Epizyme (EPZM) 10-K 12 Mar 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Mar. 06, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	EPZM
Entity Registrant Name	EPIZYME, INC.
Entity Central Index Key	1571498
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		34,472,071
Entity Public Float			$491,990,520

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current Assets:
Cash and cash equivalents	$190,095	$123,564
Accounts receivable	2,075	33,667
Prepaid expenses and other current assets	2,840	2,421
Total current assets	195,010	159,652
Property and equipment, net	3,620	2,157
Restricted cash and other assets	573	1,179
Total Assets	199,203	162,988
Current Liabilities:
Accounts payable	8,300	4,698
Accrued expenses	7,043	6,632
Current portion of deferred revenue	1,702	23,243
Total current liabilities	17,045	34,573
Deferred revenue, net of current portion	21,449	23,629
Other long-term liabilities	427	473
Commitments and contingencies
Stockholders' Equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized; 0 shares issued and outstanding	0	0
Common stock, $0.0001 par value; 125,000,000 shares authorized; 34,426,012 shares and 28,494,447 shares issued, respectively; 34,426,012 shares and 28,488,892 shares outstanding, respectively	3	3
Additional paid-in capital	271,364	160,390
Accumulated deficit	-111,085	-56,080
Total stockholders' equity	160,282	104,313
Total Liabilities and Stockholders' Equity	$199,203	$162,988

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Preferred stock, par value	$0.00	$0.00
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	$0.00	$0.00
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	34,426,012	28,494,447
Common stock, shares outstanding	34,426,012	28,488,892

Consolidated_Statements_of_Ope

Consolidated Statements of Operations and Comprehensive Loss (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Collaboration revenue	$41,411	$68,482	$45,222
Operating expenses:
Research and development	75,595	57,567	38,482
General and administrative	20,866	14,042	7,508
Total operating expenses	96,461	71,609	45,990
Operating loss	-55,050	-3,127	-768
Other income (expense):
Interest income	95	74	145
Other income (expense), net	59	-81	-78
Other income (expense), net	154	-7	67
Loss before income taxes	-54,896	-3,134	-701
Income tax expense	109	349	1
Net loss	-55,005	-3,483	-702
Less: accretion of redeemable convertible preferred stock to redemption value	0	264	486
Loss allocable to common stockholders	-55,005	-3,747	-1,188
Loss per share allocable to common stockholders:
Basic	($1.67)	($0.22)	($0.72)
Diluted	($1.67)	($0.22)	($0.72)
Weighted average shares outstanding:
Basic	33,027	17,049	1,645
Diluted	33,027	17,049	1,645
Comprehensive loss	($55,005)	($3,483)	($702)

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	($55,005)	($3,483)	($702)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation and amortization	742	703	847
Stock-based compensation	6,864	2,819	689
Loss on disposal of property and equipment	2	0	35
Changes in operating assets and liabilities:
Accounts receivable	31,592	-31,838	44
Prepaid expenses and other current assets	-419	-1,495	-607
Accounts payable	3,611	1,641	469
Accrued expenses	411	2,304	2,667
Deferred revenue	-23,721	-22,573	39,628
Restricted cash and other assets	606	-433	-415
Other long-term liabilities	-46	-1,379	1,499
Net cash (used in) provided by operating activities	-35,363	-53,734	44,154
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	-2,216	-630	-1,482
Proceeds from property insurance claim	0	0	37
Net cash used in investing activities	-2,216	-630	-1,445
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from sale of redeemable convertible preferred stock	0	0	21,961
Proceeds from public offering, net of commissions	101,283	82,491	0
Proceeds from stock options exercised	2,736	277	8
Excess tax benefit from stock option plan	17	28	0
Issuance of shares under employee stock purchase plan	454	0	0
Payment of redeemable convertible preferred stock issuance costs	0	0	-38
Payment of common stock offering costs	-649	-2,849	0
Proceeds from reimbursement of common stock offering costs	269	0	0
Net cash provided by financing activities	104,110	79,947	21,931
Net increase in cash and cash equivalents	66,531	25,583	64,640
Cash and cash equivalents, beginning of period	123,564	97,981	33,341
Cash and cash equivalents, end of period	190,095	123,564	97,981
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	81	90	0
Conversion of redeemable convertible preferred stock to common stock	0	76,420	0
Accretion of redeemable convertible preferred stock to redemption value	0	264	486
Vesting of restricted stock liability	0	0	9
Cash paid for income taxes	$963	$8	$92

Consolidated_Statements_of_Red

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity (USD $)	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Redeemable Convertible Preferred Stock [Member]
In Thousands, except Share data
Beginning Balance, Value at Dec. 31, 2011	($50,644)	$0	$0	$1,251	($51,895)	$53,747
Beginning Balance, Shares at Dec. 31, 2011		1,645,729				52,095,243
Issuance of Series C redeemable convertible preferred stock (net of issuance costs of $38), Value						21,923
Issuance of Series C redeemable convertible preferred stock (net of issuance costs of $38), Shares						9,803,922
Exercise of stock options, Value	8			8
Exercise of stock options, Shares		15,800
Vesting of restricted common stock issued to non-employees	9			9
Stock-based compensation	689			689
Stock-based compensation, Shares		33,333
Accretion of redeemable convertible preferred stock to redemption value	-486			-486		486
Net loss	-702				-702
Ending Balance, Value at Dec. 31, 2012	-51,126	0	0	1,471	-52,597	76,156
Ending Balance, Shares at Dec. 31, 2012		1,694,862				61,899,165
Issuance of common stock (net of commissions and offering costs), Value	79,642	1		79,641
Issuance of common stock (net of commissions and offering costs), Shares		5,913,300
Exercise of stock options, Value	277			277
Exercise of stock options, Shares		253,239
Stock-based compensation	2,819			2,819
Excess tax benefit from stock option plan	28			28
Accretion of redeemable convertible preferred stock to redemption value	-264			-264		264
Conversion of redeemable convertible preferred stock to common stock, Value	76,420	2		76,418		-76,420
Conversion of redeemable convertible preferred stock to common stock, Shares		20,633,046				-61,899,165
Retirement of treasury stock	0	0	0	0	0	0
Net loss	-3,483				-3,483
Ending Balance, Value at Dec. 31, 2013	104,313	3	0	160,390	-56,080
Ending Balance, Shares at Dec. 31, 2013		28,494,447
Issuance of common stock (net of commissions and offering costs), Value	100,903	0	0	100,903	0	0
Issuance of common stock (net of commissions and offering costs), Shares		3,673,901				0
Exercise of stock options, Value	2,736	0	0	2,736	0	0
Exercise of stock options, Shares	2,239,643	2,239,643				0
Stock-based compensation	6,864	0	0	6,864	0	0
Excess tax benefit from stock option plan	17	0	0	17	0	0
Issuance of shares under employee stock purchase plan, Value	454	0	0	454	0	0
Accretion of redeemable convertible preferred stock to redemption value	0
Issuance of shares under employee stock purchase plan, Shares		18,021				0
Net loss	-55,005	0	0	0	-55,005	0
Ending Balance, Value at Dec. 31, 2014	$160,282	$3	$0	$271,364	($111,085)	$0
Ending Balance, Shares at Dec. 31, 2014		34,426,012				0

Consolidated_Statements_of_Red1

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity (Parenthetical) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Statement of Stockholders' Equity [Abstract]
Issuance of Series C redeemable convertible preferred stock, issuance costs	$0	$0
Issuance of common stock, commissions and offering costs	$649	$2,849

