license and industry benchmarks on the probability of achieving development milestones. The Company then discounted these probability-weighted cash flows to their present value. The Company estimated the selling price of the research services to be provided in connection with the DOT1L Phase 1 clinical trial and related to each Option Target using management’s best estimate of selling price based on the Company’s estimated cost of providing the services plus an applicable profit margin of 10%, which is commensurate with observable market data for similar services. The Company expects to recognize the allocated arrangement consideration as follows, subject to the limitation described in ASC 605-25-30-5: • Research and development services for DOT1L: The arrangement consideration allocated to the DOT1L program was recognized as the research and development services were performed. The Company allocated $2.8 million to the remaining research services under the DOT1L program, and this amount was recognized through December 31, 2016, the period over which the related research services were provided. • Two Option Targets: The arrangement consideration allocated to these Option Targets of $70.2 million in the aggregate will be deferred until an IND filing. The Company did not recognize any revenue related to the Option Targets during 2017. • Third Option Target: The licenses and the research services provided through completion of a Phase 1 trial represent a single combined unit of accounting. Therefore, because a license is the last delivered item in this unit of accounting, the Company will defer all consideration allocated to this unit of accounting, $33.6 million, until the delivery of the license upon completion of a Phase 1 clinical trial. The Company did not recognize any revenue related to the Option Target during 2017. • If any of the programs are terminated in accordance with the amended and restated agreement, any remaining deferred revenue will be reallocated among the undelivered items based upon their relative selling prices. Collaboration Revenue Through December 31, 2017, the Company has recognized $74.3 million of total collaboration revenue since the inception of the collaboration, including $1.9 million and $1.1 million in the years ended December 31, 2016 and 2015, respectively. In addition, the Company has recognized $0.1 million and $1.1 million of total global development co-funding as a reduction to research and development expense in the years ended December 31, 2016 and 2015, respectively. The Company recognized no revenue and no global development co-funding under this collaboration for the year ended December 31, 2017. As of December 31, 2017 and December 31, 2016, the Company had deferred revenue of $28.8 million related to this agreement. 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 December 31, 2017, 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 Collaboration Revenue Through December 31, 2017, 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 and $6.0 million of milestone revenue in the year ended December 31, 2016. The Company did not have any deferred revenue related to this agreement as of December 31, 2017 or December 31, 2016 and any future revenues will relate to any milestone payments and royalties received under the agreement with respect to the two remaining targets. Eisai In April 2011, the Company entered into a collaboration and license agreement with 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. LYSA In May 2016, the Company entered into a collaboration agreement with the Lymphoma Academic Research Organisation, or LYSARC, for the first planned combination trial of tazemetostat. LYSARC is the operational arm of the Lymphoma Study Association, or LYSA, a premier cooperative group in France dedicated to clinical and translational research for lymphoma. This Phase 1b/2 study is evaluating tazemetostat in combination with R-CHOP, the standard of care first line combination treatment for diffuse large B-cell lymphoma, or DLBCL, as a first line treatment in elderly, high-risk patients with DLBCL and is being sponsored by LYSARC. LYSA is managing the study operations for the trial, and the Company is recognizing its share of the related expenses as those costs are incurred over the duration of the trial. Genentech In June 2016, the Company entered into a collaboration agreement with Genentech Inc. (“Genentech”), a member of the Roche Group, to conduct a Phase 1b clinical trial to investigate the anti-cancer effects of the Company’s EZH2 inhibitor, tazemetostat, and Genentech’s anti-PD-L1 cancer immunotherapy, atezolizumab, when used in combination. The trial is evaluating this combination regimen for the treatment of patients with relapsed or refractory DLBCL. Under the agreement, each company is supplying its respective anti-cancer agent to support the trial and sharing equally in the trial costs. Genentech is managing the study operations for the trial, and the Company is recognizing its share of the related expenses as those costs are incurred over the duration of the trial. In June 2017, the Company announced an expansion of the clinical collaboration with Genentech to investigate the combination of tazemetostat with atezolizumab in a Phase 1b/2 clinical trial for the treatment of patients with relapsed or refractory metastatic non-small cell lung cancer, or NSCLC. The trial will be part of MORPHEUS, Genentech’s open-label, multi-center, randomized umbrella trial evaluating the efficacy and safety of multiple immunotherapy-based treatment combinations for metastatic NSCLC. Under the agreement, each company is supplying its respective anti-cancer agent to support the trial and sharing equally in the trial costs. Genentech is managing the study operations for the trial, and the Company is recognizing its share of the related expenses as those costs are incurred over the duration of the trial. 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. As of December 31, 2017, the Company is responsible for the remaining development costs of $10.5 million due under the agreement. The agreement was further amended in March 2018. Under the amended agreement, the Company is responsible for remaining development costs of $10.4 million due under the agreement and Eisai has agreed to reimburse the Company $0.9 million of this amount related to a regulatory milestone for Japan. We expect 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.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Employee Benefit Plans	10. Employee Benefit Plans Stock Incentive Plans In 2008, the Company’s board of directors adopted and the Company’s stockholders approved the 2008 Stock Incentive Plan (the “2008 Plan”), which provided for the granting of certain defined stock incentive awards to employees, members of the Company’s board of directors and non-employee consultants, advisors or other service providers. In April 2013, the Company’s board of directors adopted and the Company’s stockholders approved the 2013 Stock Incentive Plan (the “2013 Plan”), which provides for the granting of certain defined stock incentive awards to employees, members of the Company’s board of directors and non-employee consultants, advisors or other service providers. Additionally, in May 2013, the Company’s board of directors adopted and the Company’s stockholders approved the 2013 Employee Stock Purchase Plan (the “2013 ESPP”), which provides participating employees the option to purchase shares of the Company’s common stock at defined purchase prices over six month offering periods. Stock incentive awards granted under the 2013 Plan may be incentive stock options, non-qualified stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock-based awards under the applicable provisions of the Internal Revenue Code. Incentive stock options are granted only to employees of the Company. Non-qualified stock options and restricted stock may be granted to officers, employees, consultants, advisors and other service providers. Incentive and non-qualified stock options and restricted stock granted to employees generally vest over four years, with 25.0% vesting upon the one-year anniversary of the grant and the remaining 75.0% vesting monthly over the following three years. Non-qualified stock options granted to consultants and other non-employees generally vest over the period of service to the Company. Initial non-qualified stock options granted to members of the Company’s board of directors generally vest over the recipient’s term of board service. Annual non-qualified stock options granted to members of the Company’s board of directors vest on the one-year anniversary of the grant. Incentive and non-qualified stock options expire ten years from the date of grant. Stock-Based Compensation Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2017 2016 2015 (In thousands) Research and development $ 5,613 $ 5,352 $ 5,155 General and administrative 5,818 5,216 4,694 Total $ 11,431 $ 10,568 $ 9,849 Stock Options The Company uses the Black-Scholes option-pricing model to measure the fair value of stock option awards. Key weighted average assumptions used in this pricing model on the date of grant for options granted to employees are as follows: Year Ended December 31, 2017 2016 2015 Risk-free interest rate 1.8 % 1.2 % 1.6 % Expected life of options 6.0 years 6.0 years 6.0 years Expected volatility of underlying stock 74.2 % 78.5 % 83.6 % Expected dividend yield 0.0 % 0.0 % 0.0 % There were no stock option awards granted to non-employees in the years ended December 31, 2017, 2016 or 2015. The risk-free interest rate is based upon the U.S. Treasury yield curve in effect at the time of grant, with a term that approximates the expected life of the option. The Company calculates the expected life of options granted to employees using the simplified method as the Company has insufficient historical information to provide a basis for estimate. The Company determines the expected volatility using a blended approach encompassing its historical experience and the historical volatility of a peer group of comparable publicly traded companies with product candidates in similar stages of development to the Company’s product candidates. The Company has applied an expected dividend yield of 0.0% as the Company has not historically declared a dividend and does not anticipate declaring a dividend during the expected life of the options. The following is a summary of stock option activity for the year ended December 31, 2017: 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, 2016 4,059 $ 14.32 Granted 2,332 13.50 Exercised (459 ) 7.15 Forfeited or expired (1,356 ) 14.49 Outstanding at December 31, 2017 4,576 $ 14.57 7.9 $ 5,814 Exercisable at December 31, 2017 1,640 $ 16.90 6.4 $ 3,379 During the years ended December 31, 2017, 2016 and 2015, the Company granted stock options to purchase an aggregate of 2,331,500 shares, 2,212,668 shares, and 1,289,755 shares, respectively, at weighted average grant date fair values per option share of $8.85, $6.42, and $14.57, respectively. The total grant date fair value of options that vested during the years ended December 31, 2017, 2016 and 2015 was $12.0 million, $12.0 million, and $9.3 million, respectively. The aggregate intrinsic value of stock options exercised was $4.1 million in 2017, $7.1 million in 2016 and $11.9 million in 2015. As of December 31, 2017, there was $22.5 million of unrecognized stock-based compensation related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.7 years. Restricted Stock The following is a summary of restricted stock unit activity for the year ended December 31, 2017: Number of Units Weighted Average Grant Date Fair Value per Unit (In thousands) Outstanding at December 31, 2016 64 $ 12.20 Granted — — Vested (20 ) 12.20 Forfeited (44 ) 12.20 Outstanding at December 31, 2017 — $ — In February 2016, the Company granted 80,732 restricted stock units with a grant date fair value of $9.29 per unit, in accordance with a former chief financial officer’s employment agreement. Of the total restricted stock awards granted, 73,779 shares vested through the termination of the officer’s employment in August 2017. As of December 31, 2017 there was no unrecognized compensation cost related to restricted stock units, which were cancelled upon termination for the remaining unvested awards. The intrinsic value of restricted stock that vested during the years ended December 31, 2017, 2016 and 2015 was 0.3 million, $0.5 million and $0.0 million, respectively. 401(k) Savings Plan The Company has a defined contribution 401(k) savings plan (the “401(k) Plan”). The 401(k) Plan covers substantially all employees, and allows participants to defer a portion of their annual compensation on a pretax basis. Company contributions to the 401(k) Plan may be made at the discretion of the board of directors. During the year ended December 31, 2014, the Company implemented a matching contribution to the 401(k) Plan, matching 50% of an employee’s contribution up to a maximum of 3% of the participant’s compensation. Company contributions to the 401(k) plan totaled $0.5 million, $0.4 million and $0.4 million in the years ended December 31, 2017, 2016 and 2015, respectively.

Loss per Share

Loss per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Loss per Share	11. Loss per Share As described in Note 2, Summary of Significant Accounting Policies Basic and diluted loss per share allocable to common stockholders are computed as follows: Year Ended December 31, 2017 2016 2015 (In thousands except per share data) Net loss $ (134,309 ) $ (110,212 ) $ (132,376 ) Weighted average shares outstanding 61,471 57,126 39,839 Basic and diluted loss per share allocable to common stockholders $ (2.18 ) $ (1.93 ) $ (3.32 ) 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 antidilutive: Year Ended December 31, 2017 2016 2015 (In thousands) Stock options 4,576 4,059 3,100 Unvested restricted stock — 64 37 Shares issuable under employee stock purchase plan 23 46 8 4,599 4,169 3,145

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	12. 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 December 31, 2017. Refer to Note 9, Collaborations, for additional information regarding the Company’s original agreement with Celgene entered into in April 2012 and the amended and restated agreement with Celgene entered into in July 2015. Under the Celgene collaboration agreement, the Company recognized $1.9 million and $1.1 million of collaboration revenue in the years ended December 31, 2016 and 2015, respectively. The Company recognized no revenue under the Celgene collaboration agreement for the year ended December 31, 2017 As of December 31, 2017 and 2016, the Company had recorded $28.8 million and $28.8 million of deferred revenue related to the Celgene collaboration arrangement, respectively. Additionally, in the years ended December 31, 2017, 2016 and 2015, the Company recorded $0.0 million, $0.1 million and $1.1 million, respectively, in global development co-funding from Celgene. As of December 31, 2017 and 2016, the Company had no accounts receivable and less than $0.1 million, respectively, related to this collaboration arrangement.

Unaudited Quarterly Results

Unaudited Quarterly Results	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Results	13. Unaudited Quarterly Results The results of operations on a quarterly basis for the years ended December 31, 2017 and 2016 are set forth below: Quarter Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 (In thousands, except per share data) Collaboration revenue $ — $ 10,000 $ — $ — Operating expenses: Research and development 24,695 27,292 28,741 28,933 General and administrative 8,269 11,170 9,311 8,431 Total operating expenses 32,964 38,462 38,052 37,364 Operating loss (32,964 ) (28,462 ) (38,052 ) (37,364 ) Other income, net 442 438 455 862 Income tax benefit — — — 336 Net loss $ (32,522 ) $ (28,024 ) $ (37,597 ) $ (36,166 ) Loss per share allocable to common stockholders: Basic $ (0.56 ) $ (0.48 ) $ (0.63 ) $ (0.52 ) Diluted $ (0.56 ) $ (0.48 ) $ (0.63 ) $ (0.52 ) Weighted average shares outstanding: Basic 58,219 58,377 59,899 69,287 Diluted 58,219 58,377 59,899 69,287 Quarter Ended March 31, 2016 June 30, 2016 September 30, 2016 December 31, 2016 (In thousands, except per share data) Collaboration revenue $ 472 $ 473 $ 6,584 $ 478 Operating expenses: Research and development 17,740 21,450 23,888 28,383 General and administrative 5,846 7,424 7,522 7,580 Total operating expenses 23,586 28,874 31,410 35,963 Operating loss (23,114 ) (28,401 ) (24,826 ) (35,485 ) Other income, net 235 420 490 469 Net loss $ (22,879 ) $ (27,981 ) $ (24,336 ) $ (35,016 ) Loss per share allocable to common stockholders: Basic $ (0.41 ) $ (0.49 ) $ (0.42 ) $ (0.60 ) Diluted $ (0.41 ) $ (0.49 ) $ (0.42 ) $ (0.60 ) Weighted average shares outstanding: Basic 55,149 57,352 57,970 58,016 Diluted 55,149 57,352 57,970 58,016