The_Company

The Company	12 Months Ended
	Dec. 31, 2014
Accounting Policies [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 discovers, develops and plans to commercialize novel epigenetic therapies for cancer patients. The Company has built a proprietary product platform that it uses to create small molecule inhibitors of a 96-member class of enzymes known as histone methyltransferases (“HMTs”). Genetic alterations can result in changes to the activity of HMTs, making them oncogenic. The Company’s therapeutic strategy is to inhibit oncogenic HMTs to treat the underlying causes of the associated cancers.
	On June 5, 2013, the Company completed an initial public offering (“IPO”) of its common stock, which resulted in the sale of 5,913,300 shares, including all additional shares available to cover over-allotments, at a price of $15.00 per share. The Company received net proceeds before expenses from the IPO of $82.5 million after deducting underwriting discounts and commissions paid by the Company. In preparation for the IPO, the Company’s Board of Directors and stockholders approved a one-for-three reverse stock split of the Company’s common stock effective May 13, 2013. All share and per share amounts in the consolidated financial statements and notes thereto have been retroactively adjusted, where necessary, to give effect to this reverse stock split. In connection with the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted to common stock at a one-for-three ratio as of June 5, 2013, resulting in an additional 20,633,046 shares of common stock of the Company becoming outstanding. Following these transactions, the Company’s total issued common stock as of December 31, 2013 was 28,494,447 shares.
	In February 2014, the Company completed a public offering of its common stock, which resulted in the sale of 3,673,901 shares, including all additional shares available to cover over-allotments, at a price of $29.25 per share. The Company received net proceeds before expenses from this offering of $101.3 million after deducting underwriting discounts and commissions paid by the Company. Following this transaction, the Company’s total issued common stock as of December 31, 2014 was 34,426,012 shares.
	The significant increases in shares outstanding in June 2013 and February 2014 are expected to impact the year-over-year comparability of the Company’s (loss) earnings per share calculations through 2015.
	The Company has generated an accumulated deficit of $111.1 million through December 31, 2014 and will require substantial additional capital to fund its research and development. It is subject to risks common to companies in the biotechnology industry, including, but not limited to, risks of failure of preclinical studies and clinical trials, 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 pilot-scale manufacturing to large-scale production of products.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
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 balances and transactions 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 of America 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.
	Fair Value Measurements
	The Company classifies fair value based measurements using a three-level hierarchy that prioritizes the inputs used to measure fair value. This hierarchy requires entities to maximize the use of observable inputs and minimize the use of unobservable inputs. The three levels of inputs used to measure fair value are as follows: Level 1, quoted market prices in active markets for identical assets or liabilities; Level 2, observable inputs other than quoted market prices included in Level 1 such as quoted market prices for markets that are not active or other inputs that are observable or can be corroborated by observable market data; and 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, including certain pricing models, discounted cash flow methodologies and similar techniques that use significant unobservable inputs.
	The Company’s financial instruments as of December 31, 2014 and 2013 consisted primarily of cash and cash equivalents, accounts receivable and accounts payable. As of December 31, 2014 and 2013, the Company’s financial assets recognized at fair value consisted of the following:
			Fair Value as of December 31, 2014
			Total		Level 1		Level 2		Level 3
			(In thousands)
	Cash equivalents		$	184,257	$	184,257	$	—	$	—
	Total		$	184,257	$	184,257	$	—	$	—
			Fair Value as of December 31, 2013
			Total		Level 1		Level 2		Level 3
			(In thousands)
	Cash equivalents		$	121,424	$	121,424	$	—	$	—
	Total		$	121,424	$	121,424	$	—	$	—
	Cash Equivalents
	The Company considers all highly liquid investments with maturities of three months or less when purchased to be cash equivalents. As of December 31, 2014 and 2013, cash equivalents consisted of interest-bearing money market accounts and prime money market funds.
	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, 2014 or 2013, 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 and cash equivalents and accounts receivable. The Company attempts to minimize the risks related to cash and cash equivalents 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.
	As of December 31, 2014 and 2013, three collaboration partners, Celgene Corporation (“Celgene”), Eisai Co. Ltd. (“Eisai”) and Glaxo Group Limted (an affiliate of GlaxoSmithKline) (“GSK”) accounted for all of the Company’s accounts receivable. Refer to Note 9, Collaborations, for additional information regarding the Company’s collaboration agreements.
	Property and Equipment
	The Company records property and equipment at cost. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:
	Asset Category		Useful Lives
	Laboratory equipment		5 - 20 years
	Office furniture and equipment		3 - 10 years
	Leasehold improvements		3 - 10 years or term of respective lease, if shorter
	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. No such impairments were recorded during 2014, 2013 or 2012.
	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.
	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 5, Income Taxes, for additional information regarding the Company’s income taxes.
	Redeemable Convertible Preferred Stock
	The Company initially records preferred stock that may be redeemed at the option of the holder or based on the occurrence of events not under the Company’s control outside of stockholders’ (deficit) equity at the value of the proceeds received or fair value, if lower, net of issuance costs. Subsequently, if it is probable that the preferred stock will become redeemable, the Company adjusts the carrying value to the redemption value over the period from the issuance date to the earliest possible redemption date using the effective interest method. If it is not probable that the preferred stock will become redeemable, the Company does not adjust the carrying value.
	Common Stock Valuation
	Prior to the completion of the Company’s IPO, due to the absence of an active market for the Company’s common stock, the Company utilized methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its common stock. The Company utilized a probability weighted expected return methodology for its common stock valuations as of February 11, 2011, April 30, 2012, November 30, 2012, February 28, 2013, April 18, 2013 and April 30, 2013 based upon an assessment of the probability of the occurrence of specific scenarios. Each valuation includes estimates and assumptions that require the Company’s judgment. These estimates include assumptions regarding future performance, including the probability of successful completion of preclinical studies and clinical trials and the probability and estimated time to completion of an initial public offering or sale. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date. Subsequent to the completion of the Company’s IPO, which occurred on June 5, 2013, the fair value of the Company’s common stock is based on observable market prices.
	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.
	Research and Development Expenses
	Research and development expenses consist of expenses incurred in performing research and development activities, including compensation and benefits, facilities expenses, overhead expenses, clinical trial and related clinical manufacturing expenses, fees paid to clinical research organizations and other outside expenses. The Company expenses research and development expenses as incurred. The Company records payments made for research and development services prior to the services being rendered as prepaid expenses on the consolidated balance sheets and expenses them as the services are provided. In circumstances where the Company’s collaboration and license agreements provide for equally co-funded global development under joint risk sharing collaborations, amounts received from collaboration partners for such co-funding are recorded as a reduction to research and development expense.
	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 generally grants stock options at exercise prices equal to the fair value of the Company’s common stock on the date of grant. Refer toCommon Stock Valuation for further information regarding the Company’s policy for determining the fair value of its common stock.
	The Company recognizes employee stock-based compensation expense, less estimated forfeitures, on a straight-line basis over the requisite service period of the awards. The Company estimates forfeitures at the time of grant and revises those estimates in subsequent periods if actual forfeitures differ from those estimates.
	Refer to Note 10, Employee Benefit Plans, for additional information regarding the measurement and recognition of expense related to the Company’s stock-based compensation awards.
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 common shares 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, for the Company’s calculation of loss per share for the periods presented.
Segment Information
The Company operates as one reportable business segment: the discovery and development of novel epigenetic therapies for cancer patients.
Recent Accounting Pronouncements
In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue From Contracts With Customers. ASU 2014-09 amends Accounting Standards Codification (“ASC”) 605, Revenue Recognition, by outlining a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers. ASU 2014-09 will be effective for the Company for interim and annual periods beginning after December 15, 2016. The Company is evaluating the impact that this ASU may have on its consolidated financial statements, if any.
In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern. ASU 2014-15 amends ASC 205-40, Presentation of Financial Statements—Going Concern, by providing guidance on determining when and how reporting entities must disclose going-concern uncertainties in their financial statements, including requiring management to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date of issuance of the entity’s financial statements and providing certain disclosures if there is substantial doubt about the entity’s ability to continue as a going concern. ASU 2014-15 will be effective for the Company for annual periods ending after December 15, 2016 and interim periods within annual periods beginning after December 15, 2016. The Company is still evaluating the impact of this ASU on its condensed consolidated financial statements; however, it is disclosure-only in nature.

Property_and_Equipment_net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Property and Equipment, net	3. Property and Equipment, net
	Property and equipment, net consists of the following:
		December 31,
		2014			2013
		(In thousands)
	Laboratory equipment	$	3,456		$	2,981
	Office furniture and equipment		2,971			1,293
	Leasehold improvements		473			430
	Property and equipment		6,900			4,704
	Less: accumulated depreciation and amortization		(3,280	)		(2,547	)
	Property and equipment, net	$	3,620		$	2,157
	Depreciation and amortization expense was $0.7 million, $0.7 million and $0.8 million for the years ended December 31, 2014, 2013 and 2012, respectively.

Accrued_Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Accrued Expenses	4. Accrued Expenses
	Accrued expenses consisted of the following:
		December 31,
		2014		2013
		(In thousands)
	Employee compensation and benefits	$	2,623	$	2,607
	Contract termination obligation		—		355
	Research and development and professional expenses		4,420		3,670
	Accrued expenses	$	7,043	$	6,632
	Contract termination obligation includes estimated lease exit charges related to the Company’s former facility at 325 Vassar Street in Cambridge, Massachusetts. As of December 31, 2013, the Company had a recorded contract termination obligation of $0.4 million. During 2014, the Company made cash payments of $0.9 million and recorded sublease income of $0.5 million. There was no remaining contract termination obligation as of December 31, 2014.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	5. Income Taxes
	The Company’s losses before income taxes consist solely of domestic losses. Income tax expense for the year ended December 31, 2014 consisted primarily of return-to-provision adjustments identified related to the year ended December 31, 2013. Income tax expense for the year ended December 31, 2013 consisted primarily of current federal tax expense as the Company was able to utilize all of its federal and state net operating loss carryforwards to offset the majority of its taxable income for the year. Income tax expense for the year ended December 31, 2012 consisted solely of current state expense as the Company was able to utilize federal net operating loss carryforwards to fully offset federal taxable income for the year. The Company had no deferred income tax expense for the years ended December 31, 2014, 2013 or 2012.
	A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows:
		Year Ended December 31,
		2014			2013			2012
	Federal statutory income tax rate		34	%		34	%		34	%
	State income taxes		4.9			5.2			5.1
	Research and development and other tax credits		13.6			115.8			68.6
	Permanent items		(5.2	)		(18.3	)		(41.7	)
	Change in valuation allowance		(44.3	)		(140.7	)		(60.9	)
	Return-to-provision adjustments		(3.1	)		1			3.6
	Change in deferred taxes		—			(9.6	)		(9.0	)
	Other		(0.1	)		1.5			0.2
	Effective income tax rate		(0.2	)%		(11.1	)%		(0.1	)%
	Deferred Tax Assets (Liabilities)
	The Company’s deferred tax assets (liabilities) consist of the following:
		December 31,
		2014			2013
		(In thousands)
	Deferred tax assets:
	Net operating loss carryforwards	$	24,166		$	—
	Research and development and other credit carryforwards		11,776			4,499
	Capitalized start-up costs		2,317			2,518
	Capitalized research and development costs		361			465
	Deferred revenue		8,869			16,748
	Accruals and allowances		1,095			1,111
	Other		2,336			1,247
	Gross deferred tax assets		50,920			26,588
	Deferred tax asset valuation allowance		(50,608	)		(26,289	)
	Total deferred tax assets		312			299
	Deferred tax liabilities:
	Depreciation and other		(312	)		(299	)
	Total deferred tax liabilities		(312	)		(299	)
	Net deferred tax asset (liability)	$	—		$	—
	The Company evaluated the expected recoverability of its net deferred tax assets as of December 31, 2014 and 2013 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 its net deferred tax assets would not be realized in the future; therefore the Company has provided a full valuation allowance against its net deferred tax asset balance as of December 31, 2014 and 2013. The valuation allowance increased by $24.3 million in 2014 compared to 2013.
	As of December 31, 2014, the Company had operating loss carryforwards of approximately $112.3 million and $115.7 million available to offset future taxable income for United States federal and state income tax purposes, respectively. The United States federal tax operating loss carryforwards expire commencing in 2029. The state tax operating loss carryforwards expire commencing in 2031. Additionally, as of December 31, 2014 the Company had research and development tax credit carryforwards of approximately $4.6 million and $1.4 million available to be used as a reduction of federal income taxes and state income taxes, respectively, which expire at various dates from 2024 through 2034, as well as federal orphan drug tax credit carryforwards of $9.1 million, which would expire at various dates from 2033 through 2034, and a $0.4 million federal alternative minimum tax credit. The federal tax credit carryforwards include approximately $0.4 million related to excess tax benefits, which have been included in the gross deferred tax asset reflected for research and development and other credit carryforwards. This amount will be recorded as an increase to additional paid-in capital on the consolidated balance sheet when the excess benefits are realized.
	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 United States 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,
		2014			2013
		(In thousands)
	Unrecognized tax benefits - as of beginning of year	$	1,829		$	—
	Gross increases - tax positions of prior periods		(66	)		1,217
	Gross increases - current period tax positions		1,299			612
	Unrecognized tax benefits - as of end of year	$	3,062		$	1,829
	None of the Company’s unrecognized tax benefits would result in income tax expense or impact the Company’s effective tax rate if recognized. Prior to 2013, the Company had no recorded unrecognized tax benefits. The Company had no accrued tax-related interest or penalties as of December 31, 2014 or 2013.
	The Company files income tax returns in the U.S. federal tax jurisdiction and Massachusetts and Indiana 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, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies
	Commitments
	In June 2012, the Company entered into an agreement to lease office and laboratory space at Technology Square in Cambridge, Massachusetts under an operating lease agreement with a term through November 30, 2017, with an option to extend the term of the lease for an additional five-year period at the then-current market rent, as defined in the lease. With the execution of this lease, the Company was required to provide a $0.5 million letter of credit as a security deposit. The Company has recorded cash held to secure this letter of credit as restricted cash in restricted cash and other assets on the consolidated balance sheet. The Company recognizes rent expense, inclusive of escalation charges, on a straight-line basis over the initial term of the lease agreement. The Company began recognizing rent expense related to the Technology Square lease in December 2012, when the Company gained access to the leased space. In September 2013, the Company entered into an amendment to the Technology Square lease, under which the Company leased additional office space. In the fourth quarter of 2014, the Company entered into three new leases relating to the lease of IT equipment and storage space. These leases have three year terms generally commencing in 2015.
	The Company’s contractual commitments under these leases, excluding common area maintenance charges and real estate taxes, as of December 31, 2014 are as follows:
		Total		2015		2016		2017
		(In thousands)
	Leases:
	Real estate	$	8,061	$	2,687	$	2,765	$	2,609
	Equipment		1,996		665		666		665
	Total commitments	$	10,057	$	3,352	$	3,431	$	3,274
	Rent expense, excluding contract termination costs related to the Vassar Street lease described in Note 4, Accrued Expenses, was $2.5 million, $2.0 million and $1.2 million for the years ended December 31, 2014, 2013 and 2012, respectively.
	Contingencies
	In January 2008, the Company entered into a license agreement with a university to obtain an exclusive license to certain patents and patent applications related to the Company’s technology (the “License Agreement”). In connection with the License Agreement, the Company is required to pay up to $1.9 million upon the achievement of specified research, development and regulatory milestones. The milestone payments are due within 60 days following the occurrence of each milestone event. In addition, the Company may be required to pay royalties in the low single-digits on worldwide net product sales of screening method technologies and related materials, but not on any drugs, during the term of the License Agreement. The Company has paid milestones of $0.1 million under the License Agreement as of December 31, 2014. The next potential milestone payment that the Company might be obligated to pay is $0.2 million that would be payable upon the initiation of a Phase 2 clinical trial for any product developed under the License Agreement.
	In October 2013, the Company entered into a license agreement with a third party to obtain a non-exclusive license to a patent related to an excipient in the formulation of a therapeutic product candidate. During the term of this license agreement, the Company may be required to make a €0.3 million milestone payment upon the first approval of a new drug application for this therapeutic product candidate and pay royalties in the low single digits on commercial net sales of the therapeutic product candidate.