Summary of Significant Accoun21

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned, controlled subsidiary, Epizyme Securities Corporation. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of these consolidated financial statements in accordance with accounting principles generally accepted in the United States requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities, as of the date of the consolidated financial statements, and the reported amounts of collaboration revenue and expenses during the reporting period. Actual results and outcomes may differ materially from management’s estimates, judgments and assumptions.
Subsequent Events	Subsequent Events The Company considers events or transactions that occur after the balance sheet date but before the consolidated financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company evaluated all events and transactions through the date these financial statements were filed with the Securities and Exchange Commission.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid securities with original final maturities of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents are comprised of funds in money market accounts, commercial paper and corporate notes.
Marketable securities	Marketable securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. The Company considers all available-for-sale securities, including those with maturity dates beyond 12 months, as available to support current operational liquidity needs and therefore classifies all securities with maturity dates beyond 90 days at the date of purchase as current assets within the consolidated balance sheets. Available-for-sale securities are maintained by the Company’s investment managers and may consist of commercial paper, high-grade corporate notes, U.S. Treasury securities, and U.S. government agency securities. Available-for-sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income (expense). The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2017 was $42,783,000, which consisted of 4 commercial paper securities, 14 corporate notes securities and 1 U.S. government agency security. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2016 was $134,860,000, which consisted of 11 commercial paper securities and 23 corporate notes securities. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, mark the investment to market through a charge to the Company’s statement of operations and comprehensive loss. 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 there was no material change in the credit risk of the above investments, and as a result, the Company determined it did not hold any investments with an other-than-temporary impairment as of December 31, 2017 and 2016. 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 following table summarizes the available for sale securities held at December 31, 2016 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 68,407 $ — $ (32 ) $ 68,375 Corporate notes 70,489 — (81 ) 70,408 U.S. government agency securities and U.S. Treasuries 25,507 7 — 25,514 Total $ 164,403 $ 7 $ (113 ) $ 164,297 The Company did not hold any available-for-sale securities prior to the second quarter of 2016. Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents within the consolidated balance sheets and are not included in the tables above. All marketable securities held at December 31, 2017 and 2016 have maturities of less than one year. The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2017, 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 year ended December 31, 2017 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 December 31, 2017 was $42.8 million. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of December 31, 2017 was less than $0.1 million. 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 December 31, 2017. The weighted-average maturity of the Company’s portfolio was approximately one month at December 31, 2017.
Fair Value Measurements	Fair Value Measurements The Financial Accounting Standards Board, or FASB, Codification Topic 820, Fair Value Measurements and Disclosures, Level 1 Quoted prices in active markets for identical assets or liabilities. Level 2 Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. The Company’s financial instruments as of December 31, 2017 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. The Company’s financial instruments as of December 31, 2016 consisted primarily of cash and cash equivalents, marketable securities, accounts receivable and accounts payable. As of December 31, 2017 and December 31, 2016, the Company’s financial assets recognized at fair value consisted of the following: 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 $ — Fair Value as of December 31, 2016 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 62,854 $ 59,862 $ 2,992 $ — Marketable securities: Commercial paper 68,375 — 68,375 — Corporate notes 70,408 — 70,408 — U.S. government agency securities and treasuries 25,514 — 25,514 — Total $ 227,151 $ 59,862 $ 167,289 $ — 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 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.
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 December 31, 2017, 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.
Accounts Receivable	Accounts Receivable Accounts receivable are amounts due from collaboration partners as a result of research and development services provided, reimbursements under equally co-funded global development arrangements or milestones achieved but not yet paid. The Company considered the need for an allowance for doubtful accounts and has concluded that no allowance was needed as of December 31, 2017 or 2016, as the estimated risk of loss on its accounts receivable was determined to be minimal.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk include cash, cash equivalents, marketable securities and accounts receivable. The Company attempts to minimize the risks related to cash, cash equivalents and marketable securities by working with highly rated financial institutions that invest in a broad and diverse range of financial instruments as defined by the Company. The Company has established guidelines relative to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. The Company maintains its funds in accordance with its investment policy, which defines allowable investments, specifies credit quality standards and is designed to limit the Company’s credit exposure to any single issuer. Accounts receivable represent amounts due from collaboration partners. The Company monitors economic conditions to identify facts or circumstances that may indicate that any of its accounts receivable are at risk of collection.
Acquired In-Process Research and Development	Acquired In-Process Research and Development The Company records upfront payments that relate to the acquisition of a development-stage product candidate as research and development expense in the period in which they are incurred, provided that the acquired development-stage product candidate did not also include processes or activities that would constitute a business, the product candidate has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use. In the first quarter of 2015, the Company accounted for the upfront payment paid to Eisai Co., Ltd., (‘Eisai”), in connection with the amended and restated collaboration and license agreement, pursuant to which the Company reacquired the worldwide rights, excluding Japan, to tazemetostat, as in-process research and development expense.
Property and Equipment	Property and Equipment The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives: Asset Category Useful Lives Laboratory equipment 5 - 10 years Office furniture and equipment 3 - 10 years Leasehold improvements 3 - 10 years or term of respective lease, if shorter Amortization of capital lease assets is included in depreciation expense. The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation, from the related accounts, and the resulting gain or loss is reflected in the results of operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews long-lived assets to be held and used, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying amount of the assets or asset group may not be recoverable. Evaluation of recoverability is based on an estimate of undiscounted future cash flows resulting from the use of the asset or asset group and its eventual disposition. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the asset or asset group, the assets are written down to their estimated fair values. No such impairments were recorded during 2017, 2016 or 2015.
Income Taxes	Income Taxes The Company records deferred income taxes to recognize the effect of temporary differences between tax and financial statement reporting. The Company calculates the deferred taxes using enacted tax rates expected to be in place when the temporary differences are realized and records a valuation allowance to reduce deferred tax assets if it is determined that it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company considers many factors when assessing the likelihood of future realization of deferred tax assets, including recent earnings results, expectations of future taxable income, carryforward periods available and other relevant factors. The Company records changes in the required valuation allowance in the period that the determination is made. The Company assesses its income tax positions and records tax benefits for all years subject to examination based upon management’s evaluation of the facts, circumstances and information available as of the reporting date. For those tax positions where it is more likely than not that a tax benefit will be sustained, the Company records the largest amount of tax benefit with a greater than 50.0% likelihood of being realized upon ultimate settlement with a taxing authority having full knowledge of all relevant information. For those income tax positions where it is not more likely than not that a tax benefit will be sustained, the Company does not recognize a tax benefit in the financial statements. The Company records interest and penalties related to uncertain tax positions, if applicable, as a component of income tax expense. Refer to Note 6, Income Taxes On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“TCJA”). This legislation makes broad and complex changes to the U.S. tax code, including, but not limited to, (i) reducing the U.S. federal statutory tax rate from 35% to 21%; (ii) eliminating the corporate alternative minimum tax (AMT) and changing how existing AMT credits can be realized; (iii) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017, and (iv) modifying the officer’s compensation limitation. The Company recognizes the effects of changes in tax law, including the TCJA, in the period the law is enacted. Accordingly, the effects of the TCJA have been recognized in the financial statements for the year ended December 31, 2017.
Revenue Recognition	Revenue Recognition The Company recognizes revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; delivery has occurred or services have been rendered; the Company’s price to the customer is fixed or determinable and collectability is reasonably assured. The Company has entered into collaboration and license agreements to discover, develop, manufacture and commercialize compounds directed to specific HMT targets. The terms of these agreements typically contain multiple deliverables, 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, option fees, exercise fees, payments for research activities, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. Multiple-Element Revenue Arrangements. The Company’s collaborations primarily represent multiple-element revenue arrangements. To account for these transactions, the Company determines the elements, or deliverables, included in the arrangement and allocates arrangement consideration to the various elements based on each element’s relative selling price. The identification of individual elements in a multiple-element arrangement and the estimation of the selling price of each element involves significant judgment, including consideration as to whether each delivered element has standalone value to the collaborator. The Company determines the estimated selling price for deliverables within each agreement using vendor-specific objective evidence (“VSOE”) of selling price, if available, or third party evidence of selling price if VSOE is not available, or the Company’s best estimate of selling price, if neither VSOE nor third party evidence is available. Determining the best estimate of selling price for a deliverable requires significant judgment. The Company typically uses its best estimate of a selling price to estimate the selling price for licenses to its proprietary technology, since it often does not have VSOE or third party evidence of selling price for these deliverables. In those circumstances where the Company applies its best estimate of selling price to determine the estimated selling price of a license to its proprietary technology, it considers market conditions as well as entity-specific factors, including those factors contemplated in negotiating the agreements as well as internally developed estimates that include assumptions related to the market opportunity, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating its best estimate of selling price, the Company evaluates whether changes in the key assumptions used to determine its best estimate of selling price will have a significant effect on the allocation of arrangement consideration between deliverables. The Company recognizes consideration allocated to an individual element when all other revenue recognition criteria are met for that element. The Company’s multiple-element revenue arrangements generally include the following: • Exclusive Licenses —The deliverables under the Company’s collaboration agreements generally include exclusive licenses to discover, develop, manufacture and commercialize compounds with respect to one or more specified HMT targets. To account for this element of the arrangement, management evaluates whether the exclusive license has standalone value from the undelivered elements to the collaboration partner based on the consideration of the relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner. The Company may recognize arrangement consideration allocated to licenses upon delivery of the license if facts and circumstances indicate that the license has standalone value from the undelivered elements, which generally include research and development services. The Company defers arrangement consideration allocated to licenses if facts and circumstances indicate that the delivered license does not have standalone value from the undelivered elements. The Company has determined that certain of its exclusive licenses lack standalone value apart from the related research and development services and is therefore recognizing collaboration revenue from non-refundable exclusive license fees on a straight-line basis over the contracted or estimated period of performance, which is generally the period over which the research and development services are to be provided. • Research and Development Services —The deliverables under the Company’s collaboration and license agreements generally include deliverables related to research and development services to be performed by the Company on behalf of the collaboration partner. As the provision of research and development services is a part of the Company’s central operations, when the Company is principally responsible for the performance of these services under the agreements, the Company recognizes revenue on a gross basis for research and development services as those services are performed. • Option Arrangements —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. The accounting for option arrangements is dependent on the nature of the options granted to the collaboration partner. Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the options to secure exclusive licenses. Factors that the Company considers in evaluating whether options are substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the options, the cost to exercise the options relative to the total upfront consideration and the additional financial commitments or economic penalties imposed on the collaborator as a result of exercising the options. For arrangements under which the option to secure licenses is considered substantive, the Company does not consider the licenses to be deliverables at the inception of the arrangement. For arrangements under which the option to secure licenses is not considered substantive, the Company considers the license to be a deliverable at the inception of the arrangement and, upon delivery of the license, would apply the multiple-element revenue arrangement criteria to the license and any other deliverables to determine the appropriate revenue recognition. None of the options to secure exclusive licenses included in the Company’s collaborative arrangements have been determined to be substantive. Milestone Revenue. The Company’s collaboration and license agreements generally include contingent milestone payments related to specified preclinical research and development milestones, clinical development milestones, regulatory milestones and sales-based milestones. Preclinical research and development milestones are typically payable upon the selection of a compound candidate for the next stage of research and development. Clinical development milestones are typically payable when a product candidate initiates or advances in clinical trial phases or achieves defined clinical events such as proof-of-concept. Regulatory milestones are typically payable upon submission for marketing approval with regulatory authorities or upon receipt of actual marketing approvals for a compound, approvals for additional indications, or upon the first commercial sale. Sales-based milestones are typically payable when annual sales reach specified levels. At the inception of each arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether (a) the consideration is commensurate with either (i) the entity’s performance to achieve the milestone or (ii) the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the entity’s performance to achieve the milestone; (b) the consideration relates solely to past performance; and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. The Company generally considers non-refundable preclinical research and development, clinical development and regulatory milestones that the Company expects to be achieved as a result of the Company’s efforts during the period of the Company’s performance obligations under the collaboration and license agreements to be substantive and recognizes them as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met. If not considered to be substantive, the Company initially defers milestones and recognizes them over the remaining term of the Company’s performance obligations. Milestones that are not considered substantive because the Company does not contribute effort to the achievement of such milestones are generally achieved after the period of the Company’s performance obligations and are recognized as revenue upon achievement, assuming all other revenue recognition criteria are met, as there are no undelivered elements remaining and no continuing performance obligations. The Company is required to adopt new revenue accounting guidance in the first quarter of 2018, which it expects will impact this policy, the amount and timing of future revenue recognition, and the amount of revenue previously recognized in our published financial statements. For further discussion, see Recent Accounting Pronouncements
Research and Development Expenses	Research and Development Expenses Research and development expenses are expensed as incurred. Research and development expenses are comprised of costs incurred in providing research and development activities, including salaries and benefits, facilities costs, overhead costs, contract research and development services, and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. External research and development expenses associated with the Company’s programs include clinical trial site costs, clinical manufacturing costs, costs incurred for consultants and other outside services, such as data management and statistical analysis support, and materials and supplies used in support of the clinical and preclinical programs. Internal costs of the Company’s clinical programs include salaries, stock-based compensation, and the portion of the Company’s facility costs allocated to research and development expense. When third-party service providers’ billing terms do not coincide with the Company’s period-end, the Company is required to make estimates of its obligations to those third parties, including clinical trial and pharmaceutical development costs, contractual services costs and costs for supply of its product candidates incurred in a given accounting period and record accruals at the end of the period. The Company bases its estimates on its knowledge of the research and development programs, services performed for the period, past history for related activities and the expected duration of the third-party service contract, where applicable. The Company generally accrues expenses related to research and development activities based on the services received and efforts expended pursuant to contracts with multiple contract research organizations that conduct and manage clinical trials, as well as other vendors that provide research and development services. The financial terms of these agreements are subject to negotiation, vary from contract to contract and may result in uneven payment flows. There may be instances in which payments made to vendors will exceed the level of services provided and result in a prepayment of the clinical expense. Payments under some of these contracts depend on factors such as the successful enrollment of subjects and the completion of clinical trial milestones. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from estimates, the Company would adjust the accrual or prepaid accordingly in future periods.
Stock-Based Compensation	Stock-Based Compensation The Company measures employee stock-based compensation based on the grant date fair value of the stock-based compensation award. The Company grants stock options at exercise prices equal to the fair value of the Company’s common stock on the date of grant, based on observable market prices. The Company recognizes employee stock-based compensation expense on a straight-line basis over the requisite service period of the awards. The Company recognizes forfeitures for employee and non-employee grants at the time they occur. The actual expense recognized over the vesting period will only represent those options that vest. Refer to Note 10, Employee Benefit Plans
Earnings (Loss) per Share	Earnings (Loss) per Share The Company computes basic earnings (loss) per share by dividing income (loss) allocable to common stockholders by the weighted average number of shares of common stock outstanding. During periods of income, the Company allocates participating securities a proportional share of income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities (the “two-class method”). The Company’s restricted stock and, prior to its automatic conversion, redeemable convertible preferred stock participate in any dividends declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, the Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The Company computes diluted earnings (loss) per share after giving consideration to the dilutive effect of stock options that are outstanding during the period, except where such non-participating securities would be anti-dilutive. Refer to Note 11, Loss per Share
Segment Information	Segment Information The Company operates as one reportable business segment: the discovery and development of novel epigenetic therapies for patients with cancer and other diseases.
Recent Accounting Pronouncements	Pending Accounting Pronouncements In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2014-09, Revenue From Contracts With Customers Revenue Recognition Revenue From Contracts with Customers Celgene Corporation (“Celgene”), Glaxo Group Limited (“GSK”) and Eisai Co. Ltd. (“Eisai”) The Company has not yet completed its assessment of the impact the adoption of this standard may have on its consolidated financial statements and internal control, but expects that the adoption will impact the amount and timing of revenues recognized related to its collaboration agreement with Celgene and the impact may be material . With regard to the collaboration agreements with GSK and Eisai, the Company currently does not expect the impact of adoption of this standard to be material. The Company is still assessing its collaboration agreement with Celgene; however, it currently expects that (a) the number of performance obligations identified under ASC 606 will be different from the number of units of accounting identified under ASC 605 and (b) the transaction price under ASC 606 will be different from the transaction price determined under ASC 605. In particular, the Company currently expects that the accounting treatment for Celgene’s option rights that were determined to be non-substantive options under ASC 605 will be different under ASC 606. The licenses and research and development services underlying Celgene’s options will not be identified as promised goods or services (or performance obligations) under ASC 606 and the consideration that would be received upon exercise of the options will not be included in the transaction price at contract inception. The Company may identify additional differences as it completes its assessment. The Company also expects the accounting for contingent milestone payments under its collaboration agreements to change under ASC 606. ASC 606 does not contain guidance specific to milestone payments, thereby requiring contingent milestone payments to be considered in accordance with the overall model of ASC 606 as variable consideration. Revenue from contingent milestone payments may be recognized earlier under ASC 606 than under ASC 605, based on an assessment at each reporting date of the probability of achievement of the underlying milestone event. This assessment may, in certain circumstances, result in the recognition of revenue related to a contingent milestone payment before the milestone event has been achieved. ASC 606 requires more robust disclosures than required by previous guidance, including disclosures related to disaggregation of revenue into appropriate categories, performance obligations, the judgments made in revenue recognition determinations, adjustments to revenue which relate to activities from previous quarters or years, any significant reversals of revenue, and costs to obtain or fulfill contracts. Expected impacts from the adoption of this standard could differ upon the final adoption and implementation of the standard. In connection with the adoption of the standard, the Company is implementing several new internal controls, including controls to monitor the probability of achievement of contingent milestone payments and the pattern of performance of certain performance obligations. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, proceeds from the settlement of corporate-owned life insurance policies, distributions received from equity method investees and beneficial interests in securitization transactions. The new standard also clarifies that an entity should determine each separately identifiable source or use within the cash receipts and cash payments on the basis of the nature of the underlying cash flows. In situations in which cash receipts and payments have aspects of more than one class of cash flows and cannot be separated by source or use, the appropriate classification should depend on the activity that is likely to be the predominant source or use of cash flows for the item. The new standard will be effective for the Company on January 1, 2018. The adoption of this standard is not expected to have a material impact on the Company’s consolidated statements of cash flows. In November 2016, the FASB issued ASU 2016-18, Restricted Cash In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting , which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. The new standard does not change the accounting for modifications but clarifies that modification accounting guidance should only be applied if the fair value, vesting conditions, or classification of the award changes as a result of the change in terms or conditions. The new standard is effective for fiscal years, and interim periods within, beginning after December 15, 2017. Early adoption is permitted. A reporting entity must apply the amendments in the ASU prospectively to an award modified on or after the adoption date. The adoption of ASU 2017-09 is not expected to have a material effect on the Company’s consolidated financial statements or disclosures. Recently Adopted Accounting Pronouncements As of January 1, 2017, the Company adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting

Summary of Significant Accoun22

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Available-for-Sale Securities Held	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 following table summarizes the available for sale securities held at December 31, 2016 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 68,407 $ — $ (32 ) $ 68,375 Corporate notes 70,489 — (81 ) 70,408 U.S. government agency securities and U.S. Treasuries 25,507 7 — 25,514 Total $ 164,403 $ 7 $ (113 ) $ 164,297
Summary of Company's Financial Assets Recognized at Fair Value	As of December 31, 2017 and December 31, 2016, the Company’s financial assets recognized at fair value consisted of the following: 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 $ — Fair Value as of December 31, 2016 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 62,854 $ 59,862 $ 2,992 $ — Marketable securities: Commercial paper 68,375 — 68,375 — Corporate notes 70,408 — 70,408 — U.S. government agency securities and treasuries 25,514 — 25,514 — Total $ 227,151 $ 59,862 $ 167,289 $ —
Useful Lives for Property, Plant and Equipment	The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives: Asset Category Useful Lives Laboratory equipment 5 - 10 years Office furniture and equipment 3 - 10 years Leasehold improvements 3 - 10 years or term of respective lease, if shorter

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, net	Property and equipment, net consists of the following: December 31, 2017 2016 (In thousands) Laboratory equipment $ 4,138 $ 3,691 Computer and office equipment, furniture (1) 4,807 4,495 Leasehold improvements 354 464 Construction in progress 71 — Property and equipment 9,370 8,650 Less: accumulated depreciation and amortization (6,843 ) (5,526 ) Property and equipment, net $ 2,527 $ 3,124 (1) In 2015, the Company acquired $1.7 million of computer hardware and equipment, pursuant to a capital lease, the term of which expires in February 2018. Accumulated depreciation related to these assets totaled $1.6 million and $1.1 million as of December 31, 2017 and 2016, respectively.

Prepaid Expenses and Other Cu24

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following: December 31, 2017 2016 (In thousands) Prepaid clinical and manufacturing costs $ 5,724 $ 3,666 Interest receivable on available for sale securities 249 295 Other prepaid expenses and other receivables 3,010 2,496 Total prepaid expenses and other current assets $ 8,983 $ 6,457

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: December 31, 2017 2016 (In thousands) Employee compensation and benefits $ 4,628 $ 4,100 Research and development expenses 11,658 10,925 Professional services and other 1,263 982 Accrued expenses $ 17,549 $ 16,007

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Provision for (Benefit from) Income Taxes	The provision for (benefit from) income taxes for the years ended December 31, 2017, 2016, and 2015 is as follows: 2017 2016 2015 (In thousands) Current $ 32.0 $ — $ — Deferred (368.0 ) — — Total (336.0 ) — — Income tax provision (benefit) $ (336.0 ) $ — $ —
Reconciliation of Federal Statutory Income Tax Rate and Effective Income Tax Rate	A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows: Year Ended December 31, 2017 2016 2015 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % State income taxes 4.8 4.5 5.2 Research and development and other tax credits 4.2 3.3 1.8 Permanent items (2.0 ) (1.9 ) (1.2 ) Change in valuation allowance 5.7 (40.1 ) (40.0 ) Return-to-provision adjustments (0.7 ) — — Change in deferred taxes — 0.2 0.2 Rate Change (45.7 ) — — Other — 0.0 — Effective income tax rate 0.3 % 0.0 % 0.0 %
Deferred Tax Assets (Liabilities)	The Company’s deferred tax assets (liabilities) consist of the following: December 31, 2017 2016 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 116,869 $ 94,390 Research and development and other credit carryforwards 24,257 18,055 Capitalized start-up costs 1,175 1,899 Capitalized research and development costs 36 154 Deferred revenue 7,766 11,231 Accruals and allowances 1,250 1,435 Eisai license payment 8,985 14,035 Other 4,702 5,607 Gross deferred tax assets 165,040 146,806 Deferred tax asset valuation allowance (164,607 ) (146,548 ) Total deferred tax assets 433 258 Deferred tax liabilities: Depreciation and other (65 ) (258 ) Total deferred tax liabilities (65 ) (258 ) Net deferred tax asset (liability) $ 368 $ —
Summary of Unrecognized Tax Benefits	The following is a rollforward of the Company’s unrecognized tax benefits: December 31, 2017 2016 (In thousands) Unrecognized tax benefits - as of beginning of year $ 4,206 $ 3,480 Gross increases - tax positions of prior periods — — Gross increases - current period tax positions 1,017 726 Unrecognized tax benefits - as of end of year $ 5,223 $ 4,206

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Contractual Commitments	The Company’s contractual commitments under these leases, excluding common area maintenance charges and real estate taxes, as of December 31, 2017 are as follows: Total 2018 2019 2020 2021 2022 (In thousands) Operating leases $ 17,262 $ 3,270 $ 3,516 $ 3,605 $ 3,538 $ 3,333 Capital lease, including amounts representing interest 111 111 — — — — Total commitments $ 17,373 $ 3,381 $ 3,516 $ 3,605 $ 3,538 $ 3,333

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense	Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2017 2016 2015 (In thousands) Research and development $ 5,613 $ 5,352 $ 5,155 General and administrative 5,818 5,216 4,694 Total $ 11,431 $ 10,568 $ 9,849
Weighted Average Assumptions Used in Applying Pricing Model	Key weighted average assumptions used in this pricing model on the date of grant for options granted to employees are as follows: Year Ended December 31, 2017 2016 2015 Risk-free interest rate 1.8 % 1.2 % 1.6 % Expected life of options 6.0 years 6.0 years 6.0 years Expected volatility of underlying stock 74.2 % 78.5 % 83.6 % Expected dividend yield 0.0 % 0.0 % 0.0 %
Summary of Stock Option Activity	The following is a summary of stock option activity for the year ended December 31, 2017: 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, 2016 4,059 $ 14.32 Granted 2,332 13.50 Exercised (459 ) 7.15 Forfeited or expired (1,356 ) 14.49 Outstanding at December 31, 2017 4,576 $ 14.57 7.9 $ 5,814 Exercisable at December 31, 2017 1,640 $ 16.90 6.4 $ 3,379
Summary of Restricted Stock Activity	The following is a summary of restricted stock unit activity for the year ended December 31, 2017: Number of Units Weighted Average Grant Date Fair Value per Unit (In thousands) Outstanding at December 31, 2016 64 $ 12.20 Granted — — Vested (20 ) 12.20 Forfeited (44 ) 12.20 Outstanding at December 31, 2017 — $ —