Redeemable_Convertible_Preferr

Redeemable Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Redeemable Convertible Preferred Stock	7. Redeemable Convertible Preferred Stock
	Prior to the completion of its IPO, the Company had outstanding Series A, Series B and Series C redeemable convertible preferred stock (collectively, the “Preferred Stock”). The Company classified the Preferred Stock outside of stockholders’ (deficit) equity because the shares contained redemption features that were not solely within the Company’s control. In connection with the closing of the Company’s IPO, all of the Company’s outstanding Preferred Stock automatically converted into common stock at a one-for-three ratio as of June 5, 2013. No Preferred Stock was outstanding as of December 31, 2013.
	In April 2012, in connection with the execution of a collaboration agreement with Celgene Corporation and Celgene International Sàrl, collectively referred to as Celgene, the Company issued and sold 9,803,922 shares of its Series C Preferred Stock, $0.0001 par value per share (the “Series C Preferred Stock”), at a price of $2.55 per share (the “Series C Original Issue Price”), for gross proceeds of $25.0 million. The Company determined that the price paid by Celgene of $2.55 per share included a premium of $0.31 over the fair value per share of the Company’s Series C Preferred Stock based on the results of a contemporaneous valuation. Accordingly, the Company considered the $3.0 million premium as additional arrangement consideration pursuant to the collaboration agreement, resulting in $22.0 million attributed to the Series C Preferred Stock. Refer to Note 9,Collaborations, for additional information regarding the Company’s collaboration agreement with Celgene. Refer to Note 12, Related Party Transactions, for additional information regarding the Company’s relationship with Celgene.
	Preferred Stock consisted of the following as of December 31, 2012:
		Preferred		Issuance		Preferred		Redemption		Carrying		Common
		Shares		Date(s)		Shares		Value /		Value		Stock Issuable
		Authorized				Issued and		Liquidation				Upon
						Outstanding		Preference				Conversion
		(Amounts in thousands except share data)
	Series A		14,000,000		2/28/08		3,756,248	$	3,756	$	3,697		1,252,081
					5/16/08		10,243,752		10,244		10,226		3,414,581
			14,000,000				14,000,000		14,000		13,923		4,666,662
	Series B		38,096,000		9/18/09		14,285,716		15,000		14,928		4,761,902
					12/4/09		5,714,286		6,000		5,961		1,904,762
					9/30/11		18,095,241		19,000		18,983		6,031,746
			38,096,000				38,095,243		40,000		39,872		12,698,410
	Series C		9,803,922		4/2/12		9,803,922		25,000		22,361		3,267,974
			61,899,922				61,899,165	$	79,000	$	76,156		20,633,046
	The differences between the respective redemption values and carrying values were being accreted over the period from the date of issuance to the earliest possible redemption date. For the years ended December 31, 2013 and 2012, the Company recorded $0.3 million and $0.5 million of accretion of redeemable convertible preferred stock to redemption value, respectively. In connection with the closing of the Company’s IPO, all of the Company’s outstanding Preferred Stock automatically converted to common stock at a one-for-three ratio as of June 5, 2013.
	For the periods during which the Preferred Stock remained outstanding, the Company had evaluated each of its series of Preferred Stock and determined that they should be considered an “equity host” and not a “debt host” as defined by ASC 815, Derivatives and Hedging. This evaluation was necessary in order to determine if any embedded features required bifurcation and, therefore, separate accounting as a derivative liability. The Company’s analysis followed the “whole instrument approach,” which compares an individual feature against the entire preferred stock instrument which includes that feature. The Company’s analysis was based on a consideration of the Preferred Stock’s economic characteristics and risks and more specifically evaluated all the stated and implied substantive terms and features including (i) whether the Preferred Stock included redemption features, (ii) whether the preferred stockholders were entitled to dividends, (iii) the voting rights of the Preferred Stock and (iv) the existence and nature of any conversion rights. As a result of the Company’s determination that the Preferred Stock was an “equity host,” the embedded conversion feature was not considered a derivative liability.

Stockholders_Deficit_Equity

Stockholders' (Deficit) Equity	12 Months Ended
	Dec. 31, 2014
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.
	As of December 31, 2014, a total of 4,698,476 shares of common stock were reserved for issuance upon (i) the exercise of outstanding stock options 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, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborations	9. Collaborations
	Celgene
	Overview
	In April 2012, the Company entered into a collaboration and license agreement with Celgene to discover, develop and commercialize, in all countries other than the United States, small molecule HMT inhibitors targeting the DOT1L HMT, including the Company’s product candidate EPZ-5676, and any other HMT targets from the Company’s platform, excluding targets covered by the Company’s two other existing therapeutic collaborations (the “available targets”).
	Under the terms of the agreement, the Company received a $65.0 million upfront payment and $25.0 million from the sale of Series C 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 recorded a $25.0 million clinical development milestone payment and $5.8 million of global development co-funding through December 31, 2014. The Company is also eligible to earn up to $35.0 million in additional substantive clinical development milestone payments and up to $100.0 million in substantive regulatory milestone payments related to DOT1L as well as up to $65.0 million in payments, including a combination of substantive clinical development milestone payments and an option exercise fee for each selected target, and up to $100.0 million in substantive regulatory milestone payments for each available target as to which Celgene exercises its option during an initial option period ending in July 2015. Celgene has the right to extend the option period until July 2016 by making a significant option extension payment. As to DOT1L and each available target as to which Celgene exercises its option, the Company retains all product rights in the United States and is 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. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any milestone or royalty payments from Celgene. The next potential milestone payment that the Company might be entitled to receive under this agreement is a $35.0 million substantive milestone for the initiation of a pivotal clinical trial, as defined in the agreement, for its DOT1L inhibitor.
	Through December 31, 2014, in addition to amounts allocated to Celgene’s purchase of shares of the Company’s Series C Preferred Stock, the Company had recorded a total of $98.8 million in cash and accounts receivable under the Celgene agreement, including the $3.0 million implied premium on Celgene’s purchase of shares of the Company’s Series C Preferred Stock described in Note 7, Redeemable Convertible Preferred Stock. The Company recognized $9.6 million, $37.8 million and $23.9 million of collaboration revenue in the consolidated statements of operations and comprehensive loss related to this agreement during the years ended December 31, 2014, 2013 and 2012, respectively, and $3.9 million and $1.9 million of global development co-funding as a reduction to research and development expense during the years ended December 31, 2014 and 2013, respectively. As of December 31, 2014, the Company had deferred revenue of $21.7 million related to this agreement.
	Agreement Structure and Accounting Analysis. The Company granted Celgene an exclusive license, for all countries other than the United States, to HMT inhibitors directed to DOT1L and an option, on a target-by-target basis, to exclusively license, for all countries of the world other than the United States, rights to HMT inhibitors directed to other available targets during an initial three year period, which period may be extended by Celgene for one year upon an additional payment (the “option period”). During the option period, Celgene has the right to exercise its option to non-U.S. rights to available targets until the effectiveness of an investigational new drug application (“IND”) for an HMT inhibitor directed to such available target. Once a target is selected, Celgene does not have the right to replace it with another target. If Celgene does not exercise its option with respect to an available target prior to the end of the option period, the Company would retain worldwide rights to HMT inhibitors directed to that target.
	For the DOT1L target, the Company is obligated to conduct and solely fund research and development costs of the Phase 1 clinical trials for EPZ-5676, after which point Celgene and the Company will equally co-fund global development and each party will solely fund territory-specific development costs for its territory. These future co-development activities were determined to be a contingent deliverable at the inception of the agreement due to the substantial clinical uncertainty that existed at agreement inception as to the success of the product candidate in planned Phase 1 clinical trials and, as a result, are being accounted for separately.
	For the available targets, the Company must conduct and fully fund research and development activities through the option period. For any available target licensed to Celgene, the Company is obligated to conduct and solely fund research and development activities through the effectiveness of the first IND for an HMT inhibitor directed to such target, after which point Celgene and the Company will equally co-fund global development and each party will solely fund territory-specific development costs for its territory for such target. These future co-development activities were determined to be a contingent deliverable at the inception of the agreement due to the substantial clinical uncertainty that existed at agreement inception as to a product candidate achieving IND effectiveness and, as a result, will be accounted for separately when the activities occur. During the option period, the Company is required to use commercially reasonable efforts to conduct platform discovery activities necessary to characterize and identify available targets and HMT inhibitors directed to available targets and targets licensed to Celgene.
	The significant deliverables of this multiple-element revenue arrangement were determined to be the DOT1L license, the licenses to available targets and the research services for DOT1L and the available targets. The license for DOT1L is a deliverable as the Company was obligated at contract inception to provide Celgene with the rights to develop, manufacture and commercialize products directed at the target. The Company concluded that the options to license available targets were not substantive as the Company was not at risk with regard to Celgene exercising its options due to the size of the upfront payment. While Celgene is not contractually required to exercise its options to acquire any licenses to available targets, the overall purpose of the agreement was for Celgene to license available targets and develop and commercialize compounds for those targets outside of the United States. Without exercising its options to license available targets, Celgene could not obtain the economic benefit needed in order to recover its significant upfront payment. Since the options are not considered substantive, the licenses for available targets were considered to be deliverables at the inception of the arrangement.
	The Company concluded that, prior to IND effectiveness, the DOT1L license did not have standalone value apart from the related research services due to the limited economic benefit that Celgene would derive from the DOT1L license if it did not obtain the research services. In particular, the Company concluded that prior to IND effectiveness, the license could not be used for its intended purpose without the highly specialized skills and know-how relating to HMT inhibitors that are only available from the Company. After IND effectiveness, the Company concluded that the DOT1L license would have standalone value apart from any remaining undelivered development services because Celgene, or other market participants, would have the ability to execute human clinical trials on the identified compound. Accordingly, the DOT1L license and related research services were accounted for as a combined unit of accounting prior to IND effectiveness. After IND effectiveness, the research services have been accounted for as separate units of accounting which have standalone value upon delivery.
	With respect to the licenses to the available targets, the Company concluded that, prior to IND effectiveness, the licenses do not have standalone value apart from the related research services due to the limited economic benefit that Celgene would derive if it did not obtain the research services. In particular, the Company concluded that prior to IND effectiveness, a license could not be used for its intended purpose without the highly specialized skills and know-how relating to HMT inhibitors that are only available from the Company. Accordingly, the licenses to the available targets and related research services have been accounted for as a combined unit of accounting prior to IND effectiveness. The Company has also concluded that the individual licenses would have standalone value from one another; accordingly, the licenses to available targets and research services will be combined into units of accounting on a license-by-license basis prior to IND effectiveness. This conclusion was based on the determination that Celgene could derive benefit from any license and research services, prior to IND effectiveness, without regard to or receipt of any other license and accompanying research services prior to IND effectiveness.
	The number and timing of the delivery of the licenses for the available targets depends upon the Company’s research progress and Celgene’s option election. Because the options to available targets are not considered substantive, any option exercise payments would be considered to be part of the total consideration for purposes of allocating the arrangement consideration. Accordingly, the Company has identified the allocable arrangement consideration as the $65.0 million upfront payment, the $3.0 million premium on Celgene’s purchase of Series C Preferred Stock and an amount for option exercise fees for Celgene’s expected selection of available targets that is based on a fixed option exercise fee for each target selected. Although there is no contractual limit to the number of licenses to available targets that could be delivered during the option period and the number of selected targets is not known, the Company has estimated the number of available targets that it believes are reasonably likely to be selected by Celgene during the option period, based on information available to management at the time the agreement was executed, including the stage of development of the Company’s available targets, for the purpose of determining the allocable arrangement consideration. The Company concluded that Celgene would select three targets based on the status of research on prioritized targets within the Company’s product platform at the inception of the agreement and the likelihood of three candidates reaching IND effectiveness within the selection period ending in July 2015. 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 DOT1L license deliverable and each available target license deliverable using management’s best estimate of selling price after considering market data regarding the pricing of development and commercialization licenses for development candidates at similar stages of development after considering the territories covered by the licenses, as well as entity-specific factors such as the pricing terms of the Company’s previous collaboration arrangements, recent research and development results related to the Company’s product platform and preclinical product candidates, the market potential for each target and the Company’s pricing practices and pricing objectives. The Company estimated the selling price of the research services to be provided in connection with the DOT1L license deliverable and each available target license deliverable using management’s best estimate of selling price based on the Company’s cost of providing the services plus an applicable profit margin of 10%, which is commensurate with observable market data for similar services. Under this method, the relative selling price of each deliverable was estimated based on the Company’s analysis of (i) the stage of development of DOT1L and the available targets at both the inception of the arrangement and the potential option exercise dates; (ii) the market potential for each target; (iii) the level of effort required to advance DOT1L through the completion of Phase 1 clinical trials and each available target to IND effectiveness and (iv) the research funding structure for each program.
	The Company expects to recognize the allocated arrangement consideration as follows:
		•	DOT1L and related research services—
		•	The Company allocated $15.7 million to the DOT1L license and related research services prior to IND effectiveness based on the factors previously described and recognized this revenue ratably over the period from contract inception through the date of IND effectiveness in July 2012, as the related research services were provided.
		•	The remaining DOT1L research services have been determined to represent two separate units of accounting. Accordingly, based on the factors previously described, $12.1 million was allocated to the post-IND research services to be provided in connection with the lead product candidate for DOT1L, EPZ-5676, and $18.8 million was allocated to the research services to be provided in connection with other potential DOT1L product candidates. The Company is recognizing revenue ratably for each unit of accounting over the period that the corresponding research services are to be provided, which for the post-IND research services for EPZ-5676 is from the date of IND effectiveness in July 2012 through the estimated date of completion of the Phase 1 clinical trial as defined in the agreement, which was initially estimated to be 17 months, and for the other potential DOT1L product candidates is from contract inception through the estimated date of IND effectiveness for a licensed compound from the other potential DOT1L product candidates, which was initially estimated to be 37 months. Revenue recognized in the year ended December 31, 2013 reflected the Company’s plan to complete the ongoing Phase 1 study of EPZ-5676 in 2014. Revenue recognized in the year ended December 31, 2014 reflects the addition of a 54mg/m2/day expansion cohort to the Phase 1 study of EPZ-5676, which is expected to enroll throughout 2015. Accordingly, the Company expects to recognize the remaining deferred revenue related to this deliverable, of approximately $0.3 million as of December 31, 2014, through December 31, 2015. Revenue recognized in the year ended December 31, 2014 also reflects the Company’s decision to discontinue development efforts on other potential DOT1L product candidates. Accordingly, the Company recognized the $4.6 million remaining balance of arrangement consideration allocated to other potential DOT1L product candidates in the year ended December 31, 2014.
	The Company’s estimates of these revenue recognition periods are subject to the risks inherent in drug discovery. The Company will continue to re-assess the periods over which such services will be provided to consider any revisions to the estimated date of completion of the Phase 1 clinical trial for EPZ-5676.
		•	Available target licenses and related research services—
		•	The Company allocated $81.4 million to the available target licenses and related research services. Because the targets which Celgene could select would be at a similar stage of development, the Company expects to recognize, on a selected target-by-selected target basis, an equal amount of the allocated arrangement consideration over the period beginning when each available target is selected through the estimated date of IND effectiveness for each selected target. No available targets have been selected as of December 31, 2014.
	If Celgene exercises its option to extend the initial three year option period for one additional year, then the Company would be entitled to a significant option term extension payment. To the extent that Celgene extends the option period for an additional year, the Company would, at the time of any exercise of the extension option, allocate such payment based on its expectation as to the number of available targets that might be selected during the additional one year period and defer any option term extension payment until such time as a license to an available target was delivered.
	Remaining eligible milestone payments under this arrangement consist of up to $135.0 million in clinical development and regulatory milestones for DOT1L and up to $165.0 million in option exercise fees and clinical development and regulatory milestones for each available target. The Company evaluated the milestones under this arrangement and believes that the milestones are substantive given the significant uncertainty as to the outcome of the substantial research efforts to be performed by the Company in order to achieve the milestones. Therefore, the milestones will be recognized as collaboration revenue upon achievement.
	On a licensed target-by-licensed target basis, the Company has the right, in its sole discretion, to opt-out of further participation in and co-funding of development, other than specified costs necessary to complete development activities in process at the time the Company exercises its opt-out right. The Company can exercise its opt-out right at specified times before the scheduled initiation of the first pivotal clinical trial or before the estimated date of filing of the first new drug application for an HMT inhibitor directed to the licensed target or any time after regulatory approval of an HMT inhibitor directed to the licensed target. Following an opt-out, the Company is no longer required to co-fund global development for the applicable program, and it is obligated to grant Celgene an exclusive license to HMT inhibitors directed to the applicable target in the United States. Following its opt-out, if any, the Company will be eligible to receive specified milestone payments and royalties based on net product sales in the United States of HMT inhibitors directed to the licensed target. The Company would recognize revenue related to the milestones and royalties on any transferred target when earned, as the Company would have no performance obligations after exercising its opt-out. Based on the terms of the opt-out, the Company may not exercise its opt-out rights prior to completing its performance obligations under any of the deliverables identified at the inception of the agreement. None of the upfront cash payments, option exercise payments or option extension payment are subject to refund as a result of the opt-out provisions.
	Agreement Termination Rights. The Company’s agreement with Celgene will expire on a product-by-product and country-by-country basis on the date of the expiration of the applicable royalty term with respect to each licensed product in each country and in its entirety upon the expiration of all applicable royalty terms for all licensed products in all countries. The royalty term for each licensed product in each country is the period commencing with first commercial sale of the applicable licensed product in the applicable country and ending on the latest of expiration of specified patent coverage, specified regulatory exclusivity or a specified period of years.
	Celgene has the right to terminate the agreement with respect to one or more licensed targets or in its entirety, upon 60 or 120 days’ notice depending on the timing of such termination. The agreement may also be terminated in its entirety during the option period, and on a licensed target-by-licensed target basis after the option term, by either Celgene or the Company in the event of a material breach by the other party, in the event the other party, or an affiliate or sublicensee of the other party, participates or actively assists in a legal challenge to specified patent(s) of the terminating party or in the event the other party becomes subject to specified bankruptcy, insolvency or similar circumstances. There are no cancellation, termination or refund provisions in this arrangement that contain material financial consequences to the Company.
	Eisai
	Overview. 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 EPZ-6438, while retaining an opt-in right to co-develop, co-commercialize and share profits with Eisai as to licensed products in the United States. Additionally, as part of the research collaboration the Company agreed to provide research and development services related to the licensed compounds through December 31, 2014 (the “research period”).
	Under the terms of the agreement, the Company recorded a $3.0 million upfront payment, $7.0 million in preclinical research and development milestone payments, a $6.0 million clinical development milestone achieved in June 2013 and $22.7 million for research and development services through December 31, 2014. The Company is eligible to earn up to $25.0 million in additional clinical development milestone payments, including substantive milestone payments of up to $10.0 million, up to $55.0 million in regulatory milestone payments and up to $115.0 million in sales-based milestone payments. The Company is also eligible to receive royalties at a percentage in the mid-single digits on any net product sales outside of the United States and at a percentage from the mid-single digits to low double-digits on any product sales in the United States, subject to reduction in specified circumstances. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone payments or royalty or profit share payments from Eisai. The next potential milestone payment that the Company might be entitled to receive under this agreement is a $10.0 million substantive milestone for the initiation of the Phase 2 portion of the Phase 1/2 clinical trial.
	Through December 31, 2014, the Company recorded a total of $38.7 million in cash and accounts receivable related to this agreement. During the years ended December 31, 2014, 2013 and 2012, the Company recognized $6.3 million, $14.3 million and $11.5 million of collaboration revenue, respectively, related to this agreement. As of December 31, 2014, the Company had no remaining deferred revenue related to this agreement.
	Agreement Structure and Accounting Analysis. The significant deliverables of this multiple-element revenue arrangement were determined to be the worldwide license rights to EZH2 compounds and the research and development services. At the inception of the arrangement, the Company concluded that the license cannot be used for its intended purpose without the highly specialized skills and know-how relating to HMT inhibitors that is only available from the Company. The Company has therefore concluded that the delivered exclusive license lacked standalone value apart from the research and development services due to the limited economic benefit that Eisai would derive from the license if it did not obtain the Company’s research and development services. Consequently, the Company is accounting for these deliverables as a combined unit of accounting and is recognizing the $3.0 million upfront payment received from Eisai related to this agreement ratably over the research period. Funding for research and development services is being recognized as collaboration revenue in the period in which the related research and development costs are incurred.
	Upon the execution of this agreement, in addition to the $3.0 million upfront payment, the Company received another $3.0 million payment for a preclinical research and development milestone that was deemed to have already been achieved. Because this initial $3.0 million milestone was certain at the execution of the agreement and did not require substantive effort by the Company, it has been combined with the upfront payment and is being recognized as collaboration revenue ratably over the research period. The Company has evaluated the remaining milestones under this agreement and determined that the milestones through human proof-of-concept are substantive, given the significant uncertainty as to the outcome of the substantial research efforts to be performed by the Company in order to achieve the milestones. Therefore, payments for the achievement of any milestones through human proof-of-concept are being recognized as revenue upon achievement, assuming all other revenue recognition criteria are met. In the first quarter of 2012, the Company commenced a study for the lead product candidate, EPZ-6438, representing the first substantive research milestone under this arrangement. Accordingly, the $4.0 million milestone payment received from Eisai upon the achievement of this preclinical research and development milestone was recognized as revenue upon achievement. In the second quarter of 2013, the first patient was enrolled in the Phase 1/2 clinical trial of EPZ-6438, representing the first substantive clinical development milestone under this arrangement. Accordingly, the $6.0 million milestone payment due from Eisai upon the achievement of this milestone was recognized as revenue upon achievement. Evaluation of milestones after human proof-of-concept will be dependent upon the Company’s decision to participate or not participate in the profit share and co-commercialization arrangement with Eisai.
	Eisai solely funds all research, development and commercialization costs for licensed compounds, except for the cost obligations that the Company will undertake if it exercises its opt-in right to co-develop, co-commercialize and share profits with Eisai as to licensed products in the United States. The Company’s opt-in right to co-commercialize and share profits may be exercised on a licensed compound-by-licensed compound basis prior to the end of a specified period following Eisai’s provision to the Company of specified information following the licensed compound’s achievement of clinical proof-of-concept. If the Company exercises its opt-in right as to a licensed compound, the licensed compound becomes a shared product as to which: (i) Eisai’s obligation to pay royalties to the Company as to such shared product in the United States will terminate; (ii) Eisai and the Company will share in net profits or losses with respect to such shared product in the United States; (iii) 25.0% of specified past development costs will become creditable by Eisai against future milestone payments or royalties due to the Company, subject to certain limitations specified in the agreement; (iv) all subsequent milestones that become payable by Eisai after the Company exercises its opt-in right will be decreased by 50.0% in certain circumstances; and (v) Eisai and the Company will share equally in subsequent development costs allocated to the United States. All previous milestones earned by the Company are not subject to reimbursement.
	If the Company elects to exercise its opt-in right, (i) future research and development costs for the shared product would be recorded on a collaboration basis, in which case the Company’s 50.0% share of the costs would be recorded as research and development expense and (ii) the recognition of future milestones may be re-evaluated. If the Company does not exercise its opt-in right, the remaining milestones will not be considered substantive, as Eisai would then control the development leading to the achievement of such milestones, which would generally be achieved after the Company’s performance obligations are complete.
	Agreement Termination Rights. The Company’s agreement with Eisai will remain in effect until the later of expiration of all royalty obligations under the agreement with respect to all licensed products or, if the Company exercises its option, until the shared product is no longer being developed or commercialized by the parties in or for the United States or the parties’ agreement with respect to co-commercialization and profit sharing otherwise terminates. The royalty term for each licensed product in each country, other than shared products in the United States, is the period commencing with first commercial sale of the applicable licensed product in the applicable country and ending on the latest of expiration of specified patent coverage, specified regulatory exclusivity or a specified period of years.
	Eisai may terminate the agreement for its convenience in its entirety or as to one or more major market countries, as defined in the agreement, upon 90 days’ prior written notice to the Company. Eisai also has the right to terminate the agreement in its entirety immediately if, in good faith, it believes that it is not advisable for it to continue to develop or commercialize the licensed products from a scientific, regulatory or ethical perspective as a result of a bona fide serious safety issue regarding the use of any licensed product. The agreement may also be terminated by either party in the event of a material breach by the other party or by the Company in the event Eisai, or an affiliate or sublicensee, participates or actively assists in an action or proceeding challenging or denying the validity of one of the Company’s patents. There are no cancellation, termination or refund provisions in this arrangement that contain material financial consequences to the Company.
	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 EPZ-6438. Refer to Footnote 14, Subsequent Event.
	GlaxoSmithKline
	Overview. 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 the option to obtain exclusive worldwide license rights to HMT inhibitors directed to up to three targets. GSK selected and licensed three targets and the term during which it was entitled to select targets expired in July 2012. In March 2014, the Company and GSK amended certain terms of this agreement for the third 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. In connection with the execution of this amendment, the Company recorded a $3.0 million upfront payment.
	Under the terms of the agreement, the Company recorded 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, $15.0 million of preclinical research and development milestone payments and $9.0 million for research and development services through December 31, 2014. The Company is eligible to receive up to $18.0 million in additional substantive preclinical research and development milestone payments, up to $109.0 million in clinical development milestone payments, up to $275.0 million in regulatory milestone payments and up to $218.0 million in sales-based milestone payments. In addition, GSK is required to pay the Company royalties at percentages between the mid-single digits to the low double-digits, on a licensed product-by-licensed product basis, on worldwide net product sales, subject to reduction in certain specified circumstances. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone payments or royalty payments from GSK. Due to the varying stages of development of each licensed target, the Company is not able to determine the next milestone that might be achieved under this agreement, if any.
	Through December 31, 2014, the Company recorded a total of $53.0 million in cash and accounts receivable under the GSK agreement. During the years ended December 31, 2014, 2013 and 2012, the Company recognized $25.5 million, $16.4 million and $9.7 million of collaboration revenue, respectively, related to this agreement. As of December 31, 2014, the Company had deferred revenue of $1.4 million related to this agreement.
	Agreement Structure and Accounting Analysis. For each selected target in the collaboration, the Company was primarily responsible for research until the earlier of the selection of a development candidate for the target or January 8, 2015, and GSK is solely responsible for subsequent development and commercialization. The Company was responsible for providing research and development services with respect to the selected targets pursuant to agreed-upon research plans during a research term that ended in January 2015, or earlier if a target reaches development candidate selection, at which point GSK is solely responsible for development and commercialization. GSK provided a fixed amount of research funding during the second and third years of the research term. GSK was obligated to provide research funding equal to 100.0% of research and development costs, subject to specified limitations, during the fourth year of the research term.
	The significant deliverables of this multiple-element revenue arrangement were determined to be exclusive licenses to three targets and corresponding research services for each target. At the inception of the arrangement, the Company concluded that the licenses cannot be used for their intended purpose without the highly specialized skills and know-how relating to HMT inhibitors that is only available from the Company. The Company therefore concluded that the target licenses lacked standalone value apart from the related research services due to the limited economic benefit that GSK would derive from the licenses if it did not obtain the Company’s research services and due to the lack of transferability of the exclusive licenses. The Company is therefore accounting for these deliverables, on a license-by-license basis, as a combined unit of accounting. The Company concluded that the option to secure licenses for three targets was not substantive, as the Company was not at risk with regard to GSK exercising its option due to the size of the upfront payment and the research funding commitment. Since the option was not considered substantive, the Company considered the licenses to be deliverables at the inception of the agreement. While the Company concluded that there were three units of accounting, each consisting of a license to a target and the research and development services related to that target, because the targets were at similar stages of development at the inception of the agreement, had equal probabilities of success and the research services in each unit of accounting were initially expected to be performed concurrently on a ratable basis over the research term, the Company allocated the arrangement consideration equally across the three targets. Accordingly, the $30.0 million of allocable arrangement consideration, consisting of the $20.0 million upfront payment, $4.0 million in milestone payments achieved during the selection term and the $6.0 million fixed research funding, is being recognized as collaboration revenue, on a target-by-target basis, ratably from the conclusion of the selection term, in July 2012, through the end of the research term, or earlier if a target reaches development candidate selection, at which point GSK is solely responsible for development and commercialization. In December 2013, the Company and GSK agreed to the selection of a development candidate for one of the three targets under the agreement, earning the Company a $4.0 million milestone payment and reducing the period over which the Company is recognizing revenue for this target by nine months. Accordingly, the Company recognized the remaining deferred revenue related to this target during the first quarter of 2014. As to this target, GSK is solely responsible for subsequent development and commercialization.
	The $3.0 million upfront payment received in connection with the March 2014 amendment was allocated equally to the remaining two targets for which the Company was actively providing research and development services, as these remaining two targets were at similar stages of development, had equal probabilities of success and the remaining research services were expected to be performed concurrently on a ratable basis over the research term. The $3.0 million is being recognized as collaboration revenue, on a target-by-target basis, ratably from the execution of the amendment, in March 2014, through the end of the research term. In the fourth quarter of 2014, the Company agreed to perform additional preclinical research and development studies related to the third target under the agreement, which extends the research term for this target through June 2015. Accordingly, the Company expects to recognize the remaining deferred revenue related to this deliverable, of approximately $1.2 million as of December 31, 2014, through June 30, 2015.
	During the selection term, the Company received $4.0 million upon the achievement of preclinical research and development milestones which required effort in the form of research activities by the Company and was not certain to be achieved at the execution of the agreement. However, because GSK had the right to drop a target and select a replacement target at any point during the selection term, the Company, in such a case, would have been obligated to perform the validation work for a replacement target. Consequently, this $4.0 million in preclinical research and development milestones has been combined with the upfront payment and fixed research funding and is being recognized as collaboration revenue ratably over the research term. The Company has evaluated the remaining milestones under this agreement and determined that the milestones through development candidate selection are substantive given the significant uncertainty as to the outcome of the substantial research efforts to be performed by the Company in order to achieve the milestones and will be recognized as revenue upon achievement, assuming all other revenue recognition criteria are met. The milestones after development candidate selection are not considered substantive because the Company does not contribute effort to the achievement of such milestones, which would generally be achieved after the research term. In 2014, the Company achieved $3.0 million in preclinical research and development milestones upon the selection of lead candidates for the second and third targets under the agreement. In the fourth quarter of 2013, the Company achieved a $4.0 million preclinical research and development milestone upon the selection of a development candidate for one of the three targets under the agreement. In the third quarter of 2012, the Company achieved two additional preclinical research and development milestones and received payments totaling $4.0 million. The preclinical research and development milestones achieved in 2014, 2013 and 2012 required effort in the form of research activities by the Company and were not certain to be achieved at the execution of the agreement. Additionally, at the time of the achievement of these preclinical research and development milestones, the selection term had expired and, as such, these milestones were determined to be substantive, and the milestones were recognized as revenue upon achievement.
	Under the agreement, the Company also granted GSK the option to acquire up to 10.0% of the securities issued in its next qualified venture capital financing, if any, which meets conditions set forth in the agreement. The Company is not obligated to undertake any such financing and one has not occurred since the Company granted GSK this right.
	Agreement Termination Rights. The agreement will expire on a product-by-product and country-by-country basis on the date of the expiration of the applicable royalty term with respect to each licensed product in each country and in its entirety upon the expiration of all applicable royalty terms for all licensed products in all countries. The royalty term for each licensed product in each country is the period commencing with the first commercial sale of the applicable licensed product in the applicable country and ending on the later of expiration of specified patent coverage or a specified period of years.
	GSK has the right to terminate the agreement at any time with respect to one or more selected targets or in its entirety, upon 90 days’ prior written notice to the Company. The agreement may also be terminated with respect to one or more selected targets or in its entirety by either GSK or the Company in the event of a material breach by the other party. The agreement may be terminated with respect to selected targets by the Company in the event GSK, or an affiliate or sublicensee of GSK, participates or actively assists in a legal challenge to one of the patents exclusively licensed to GSK under the agreement with respect to the applicable target.
	The Leukemia & Lymphoma Society
	In June 2011, The Leukemia & Lymphoma Society (“LLS”) and the Company entered into an arrangement to support preclinical and Phase 1 development of the Company’s DOT1L-targeted HMT inhibitors for mixed lineage leukemia. Under this arrangement, LLS committed to provide up to $7.5 million in development milestone-based payments to the Company to support the program through Phase 1 clinical trials in exchange for defined future royalties and transfer payments.
	The Company received $2.6 million in funding from LLS through May 2012, including upfront payments of $1.1 million and a milestone payment of $1.5 million received in March 2012 for a clinical candidate declaration. The Company paid LLS $0.4 million as a transfer payment relating to the Company’s entry into its collaboration agreement with Celgene.
	In June 2012, the Company exercised its option to terminate this arrangement and repaid LLS $0.8 million, representing the portion of the upfront payment that had not yet been spent on research. Upon the Company’s exercise of its termination option, LLS elected to receive a termination fee equal to the aggregate amount of funding provided by LLS through the termination date, less any amounts previously repaid. Accordingly, the Company did not recognize any revenue in 2012 related to either the LLS upfront payment or the milestone achieved. Except for certain acceleration provisions described in the agreement, the termination fee was payable in three equal annual installments plus 10.0% interest. Accordingly, the Company had accrued $2.0 million as of December 31, 2012, representing the present value of its obligation to LLS in connection with the termination of this agreement. Upon the closing of the Company’s IPO, the Company’s obligation to LLS was accelerated, and, as a result, this termination obligation was paid in full in June 2013. Refer to Celgene for additional information regarding the collaboration agreement with Celgene.
	Roche
	In December 2012, Eisai and the Company entered into an agreement with Roche Molecular Systems, Inc. (“Roche”) under which Eisai and the Company are funding Roche’s development of a companion diagnostic to identify patients who possess certain point mutations in EZH2. In October 2013, this agreement was amended to include additional point mutations in EZH2. The development costs under the agreement with Roche will be the responsibility of Eisai until such time, if any, as the Company exercises its opt-in right under its collaboration agreement with Eisai. Under the terms of the amended agreement, Eisai agreed to pay Roche defined milestone payments of up to $21.5 million to develop and to make commercially available the companion diagnostic. As a result, the cost of the companion diagnostic agreement prior to the Company’s potential future exercise of its opt-in right under the Eisai collaboration will not be reflected in the Company’s consolidated statements of operations and comprehensive loss. If the Company exercises its opt-in right to co-develop, co- commercialize and share profits in the United States as to EPZ-6438, Eisai will be entitled to offset up to 25.0% of the funding amount it has previously paid to Roche against future milestone payments and royalties that Eisai may be obligated to pay to the Company under the Eisai collaboration and license agreement, and the Company will become obligated to fund up to half of the defined milestones that remain payable to Roche as of the time the Company opts-in.
	The Company’s agreement with Roche will expire when Eisai or the Company are no longer developing or commercializing the Company’s product directed to EZH2. Eisai and the Company may terminate the agreement by giving Roche 90 days’ written notice if the Company and Eisai discontinue development and commercialization of the Company’s product directed to EZH2 or determine, in conjunction with Roche, that the diagnostic is not needed for use with the Company’s product directed to EZH2. Either Eisai and the Company or Roche 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 may become entitled to specified termination fees, which Eisai and the Company would be obligated to bear in the same manner that they bear the funding amounts payable to Roche.
	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 EPZ-6438. Refer to Footnote 14, Subsequent Event.
	Abbott
	In February 2013, the Company entered into an agreement with Abbott Molecular Inc. (“Abbott”) under which the Company agreed to fund Abbott’s development of a companion diagnostic to identify patients with the mixed lineage leukemia (“MLL-r”) genetic alteration targeted by the Company’s EPZ-5676 product candidate. Under the terms of the agreement, the Company paid Abbott an upfront payment of $0.9 million upon the execution of the agreement, and agree dto make aggregate milestone-based development payments of up to $6.0 million and to reimburse Abbott for specified costs not to exceed $0.9 million. In October 2014, the Company voluntarily terminated this agreement with Abbott, discontinuing development of this proposed companion diagnostic. The termination of this agreement did not have a material impact on the Company’s financial statements.