Loss per Share (Tables)

Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Basic and Diluted Loss per Share	Basic and diluted loss per share allocable to common stockholders are computed as follows: Year Ended December 31, 2017 2016 2015 (In thousands except per share data) Net loss $ (134,309 ) $ (110,212 ) $ (132,376 ) Weighted average shares outstanding 61,471 57,126 39,839 Basic and diluted loss per share allocable to common stockholders $ (2.18 ) $ (1.93 ) $ (3.32 )
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 antidilutive: Year Ended December 31, 2017 2016 2015 (In thousands) Stock options 4,576 4,059 3,100 Unvested restricted stock — 64 37 Shares issuable under employee stock purchase plan 23 46 8 4,599 4,169 3,145

Unaudited Quarterly Results (Ta

Unaudited Quarterly Results (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Results of Operations on Quarterly Basis	The results of operations on a quarterly basis for the years ended December 31, 2017 and 2016 are set forth below: Quarter Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 (In thousands, except per share data) Collaboration revenue $ — $ 10,000 $ — $ — Operating expenses: Research and development 24,695 27,292 28,741 28,933 General and administrative 8,269 11,170 9,311 8,431 Total operating expenses 32,964 38,462 38,052 37,364 Operating loss (32,964 ) (28,462 ) (38,052 ) (37,364 ) Other income, net 442 438 455 862 Income tax benefit — — — 336 Net loss $ (32,522 ) $ (28,024 ) $ (37,597 ) $ (36,166 ) Loss per share allocable to common stockholders: Basic $ (0.56 ) $ (0.48 ) $ (0.63 ) $ (0.52 ) Diluted $ (0.56 ) $ (0.48 ) $ (0.63 ) $ (0.52 ) Weighted average shares outstanding: Basic 58,219 58,377 59,899 69,287 Diluted 58,219 58,377 59,899 69,287 Quarter Ended March 31, 2016 June 30, 2016 September 30, 2016 December 31, 2016 (In thousands, except per share data) Collaboration revenue $ 472 $ 473 $ 6,584 $ 478 Operating expenses: Research and development 17,740 21,450 23,888 28,383 General and administrative 5,846 7,424 7,522 7,580 Total operating expenses 23,586 28,874 31,410 35,963 Operating loss (23,114 ) (28,401 ) (24,826 ) (35,485 ) Other income, net 235 420 490 469 Net loss $ (22,879 ) $ (27,981 ) $ (24,336 ) $ (35,016 ) Loss per share allocable to common stockholders: Basic $ (0.41 ) $ (0.49 ) $ (0.42 ) $ (0.60 ) Diluted $ (0.41 ) $ (0.49 ) $ (0.42 ) $ (0.60 ) Weighted average shares outstanding: Basic 55,149 57,352 57,970 58,016 Diluted 55,149 57,352 57,970 58,016

The Company - Additional Inform

The Company - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basis Of Presentation [Line Items]
Sale of common stock in public offering	$ 152,534	$ 129,955	$ 130,345
Proceeds from sale of redeemable convertible preferred stock	$ 76,000
Initial public offering completion date	2013-05
Cash, cash equivalents, and marketable securities	$ 276,400
Accumulated deficit	(488,097)	$ (353,673)
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 Accoun32

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017USD ($)SecuritySegment	Dec. 31, 2016USD ($)Security	Dec. 31, 2015USD ($)	Mar. 31, 2016USD ($)
Accounting Policies [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		$ 42,783,000	$ 134,860,000
Investments with an other-than-temporary impairment		0	0
Available-For-Sale Securities Held, Fair Value		49,775,000	164,297,000		$ 0
Realized gains (losses) recognized on sale or maturity of marketable equity securities		$ 0
Weighted average maturity period		1 month
Allowance for doubtful accounts		$ 0	0
Impairment of long-lived assets		$ 0	$ 0	$ 0
Largest amount of tax benefit		50.00%
Federal statutory income tax rate		34.00%	34.00%	34.00%
Reportable business segment | Segment		1
Change in amount of valuation allowance and deferred tax asset		$ 25,700,000
Amount charged to retained earnings		100,000
Scenario, Plan [Member]
Accounting Policies [Line Items]
Federal statutory income tax rate	21.00%
Maximum [Member]
Accounting Policies [Line Items]
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss		$ 100,000
Federal statutory income tax rate		35.00%
U.S. Government Agency [Member]
Accounting Policies [Line Items]
Number of securities | Security		1
Available-For-Sale Securities Held, Fair Value		$ 1,250,000	$ 25,514,000
Corporate Notes [Member]
Accounting Policies [Line Items]
Number of securities | Security		14	23
Available-For-Sale Securities Held, Fair Value		$ 31,567,000	$ 70,408,000
Commercial Paper [Member]
Accounting Policies [Line Items]
Number of securities | Security		4	11
Available-For-Sale Securities Held, Fair Value		$ 16,958,000	$ 68,375,000

Summary of Significant Accoun33

Summary of Significant Accounting Policies - Summary of Available-for-Sale Securities Held (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 49,824,000	$ 164,403,000
Available-For-Sale Securities Held, Unrealized Gains		7,000
Available-For-Sale Securities Held, Unrealized Losses	(49,000)	(113,000)
Available-For-Sale Securities Held, Fair Value	49,775,000	164,297,000	$ 0
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	16,964,000	68,407,000
Available-For-Sale Securities Held, Unrealized Losses	(6,000)	(32,000)
Available-For-Sale Securities Held, Fair Value	16,958,000	68,375,000
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	31,610,000	70,489,000
Available-For-Sale Securities Held, Unrealized Losses	(43,000)	(81,000)
Available-For-Sale Securities Held, Fair Value	31,567,000	70,408,000
U.S. Government Agency Securities and Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	1,250,000	25,507,000
Available-For-Sale Securities Held, Unrealized Gains		7,000
Available-For-Sale Securities Held, Fair Value	$ 1,250,000	$ 25,514,000

Summary of Significant Accoun34

Summary of Significant Accounting Policies - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 207,251	$ 62,854
Total	257,026	227,151
Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	16,958	68,375
Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	31,567	70,408
U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	1,250	25,514
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	207,251	59,862
Total	207,251	59,862
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents		2,992
Total	49,775	167,289
Level 2 [Member] | Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	16,958	68,375
Level 2 [Member] | Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	31,567	70,408
Level 2 [Member] | U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	$ 1,250	$ 25,514

Summary of Significant Accoun35

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets Acquired (Detail)	12 Months Ended
	Dec. 31, 2017
Laboratory Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	5 years
Laboratory Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years
Office Furniture and Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Office Furniture and Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years
Leasehold Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Leasehold Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment	$ 9,370	$ 8,650
Less: accumulated depreciation and amortization	(6,843)	(5,526)
Property and equipment, net	2,527	3,124
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	4,138	3,691
Computer Office Equipment and Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	4,807	4,495
Less: accumulated depreciation and amortization	(1,600)	(1,100)
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	354	$ 464
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 71

Property and Equipment, Net -37

Property and Equipment, Net - Schedule of Property and Equipment, Net (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 6,843	$ 5,526
Computer Office Equipment and Furniture [Member]
Property, Plant and Equipment [Line Items]
Computer hardware and equipment acquired			$ 1,700
Expiration of capital lease term	Feb. 28, 2018
Accumulated depreciation	$ 1,600	$ 1,100