Employee_Benefit_Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based 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. Upon the closing of the IPO, the Company ceased granting stock incentive awards under the 2008 Plan, and any shares of common stock that remained available for grant under the 2008 Plan upon the closing of the IPO became available for issuance under the 2013 Plan. In addition, any shares of common stock subject to awards under the 2008 Plan that expire, terminate, or are otherwise surrendered, canceled, forfeited or repurchased without having been fully exercised or resulting in any common stock being issued will become available for issuance under the 2013 Plan. 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. Incentive and non-qualified stock options expire ten years from the date of grant. 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.
	Stock-Based Compensation
	Total stock-based compensation related to stock options, restricted stock and the employee stock purchase plan was $6.9 million, $2.8 million and $0.7 million in the years ended December 31, 2014, 2013 and 2012, respectively.
	Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows:
		Year Ended December 31,
		2014			2013			2012
		(In thousands)
	Research and development	$	3,299		$	1,072		$	448
	General and administrative		3,565			1,747			241
	Total	$	6,864		$	2,819		$	689
	Stock Options
	The Company uses the Black-Scholes option-pricing model to measure the fair value of stock option awards. Key assumptions used in this pricing model on the date of grant for options granted to employees are as follows:
		Year Ended December 31,
		2014			2013			2012
	Risk-free interest rate		1.6	%		1	%		0.7	%
	Expected life of options		6.0 years			6.0 years			6.0 years
	Expected volatility of underlying stock		92.1	%		94.7	%		98.9	%
	Expected dividend yield		0	%		0	%		0	%
	Key assumptions used in this pricing model on the date of grant for options granted to non-employees in 2013 are as follows:
		Year Ended December 31,
		2013
	Risk-free interest rate		3	%
	Expected life of options		10.0 years
	Expected volatility of underlying stock		86.3	%
	Expected dividend yield		0	%
	There were no stock option awards granted to non-employees in 2014 or 2012.
	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 based on 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, 2014:
		Number of			Weighted			Weighted			Aggregate
		Options			Average			Average			Intrinsic
					Exercise			Remaining			Value
					Price per			Contractual
					Share			Term
								(In years)			(In thousands)
	Outstanding at December 31, 2013		4,728,503		$	3.13
	Granted		876,385			28.88
	Exercised		(2,239,643	)		1.22
	Forfeited or expired		(405,739	)		14.32
	Outstanding at December 31, 2014		2,959,506		$	10.66			6.8		$	33,538
	Exercisable at December 31, 2014		1,393,210		$	2.7			5.5		$	23,112
	During the years ended December 31, 2014, 2013 and 2012, the Company granted stock options to purchase an aggregate of 876,385 shares, 1,514,828 shares and 766,297 shares of its common stock, including, in 2013, a stock option to purchase 10,000 shares of its common stock granted to a non-employee, at weighted average grant date fair values per option share of $21.73, $7.85 and $1.83, respectively. The total grant date fair value of options that vested during the years ended December 31, 2014, 2013 and 2012 was $4.9 million, $0.6 million and $0.6 million, respectively. The aggregate intrinsic value of stock options exercised was $59.4 million in 2014, $2.1 million in 2013 and insignificant during 2012.
	As of December 31, 2014, there was $16.5 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 2.5 years.
	Restricted Stock
	The following is a summary of restricted stock activity for the year ended December 31, 2014:
		Number of			Weighted
		Shares			Average Grant
					Date Fair Value
					per Share
	Outstanding at December 31, 2013		5,555		$	0.6
	Vested		(5,555	)		0.6
	Outstanding at December 31, 2014		—		$	—
	During the year ended December 31, 2012 the Company granted 33,333 shares of restricted stock with a weighted-average grant date fair value per share of $0.60. The Company did not grant any restricted stock during the years ended December 31, 2014 or 2013. The intrinsic value of restricted stock that vested during the years ended December 31, 2014, 2013 and 2012 was $0.1 million, $0.4 million and $0.1 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 $0.2 million in the year ended December 31, 2014.

Loss_per_Share

Loss per Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Loss per Share	11. Loss per Share
	As described in Note 2, Summary of Significant Accounting Policies, the Company computes basic and diluted earnings (loss) per share using a methodology that gives effect to the impact of outstanding participating securities (the “two-class method”)
	Basic and diluted loss per share allocable to common stockholders are computed as follows:
		Year Ended December 31,
		2014			2013			2012
		(In thousands except per share data)
	Net loss	$	(55,005	)	$	(3,483	)	$	(702	)
	Less: accretion of redeemable convertible preferred stock to redemption value		—			264			486
	Loss allocable to common stockholders	$	(55,005	)	$	(3,747	)	$	(1,188	)
	Weighted average shares outstanding		33,027			17,049			1,645
	Basic and diluted loss per share allocable to common stockholders	$	(1.67	)	$	(0.22	)	$	(0.72	)
	In June 2013, the Company issued 5,913,300 shares of common stock in connection with its IPO and 20,633,046 shares of common stock in connection with the automatic conversion of its Preferred Stock upon the closing of the IPO. In February 2014, the Company issued an additional 3,673,901 shares of common stock in connection with a public offering. The issuance of these shares contributed to a significant increase in the Company’s shares outstanding, to 34,426,012 shares as of December 31, 2014, and in the weighted average shares outstanding for the years ended December 31, 2014 and 2013 when compared to the comparable prior year periods and is expected to continue to impact the year-over-year comparability of the Company’s (loss) earnings per share calculations through 2015.
	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. The redeemable convertible preferred stock amounts shown in the table are on a common stock equivalent basis as a result of the reverse stock split described in Note 1, The Company:
		Year Ended December 31,
		2014			2013			2012
		(In thousands)
	Redeemable convertible preferred stock		—			—			20,633
	Stock options		2,960			4,729			3,493
	Unvested restricted stock		—			6			22
	Shares issuable under employee stock purchase plan		6			48			—
			2,966			4,783			24,148

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions
	In connection with its entry into the collaboration agreement with Celgene, on April 2, 2012, the Company sold Celgene 9,803,922 shares of its Series C Preferred Stock. As a result of this transaction, Celgene owned 12.5% of the Company’s fully diluted equity as of December 31, 2012. Refer to Note 9, Collaborations, for additional information regarding this collaboration agreement. In the second quarter of 2013, in connection with the IPO, Celgene made an additional investment in the Company, acquiring an additional 66,666 shares of the Company’s common stock. Additionally, as a result of the IPO, Celgene’s shares of Series C Preferred Stock automatically converted to common stock of the Company at a one-for-three ratio, collectively resulting in Celgene owning 3,334,640 shares of the Company’s common stock as of December 31, 2013. In the first quarter of 2014, in connection with the Company’s public offering of common stock, Celgene made an additional investment in the Company, acquiring an additional 340,000 shares of the Company’s common stock, maintaining an ownership percentage representing 9.8% of the Company’s fully diluted equity and 10.7% of the voting interests of the Company as of December 31, 2014.
	Under the Celgene collaboration agreement, the Company recognized $9.6 million, $37.8 million and $23.9 million of collaboration revenue in the years ended December 31, 2014, 2013 and 2012, respectively, and as of December 31, 2014 and December 31, 2013, had recorded $21.7 million and $31.3 million of deferred revenue related to the Celgene collaboration arrangement, respectively. Additionally, in the years ended December 31, 2014 and 2013, the Company recorded $3.9 million and $1.9 million, respectively, in global development co-funding from Celgene. As of December 31, 2014 and 2013, the Company had accounts receivable of $1.1 million and $26.2 million, respectively, related to this collaboration arrangement.