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 1,639	$ 1,589	$ 1,419

Prepaid Expenses and Other Cu39

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid clinical and manufacturing costs	$ 5,724	$ 3,666
Interest receivable on available for sale securities	249	295
Other prepaid expenses and other receivables	3,010	2,496
Total prepaid expenses and other current assets	$ 8,983	$ 6,457

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities Current [Abstract]
Employee compensation and benefits	$ 4,628	$ 4,100
Research and development expenses	11,658	10,925
Professional services and other	1,263	982
Accrued expenses	$ 17,549	$ 16,007

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for (Benefit from) Income Taxes (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Current		$ 32,000
Deferred		(368,000)
Total	$ (336,000)	(336,000)	$ 0	$ 0
Income tax provision (benefit)	$ (336,000)	$ (336,000)	$ 0	$ 0

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Federal Statutory Income Tax Rate and Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	34.00%	34.00%	34.00%
State income taxes	4.80%	4.50%	5.20%
Research and development and other tax credits	4.20%	3.30%	1.80%
Permanent items	(2.00%)	(1.90%)	(1.20%)
Change in valuation allowance	5.70%	(40.10%)	(40.00%)
Return-to-provision adjustments	(0.70%)
Change in deferred taxes		0.20%	0.20%
Rate Change	(45.70%)
Other		0.00%
Effective income tax rate	0.30%	0.00%	0.00%

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 116,869	$ 94,390
Research and development and other credit carryforwards	24,257	18,055
Capitalized start-up costs	1,175	1,899
Capitalized research and development costs	36	154
Deferred revenue	7,766	11,231
Accruals and allowances	1,250	1,435
Eisai license payment	8,985	14,035
Other	4,702	5,607
Gross deferred tax assets	165,040	146,806
Deferred tax asset valuation allowance	(164,607)	(146,548)
Total deferred tax assets	433	258
Deferred tax liabilities:
Depreciation and other	(65)	(258)
Total deferred tax liabilities	(65)	(258)
Net deferred tax asset (liability)	$ 368

Income Taxes (Tax Cuts and Jobs

Income Taxes (Tax Cuts and Jobs Act) - Additional Information (Detail) - USD ($)	Dec. 22, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Reversal of deferred tax asset valuation allowance		$ 368,000		$ 368,000
Income tax benefit		$ (336,000)		$ (336,000)	$ 0	$ 0
Federal statutory income tax rate				34.00%	34.00%	34.00%
Tax cuts and jobs act, provision of income tax expense in continuing operations	$ 61,600,000
Scenario, Plan [Member]
Income Tax Contingency [Line Items]
Federal statutory income tax rate			21.00%

Income Taxes (Operating Loss Ca

Income Taxes (Operating Loss Carryforwards) - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
United States, Federal [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 428.5
Operating loss carryforwards, expiration beginning year	2,029
United States, State [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 428.7
Operating loss carryforwards, expiration beginning year	2,030

Income Taxes (Research and Deve

Income Taxes (Research and Development Tax Credit) - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Research and development tax credit carryforward expiration beginning year	2,028
Research and development tax credit carryforward expiration ending year	2,037
Federal orphan drug tax credit carryforwards expiration beginning year	2,033
Expiry period of federal orphan drug tax credit carryforwards	2,037
Federal orphan drug tax credit carryforwards	$ 14,700,000
Federal alternative minimum tax credit	400,000
Accrued tax-related interest or penalties	0	$ 0
Research Tax Credit Carryforward [Member] | United States, Federal [Member]
Tax Credit Carryforward [Line Items]
Research and development credit carryforwards	7,200,000
Research Tax Credit Carryforward [Member] | United States, State [Member]
Tax Credit Carryforward [Line Items]
Research and development credit carryforwards	$ 2,500,000

Income Taxes - Summary of Unrec

Income Taxes - Summary of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Unrecognized tax benefits - as of beginning of year	$ 4,206	$ 3,480
Gross increases - current period tax positions	1,017	726
Unrecognized tax benefits - as of end of year	$ 5,223	$ 4,206

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2018
Other Commitments [Line Items]
Monthly base rent	$ 0.2
Letter of credit as a security deposit	0.5
Rental expense	3.3	$ 2.8	$ 2.7
Roche [Member]
Other Commitments [Line Items]
Payment obligation incurred and paid	1.5
Collaborative Arrangement [Member] | Roche [Member]
Other Commitments [Line Items]
Remaining development costs	$ 10.5
Collaborative Arrangement [Member] | Roche [Member] | Scenario, Forecast [Member]
Other Commitments [Line Items]
Remaining development costs				$ 10.4
Initial Term [Member]
Other Commitments [Line Items]
Lease expiration date	May 31, 2018
Extended Term [Member]
Other Commitments [Line Items]
Lease expiration date	Nov. 30, 2022

Commitments and Contingencies49

Commitments and Contingencies - Contractual Commitments (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Commitments And Contingencies Disclosure [Abstract]
Operating lease, Total	$ 17,262
Operating lease, 2018	3,270
Operating lease, 2019	3,516
Operating lease, 2020	3,605
Operating lease, 2021	3,538
Operating lease, 2022	3,333
Capital lease, including amounts representing interest, Total	111
Capital lease, including amounts representing interest, 2018	111
Commitments, Total	17,373
Commitments, 2018	3,381
Commitments, 2019	3,516
Commitments, 2020	3,605
Commitments, 2021	3,538
Commitments, 2022	$ 3,333

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity - Additional Information (Detail) $ in Thousands	Mar. 10, 2017shares	Sep. 30, 2017shares	Apr. 30, 2016	Jan. 31, 2016shares	Dec. 31, 2016USD ($)	Dec. 31, 2017USD ($)Voteshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)
Equity [Line Items]
Number of votes for each share of common stock | Vote						1
Common Stock Sold through ATM Facility | shares		10,557,000		15,333,334
Proceeds from Issuance of Common Stock						$ 152,922	$ 130,438	$ 130,712
Common Stock Issuance Cost Incurred						$ 388	$ 483	$ 367
Common stock reserved for future issuance | shares						8,890,201
At The Market Facility [Member]
Equity [Line Items]
Common Stock Sold through ATM Facility | shares	155,834					132,253	23,581
Commission rate for sales agent			3.00%
Proceeds from Issuance of Common Stock						$ 1,600	$ 300
Common Stock Issuance Cost Incurred					$ 100
Equity offering sales agreement termination date						Mar. 10, 2017

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Jul. 08, 2015USD ($)OptionTarget	Jan. 01, 2011OptionTarget	May 31, 2017USD ($)	Mar. 31, 2015USD ($)	Mar. 31, 2014USD ($)	Jun. 30, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2017USD ($)	Mar. 31, 2018USD ($)
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue						$ 10,000,000	$ 478,000	$ 6,584,000	$ 473,000	$ 472,000		$ 10,000,000	$ 8,007,000	$ 2,560,000
Upfront payment made														40,000,000
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
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
Clinical development milestone achieved																25,000,000
Global development co-funding												0	100,000	1,100,000		7,000,000
Number of option targets | OptionTarget	3
Deferred revenue recognized	$ 21,600,000
Upfront payment made as part of the amendment	10,000,000
License and research services related cost	106,600,000
Collaboration revenue												0	1,900,000	$ 1,100,000		74,300,000
Deferred revenue							28,800,000					28,800,000	28,800,000			28,800,000	28,800,000
Celgene [Member] | Third Option Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
License and research services related cost	33,600,000
Celgene [Member] | License and Pre-IND Services [Member] | Two Option Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
License and research services related cost	$ 70,200,000
Number of option targets with no standalone value | OptionTarget	2
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 payment												35,000,000				35,000,000	35,000,000
Additional milestone payments												100,000,000				100,000,000	100,000,000
License and research services related cost	$ 2,800,000
Applicable profit margin	10.00%
Celgene [Member] | DOT1L [Member] | License and Pre-IND Services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets with standalone value | OptionTarget	2
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
Development milestone and license payments associated with the Option Targets	$ 75,000,000
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
Additional milestone payments											197,000,000
Number of option targets | OptionTarget		3
Sales-based milestone payments											128,000,000
Collaboration revenue												10,000,000	6,000,000
Deferred revenue							$ 0					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												10,500,000				10,500,000	10,500,000
Reimbursements receivable of development costs												$ 900,000				$ 900,000	$ 900,000
Collaborative Arrangement [Member] | Roche [Member] | Scenario, Forecast [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments																		$ 10,400,000