Unaudited_Quarterly_Results

Unaudited Quarterly Results	12 Months Ended
	Dec. 31, 2014
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, 2014 and 2013 are set forth below:
		Quarter Ended
		March 31,			June 30,			September 30,			December 31,
		2014			2014			2014			2014
	Collaboration revenue	$	13,391		$	9,494		$	8,177		$	10,349
	Operating expenses:
	Research and development		15,347			17,499			22,244			20,505
	General and administrative		4,956			5,306			5,669			4,935
	Total operating expenses		20,303			22,805			27,913			25,440
	Operating loss		(6,912	)		(13,311	)		(19,736	)		(15,091	)
	Other income, net		28			38			41			47
	Loss before income taxes		(6,884	)		(13,273	)		(19,695	)		(15,044	)
	Income tax expense (benefit)		—			113			5			(9	)
	Net loss	$	(6,884	)	$	(13,386	)	$	(19,700	)	$	(15,035	)
	Loss per share allocable to common stockholders:
	Basic	$	(0.22	)	$	(0.40	)	$	(0.58	)	$	(0.44	)
	Diluted	$	(0.22	)	$	(0.40	)	$	(0.58	)	$	(0.44	)
	Weighted average shares outstanding:
	Basic		30,959			33,156			33,676			34,273
	Diluted		30,959			33,156			33,676			34,273
		Quarter Ended
		March 31,			June 30,			September 30,			December 31,
		2013			2013			2013			2013
	Collaboration revenue	$	8,882		$	14,839		$	8,444		$	36,317
	Operating expenses:
	Research and development		13,361			13,937			14,584			15,685
	General and administrative		2,998			3,079			3,587			4,378
	Total operating expenses		16,359			17,016			18,171			20,063
	Operating (loss) income		(7,477	)		(2,177	)		(9,727	)		16,254
	Other (expense) income, net		(20	)		(35	)		23			25
	(Loss) income before income taxes		(7,497	)		(2,212	)		(9,704	)		16,279
	Income tax expense		—			—			—			349
	Net (loss) income	$	(7,497	)	$	(2,212	)	$	(9,704	)	$	15,930
	Less: accretion of redeemable convertible preferred stock to redemption value		157			107			—			—
	Less: income allocable to participating securities		—			—			—			4
	(Loss) income allocable to common stockholders	$	(7,654	)	$	(2,319	)	$	(9,704	)	$	15,926
	(Loss) earnings per share allocable to common stockholders:
	Basic	$	(4.27	)	$	(0.25	)	$	(0.34	)	$	0.56
	Diluted	$	(4.27	)	$	(0.25	)	$	(0.34	)	$	0.52
	Weighted average shares outstanding:
	Basic		1,791			9,146			28,406			28,434
	Diluted		1,791			9,146			28,406			30,901

Subsequent_Event

Subsequent Event	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Event	14. Subsequent Event
	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 EPZ-6438. Under the amended and restated collaboration agreement, the Company will be responsible for global development, manufacturing and commercialization outside of Japan of EPZ-6438 and any other EZH2 product candidates, with Eisai retaining development and commercialization rights in Japan, as well as a right to elect to manufacture EPZ-6438 and any other EZH2 product candidates in Japan. Under the original collaboration and license agreement, the Company had granted Eisai an exclusive worldwide license to our small molecule HMT inhibitors directed to EZH2, including EPZ-6438, while retaining an opt-in right to co-develop, co-commercialize and share profits with Eisai as to licensed products in the United States.
	Upon the execution of the amended and restated collaboration and license agreement, the Company agreed to pay Eisai a $40.0 million upfront payment. The Company also agreed to pay Eisai up to $20.0 million in clinical development milestone payments, up to $50.0 million in regulatory milestone payments 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.
	Under the original agreement, Eisai was solely responsible for funding all research, development and commercialization costs for licensed compounds. Under the amended agreement, the Company will be solely responsible for funding global development and commercialization costs for EZH2 compounds outside of Japan, including $8.5 million of the remaining milestone payments due under the Roche companion diagnostic agreement, and Eisai will be solely responsible for funding Japan-specific development and commercialization costs for EZH2 compounds. In connection with the amendment and restatement of our collaboration and license agreement with Eisai, the Company and Eisai have agreed upon a transition to the Company of ongoing development and manufacturing activities being conducted by or on behalf of Eisai.
	In the event that the Company seeks to license rights to a third party to develop or commercialize an EZH2 product in any country in Asia other than Japan, Eisai has a limited right of first negotiation for such rights. In the event that the Company is awarded a priority review voucher from the FDA with respect to an EZH2 product, Eisai is entitled to specified compensation if the Company uses the voucher on a non-EZH2 program or sells the voucher to a third party.
	The Company’s amended and restated collaboration and license agreement with Eisai will remain in effect until the expiration of all payment obligations under the agreement with respect to all licensed products. The royalty term for each licensed product in each country commences on the first commercial sale of the applicable licensed product in the applicable country and ends on the latest of expiration of specified patent coverage, expiration of specified regulatory exclusivity or ten years following the first commercial sale. The Company or Eisai may terminate the agreement for convenience as to their respective territories, upon 90 days’ prior written notice. The agreement will also terminate as to the Company’s territory if the Company ceases all development and commercialization activities for the United States and specified major countries in Europe and as to Eisai’s territory if Eisai ceases all development and commercialization activities for Japan. The agreement may also be terminated by either party in the event of an uncured material breach by the other party or by the Company in the event Eisai, or an affiliate or sublicensee, participates or actively assists in an action or proceeding challenging or denying the validity of one of the Company’s patents. If the Company terminates the agreement for its convenience, the agreement terminates as a result of the Company’s cessation of development and commercialization activities or Eisai terminates the agreement for the Company’s uncured material breach, Eisai may elect to have worldwide development and commercialization rights revert to Eisai, and if Eisai so elects, Eisai will be required to pay the Company specified royalties on net sales of the licensed products and reimburse certain development expenses incurred by the Company. If Eisai terminates the agreement for its convenience, the agreement terminates as a result of Eisai’s cessation of development and commercialization activities or the Company terminates the agreement for Eisai’s uncured material breach or Eisai’s, or its affiliate’s or sublicensee’s, participation in, or assistance with, an action or proceeding challenging or denying the validity of one of the Company’s patents, Japanese development and commercialization rights to the licensed products revert to the Company, and the Company will be required to pay Eisai specified royalties on net sales of licensed products in Japan.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
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 balances and transactions 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 of America 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.
Fair Value Measurements	Fair Value Measurements
	The Company classifies fair value based measurements using a three-level hierarchy that prioritizes the inputs used to measure fair value. This hierarchy requires entities to maximize the use of observable inputs and minimize the use of unobservable inputs. The three levels of inputs used to measure fair value are as follows: Level 1, quoted market prices in active markets for identical assets or liabilities; Level 2, observable inputs other than quoted market prices included in Level 1 such as quoted market prices for markets that are not active or other inputs that are observable or can be corroborated by observable market data; and 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, including certain pricing models, discounted cash flow methodologies and similar techniques that use significant unobservable inputs.
	The Company’s financial instruments as of December 31, 2014 and 2013 consisted primarily of cash and cash equivalents, accounts receivable and accounts payable. As of December 31, 2014 and 2013, the Company’s financial assets recognized at fair value consisted of the following:
			Fair Value as of December 31, 2014
			Total		Level 1		Level 2		Level 3
			(In thousands)
	Cash equivalents		$	184,257	$	184,257	$	—	$	—
	Total		$	184,257	$	184,257	$	—	$	—
			Fair Value as of December 31, 2013
			Total		Level 1		Level 2		Level 3
			(In thousands)
	Cash equivalents		$	121,424	$	121,424	$	—	$	—
	Total		$	121,424	$	121,424	$	—	$	—
Cash Equivalents	Cash Equivalents
	The Company considers all highly liquid investments with maturities of three months or less when purchased to be cash equivalents. As of December 31, 2014 and 2013, cash equivalents consisted of interest-bearing money market accounts and prime money market funds.
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, 2014 or 2013, 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 and cash equivalents and accounts receivable. The Company attempts to minimize the risks related to cash and cash equivalents 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.
	As of December 31, 2014 and 2013, three collaboration partners, Celgene Corporation (“Celgene”), Eisai Co. Ltd. (“Eisai”) and Glaxo Group Limted (an affiliate of GlaxoSmithKline) (“GSK”) accounted for all of the Company’s accounts receivable. Refer to Note 9, Collaborations, for additional information regarding the Company’s collaboration agreements.
Property and Equipment	Property and Equipment
	The Company records property and equipment at cost. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:
	Asset Category		Useful Lives
	Laboratory equipment		5 - 20 years
	Office furniture and equipment		3 - 10 years
	Leasehold improvements		3 - 10 years or term of respective lease, if shorter
	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. No such impairments were recorded during 2014, 2013 or 2012.
	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.
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 5, Income Taxes, for additional information regarding the Company’s income taxes.
Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock
	The Company initially records preferred stock that may be redeemed at the option of the holder or based on the occurrence of events not under the Company’s control outside of stockholders’ (deficit) equity at the value of the proceeds received or fair value, if lower, net of issuance costs. Subsequently, if it is probable that the preferred stock will become redeemable, the Company adjusts the carrying value to the redemption value over the period from the issuance date to the earliest possible redemption date using the effective interest method. If it is not probable that the preferred stock will become redeemable, the Company does not adjust the carrying value.
Common Stock Valuation	Common Stock Valuation
	Prior to the completion of the Company’s IPO, due to the absence of an active market for the Company’s common stock, the Company utilized methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its common stock. The Company utilized a probability weighted expected return methodology for its common stock valuations as of February 11, 2011, April 30, 2012, November 30, 2012, February 28, 2013, April 18, 2013 and April 30, 2013 based upon an assessment of the probability of the occurrence of specific scenarios. Each valuation includes estimates and assumptions that require the Company’s judgment. These estimates include assumptions regarding future performance, including the probability of successful completion of preclinical studies and clinical trials and the probability and estimated time to completion of an initial public offering or sale. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date. Subsequent to the completion of the Company’s IPO, which occurred on June 5, 2013, the fair value of the Company’s common stock is based on observable market prices.
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.
Research and Development Expenses	Research and Development Expenses
	Research and development expenses consist of expenses incurred in performing research and development activities, including compensation and benefits, facilities expenses, overhead expenses, clinical trial and related clinical manufacturing expenses, fees paid to clinical research organizations and other outside expenses. The Company expenses research and development expenses as incurred. The Company records payments made for research and development services prior to the services being rendered as prepaid expenses on the consolidated balance sheets and expenses them as the services are provided. In circumstances where the Company’s collaboration and license agreements provide for equally co-funded global development under joint risk sharing collaborations, amounts received from collaboration partners for such co-funding are recorded as a reduction to research and development expense.
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 generally grants stock options at exercise prices equal to the fair value of the Company’s common stock on the date of grant. Refer to Common Stock Valuation for further information regarding the Company’s policy for determining the fair value of its common stock.
	The Company recognizes employee stock-based compensation expense, less estimated forfeitures, on a straight-line basis over the requisite service period of the awards. The Company estimates forfeitures at the time of grant and revises those estimates in subsequent periods if actual forfeitures differ from those estimates.
	Refer to Note 10, Employee Benefit Plans, for additional information regarding the measurement and recognition of expense related to the Company’s stock-based compensation awards.
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 common shares 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, for the Company’s calculation of loss per share for the periods presented.
Segment Information	Segment Information
	The Company operates as one reportable business segment: the discovery and development of novel epigenetic therapies for cancer patients.
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue From Contracts With Customers. ASU 2014-09 amends Accounting Standards Codification (“ASC”) 605, Revenue Recognition, by outlining a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers. ASU 2014-09 will be effective for the Company for interim and annual periods beginning after December 15, 2016. The Company is evaluating the impact that this ASU may have on its consolidated financial statements, if any.
	In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern. ASU 2014-15 amends ASC 205-40, Presentation of Financial Statements—Going Concern, by providing guidance on determining when and how reporting entities must disclose going-concern uncertainties in their financial statements, including requiring management to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date of issuance of the entity’s financial statements and providing certain disclosures if there is substantial doubt about the entity’s ability to continue as a going concern. ASU 2014-15 will be effective for the Company for annual periods ending after December 15, 2016 and interim periods within annual periods beginning after December 15, 2016. The Company is still evaluating the impact of this ASU on its condensed consolidated financial statements; however, it is disclosure-only in nature.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of December 31, 2014 and 2013, the Company’s financial assets recognized at fair value consisted of the following:
		Fair Value as of December 31, 2014
		Total		Level 1		Level 2		Level 3
		(In thousands)
	Cash equivalents	$	184,257	$	184,257	$	—	$	—
	Total	$	184,257	$	184,257	$	—	$	—
		Fair Value as of December 31, 2013
		Total		Level 1		Level 2		Level 3
		(In thousands)
	Cash equivalents	$	121,424	$	121,424	$	—	$	—
	Total	$	121,424	$	121,424	$	—	$	—
Useful Lives for Property, Plant and Equipment	The Company records property and equipment at cost. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:
	Asset Category	Useful Lives
	Laboratory equipment	5 - 20 years
	Office furniture and equipment	3 - 10 years
	Leasehold improvements	3 - 10 years or term of respective lease, if shorter