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Aug. 31, 2017	Feb. 29, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incentive and non-qualified stock options and restricted stock, vesting period			4 years
Vesting terms			25.0% vesting upon the one-year anniversary of the grant and the remaining 75.0% vesting monthly over the following three years
Incentive and non-qualified stock options, expiration period			10 years
Non-employee stock option awards granted			0	0	0
Expected dividend yield			0.00%
Aggregate stock options granted to employees, including a non-employee			2,331,500	2,212,668	1,289,755
Weighted-average fair value of options granted			$ 8.85	$ 6.42	$ 14.57
Grant date fair value of options vested			$ 12,000,000	$ 12,000,000	$ 9,300,000
Aggregate intrinsic value of stock option exercised			4,100,000	7,100,000	11,900,000
Unrecognized compensation cost related to stock options			$ 22,500,000
Expected weighted average period for recognition of compensation cost			2 years 8 months 12 days
Percentage of employer's matching contribution						50.00%
Maximum percentage of employee contribution						3.00%
Contribution to 401 (k) plan			$ 500,000	400,000	400,000
Unvested Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares vested			20,000
Unrecognized compensation cost related to restricted stock units			$ 0
Unvested Restricted Stock [Member] | February 9, 2016 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Granted		80,732
Weighted Average Grant Date Fair Value per Share, Granted		$ 9.29
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares vested	73,779
Intrinsic value of restricted stock vested during period			$ 300,000	$ 500,000	$ 0
Vesting Year One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incentive and non-qualified stock options and restricted stock, vesting percentage			25.00%
Vesting Remaining Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incentive and non-qualified stock options and restricted stock, vesting percentage			75.00%

Employee Benefit Plans - Schedu

Employee Benefit Plans - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 11,431	$ 10,568	$ 9,849
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	5,613	5,352	5,155
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 5,818	$ 5,216	$ 4,694

Employee Benefit Plans - Weight

Employee Benefit Plans - Weighted Average Assumptions Used in Applying Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.80%	1.20%	1.60%
Expected life of options	6 years	6 years	6 years
Expected volatility of underlying stock	74.20%	78.50%	83.60%
Expected dividend yield	0.00%	0.00%	0.00%

Employee Benefit Plans - Summar

Employee Benefit Plans - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding, Beginning balance	4,059,000
Number of Options, Granted	2,331,500	2,212,668	1,289,755
Number of Options, Exercised	(459,000)
Number of Options, Forfeited or expired	(1,356,000)
Number of Options, Outstanding, Ending balance	4,576,000	4,059,000
Number of Options, Exercisable	1,640,000
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$ 14.32
Weighted Average Exercise Price per Share, Granted	13.50
Weighted Average Exercise Price per Share, Exercised	7.15
Weighted Average Exercise Price per Share, Forfeited or expired	14.49
Weighted Average Exercise Price per Share, Outstanding, Ending balance	14.57	$ 14.32
Weighted Average Exercise Price per Share, Exercisable	$ 16.90
Weighted Average Remaining Contractual Term (In Years), Outstanding	7 years 10 months 24 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	6 years 4 months 24 days
Aggregate Intrinsic Value, Outstanding	$ 5,814
Aggregate Intrinsic Value, Exercisable	$ 3,379

Employee Benefit Plans - Summ56

Employee Benefit Plans - Summary of Restricted Stock Activity (Detail) - Unvested Restricted Stock [Member] shares in Thousands	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Outstanding, Beginning balance | shares	64
Number of Units, Vested | shares	(20)
Number of Units, Forfeited | shares	(44)
Weighted Average Grant Date Fair Value per Unit, Beginning balance | $ / shares	$ 12.20
Weighted Average Grant Date Fair Value per Unit, Vested | $ / shares	12.20
Weighted Average Grant Date Fair Value per Unit, Forfeited | $ / shares	$ 12.20

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								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net loss	$ (36,166)	$ (37,597)	$ (28,024)	$ (32,522)	$ (35,016)	$ (24,336)	$ (27,981)	$ (22,879)	$ (134,309)	$ (110,212)	$ (132,376)
Weighted average shares outstanding									61,471	57,126	39,839
Basic and diluted loss per share allocable to common stockholders									$ (2.18)	$ (1.93)	$ (3.32)

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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	4,599	4,169	3,145
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	4,576	4,059	3,100
Unvested Restricted Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share		64	37
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	23	46	8

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended					12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Related Party Transaction [Line Items]
Equity investments in common stock		58,050,000				69,302,000	58,050,000
Collaboration revenue related to agreement	$ 10,000,000	$ 478,000	$ 6,584,000	$ 473,000	$ 472,000	$ 10,000,000	$ 8,007,000	$ 2,560,000
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%
Collaboration revenue related to agreement						$ 0	1,900,000	1,100,000
Deferred revenue		28,800,000				28,800,000	28,800,000
Global development co-funding from Celgene						0	100,000	$ 1,100,000
Accounts receivable related to collaboration arrangement		$ 100,000				$ 0	$ 100,000

Unaudited Quarterly Results - S

Unaudited Quarterly Results - Summary of Results of Operations on a Quarterly Basis (Detail) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue			$ 10,000,000		$ 478,000	$ 6,584,000	$ 473,000	$ 472,000	$ 10,000,000	$ 8,007,000	$ 2,560,000
Operating expenses:
Research and development	$ 28,933,000	$ 28,741,000	27,292,000	$ 24,695,000	28,383,000	23,888,000	21,450,000	17,740,000	109,661,000	91,461,000	111,209,000
General and administrative	8,431,000	9,311,000	11,170,000	8,269,000	7,580,000	7,522,000	7,424,000	5,846,000	37,181,000	28,372,000	23,900,000
Total operating expenses	37,364,000	38,052,000	38,462,000	32,964,000	35,963,000	31,410,000	28,874,000	23,586,000	146,842,000	119,833,000	135,109,000
Operating loss	(37,364,000)	(38,052,000)	(28,462,000)	(32,964,000)	(35,485,000)	(24,826,000)	(28,401,000)	(23,114,000)	(136,842,000)	(111,826,000)	(132,549,000)
Other income, net	862,000	455,000	438,000	442,000	469,000	490,000	420,000	235,000	2,197,000	1,614,000	173,000
Income tax benefit	336,000								336,000	0	0
Net loss	$ (36,166,000)	$ (37,597,000)	$ (28,024,000)	$ (32,522,000)	$ (35,016,000)	$ (24,336,000)	$ (27,981,000)	$ (22,879,000)	$ (134,309,000)	$ (110,212,000)	$ (132,376,000)
Loss per share allocable to common stockholders:
Basic	$ (0.52)	$ (0.63)	$ (0.48)	$ (0.56)	$ (0.60)	$ (0.42)	$ (0.49)	$ (0.41)	$ (2.18)	$ (1.93)	$ (3.32)
Diluted	$ (0.52)	$ (0.63)	$ (0.48)	$ (0.56)	$ (0.60)	$ (0.42)	$ (0.49)	$ (0.41)	$ (2.18)	$ (1.93)	$ (3.32)
Weighted average shares outstanding:
Basic	69,287	59,899	58,377	58,219	58,016	57,970	57,352	55,149	61,471	57,126	39,839
Diluted	69,287	59,899	58,377	58,219	58,016	57,970	57,352	55,149	61,471	57,126	39,839