Property_and_Equipment_net_Tab

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, net	Property and equipment, net consists of the following:
		December 31,
		2014			2013
		(In thousands)
	Laboratory equipment	$	3,456		$	2,981
	Office furniture and equipment		2,971			1,293
	Leasehold improvements		473			430
	Property and equipment		6,900			4,704
	Less: accumulated depreciation and amortization		(3,280	)		(2,547	)
	Property and equipment, net	$	3,620		$	2,157

Accrued_Expenses_Tables

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following:
		December 31,
		2014		2013
		(In thousands)
	Employee compensation and benefits	$	2,623	$	2,607
	Contract termination obligation		—		355
	Research and development and professional expenses		4,420		3,670
	Accrued expenses	$	7,043	$	6,632

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
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,
		2014			2013			2012
	Federal statutory income tax rate		34	%		34	%		34	%
	State income taxes		4.9			5.2			5.1
	Research and development and other tax credits		13.6			115.8			68.6
	Permanent items		(5.2	)		(18.3	)		(41.7	)
	Change in valuation allowance		(44.3	)		(140.7	)		(60.9	)
	Return-to-provision adjustments		(3.1	)		1			3.6
	Change in deferred taxes		—			(9.6	)		(9.0	)
	Other		(0.1	)		1.5			0.2
	Effective income tax rate		(0.2	)%		(11.1	)%		(0.1	)%
Deferred Tax Assets (Liabilities)	The Company’s deferred tax assets (liabilities) consist of the following:
		December 31,
		2014			2013
		(In thousands)
	Deferred tax assets:
	Net operating loss carryforwards	$	24,166		$	—
	Research and development and other credit carryforwards		11,776			4,499
	Capitalized start-up costs		2,317			2,518
	Capitalized research and development costs		361			465
	Deferred revenue		8,869			16,748
	Accruals and allowances		1,095			1,111
	Other		2,336			1,247
	Gross deferred tax assets		50,920			26,588
	Deferred tax asset valuation allowance		(50,608	)		(26,289	)
	Total deferred tax assets		312			299
	Deferred tax liabilities:
	Depreciation and other		(312	)		(299	)
	Total deferred tax liabilities		(312	)		(299	)
	Net deferred tax asset (liability)	$	—		$	—
Summary of Unrecognized Tax Benefits	The following is a rollforward of the Company’s unrecognized tax benefits:
		December 31,
		2014			2013
		(In thousands)
	Unrecognized tax benefits - as of beginning of year	$	1,829		$	—
	Gross increases - tax positions of prior periods		(66	)		1,217
	Gross increases - current period tax positions		1,299			612
	Unrecognized tax benefits - as of end of year	$	3,062		$	1,829

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014 are as follows:
		Total		2015		2016		2017
		(In thousands)
	Leases:
	Real estate	$	8,061	$	2,687	$	2,765	$	2,609
	Equipment		1,996		665		666		665
	Total commitments	$	10,057	$	3,352	$	3,431	$	3,274

Redeemable_Convertible_Preferr1

Redeemable Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Summary of Preferred Stock	Preferred Stock consisted of the following as of December 31, 2012:
		Preferred		Issuance		Preferred		Redemption		Carrying		Common
		Shares		Date(s)		Shares		Value /		Value		Stock Issuable
		Authorized				Issued and		Liquidation				Upon
						Outstanding		Preference				Conversion
		(Amounts in thousands except share data)
	Series A		14,000,000		2/28/08		3,756,248	$	3,756	$	3,697		1,252,081
					5/16/08		10,243,752		10,244		10,226		3,414,581
			14,000,000				14,000,000		14,000		13,923		4,666,662
	Series B		38,096,000		9/18/09		14,285,716		15,000		14,928		4,761,902
					12/4/09		5,714,286		6,000		5,961		1,904,762
					9/30/11		18,095,241		19,000		18,983		6,031,746
			38,096,000				38,095,243		40,000		39,872		12,698,410
	Series C		9,803,922		4/2/12		9,803,922		25,000		22,361		3,267,974
			61,899,922				61,899,165	$	79,000	$	76,156		20,633,046

Employee_Benefit_Plans_Tables

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2014
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,
		2014			2013			2012
		(In thousands)
	Research and development	$	3,299		$	1,072		$	448
	General and administrative		3,565			1,747			241
	Total	$	6,864		$	2,819		$	689
Assumptions Used in Applying Pricing Model	The Company uses the Black-Scholes option-pricing model to measure the fair value of stock option awards. Key assumptions used in this pricing model on the date of grant for options granted to employees are as follows:
		Year Ended December 31,
		2014			2013			2012
	Risk-free interest rate		1.6	%		1	%		0.7	%
	Expected life of options		6.0 years			6.0 years			6.0 years
	Expected volatility of underlying stock		92.1	%		94.7	%		98.9	%
	Expected dividend yield		0	%		0	%		0	%
Summary of Stock Option Activity	The following is a summary of stock option activity for the year ended December 31, 2014:
		Number of			Weighted			Weighted			Aggregate
		Options			Average			Average			Intrinsic
					Exercise			Remaining			Value
					Price per			Contractual
					Share			Term
								(In years)			(In thousands)
	Outstanding at December 31, 2013		4,728,503		$	3.13
	Granted		876,385			28.88
	Exercised		(2,239,643	)		1.22
	Forfeited or expired		(405,739	)		14.32
	Outstanding at December 31, 2014		2,959,506		$	10.66			6.8		$	33,538
	Exercisable at December 31, 2014		1,393,210		$	2.7			5.5		$	23,112
Summary of Restricted Stock Activity	The following is a summary of restricted stock activity for the year ended December 31, 2014:
		Number of			Weighted
		Shares			Average Grant
					Date Fair Value
					per Share
	Outstanding at December 31, 2013		5,555		$	0.6
	Vested		(5,555	)		0.6
	Outstanding at December 31, 2014		—		$	—
Non Employee Stock Option [Member]
Assumptions Used in Applying Pricing Model	Key assumptions used in this pricing model on the date of grant for options granted to non-employees in 2013 are as follows:
		Year Ended December 31,
		2013
	Risk-free interest rate		3	%
	Expected life of options		10.0 years
	Expected volatility of underlying stock		86.3	%
	Expected dividend yield		0	%

Loss_per_Share_Tables

Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2014
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,
		2014			2013			2012
		(In thousands except per share data)
	Net loss	$	(55,005	)	$	(3,483	)	$	(702	)
	Less: accretion of redeemable convertible preferred stock to redemption value		—			264			486
	Loss allocable to common stockholders	$	(55,005	)	$	(3,747	)	$	(1,188	)
	Weighted average shares outstanding		33,027			17,049			1,645
	Basic and diluted loss per share allocable to common stockholders	$	(1.67	)	$	(0.22	)	$	(0.72	)
Common Stock Equivalents Excluded from Calculation of Diluted Loss Per Share Attributable to Common Stockholders	The redeemable convertible preferred stock amounts shown in the table are on a common stock equivalent basis as a result of the reverse stock split described in Note 1, The Company:
		Year Ended December 31,
		2014			2013			2012
		(In thousands)
	Redeemable convertible preferred stock		—			—			20,633
	Stock options		2,960			4,729			3,493
	Unvested restricted stock		—			6			22
	Shares issuable under employee stock purchase plan		6			48			—
			2,966			4,783			24,148

Unaudited_Quarterly_Results_Ta

Unaudited Quarterly Results (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014 and 2013 are set forth below:
		Quarter Ended
		March 31,			June 30,			September 30,			December 31,
		2014			2014			2014			2014
	Collaboration revenue	$	13,391		$	9,494		$	8,177		$	10,349
	Operating expenses:
	Research and development		15,347			17,499			22,244			20,505
	General and administrative		4,956			5,306			5,669			4,935
	Total operating expenses		20,303			22,805			27,913			25,440
	Operating loss		(6,912	)		(13,311	)		(19,736	)		(15,091	)
	Other income, net		28			38			41			47
	Loss before income taxes		(6,884	)		(13,273	)		(19,695	)		(15,044	)
	Income tax expense (benefit)		—			113			5			(9	)
	Net loss	$	(6,884	)	$	(13,386	)	$	(19,700	)	$	(15,035	)
	Loss per share allocable to common stockholders:
	Basic	$	(0.22	)	$	(0.40	)	$	(0.58	)	$	(0.44	)
	Diluted	$	(0.22	)	$	(0.40	)	$	(0.58	)	$	(0.44	)
	Weighted average shares outstanding:
	Basic		30,959			33,156			33,676			34,273
	Diluted		30,959			33,156			33,676			34,273
		Quarter Ended
		March 31,			June 30,			September 30,			December 31,
		2013			2013			2013			2013
	Collaboration revenue	$	8,882		$	14,839		$	8,444		$	36,317
	Operating expenses:
	Research and development		13,361			13,937			14,584			15,685
	General and administrative		2,998			3,079			3,587			4,378
	Total operating expenses		16,359			17,016			18,171			20,063
	Operating (loss) income		(7,477	)		(2,177	)		(9,727	)		16,254
	Other (expense) income, net		(20	)		(35	)		23			25
	(Loss) income before income taxes		(7,497	)		(2,212	)		(9,704	)		16,279
	Income tax expense		—			—			—			349
	Net (loss) income	$	(7,497	)	$	(2,212	)	$	(9,704	)	$	15,930
	Less: accretion of redeemable convertible preferred stock to redemption value		157			107			—			—
	Less: income allocable to participating securities		—			—			—			4
	(Loss) income allocable to common stockholders	$	(7,654	)	$	(2,319	)	$	(9,704	)	$	15,926
	(Loss) earnings per share allocable to common stockholders:
	Basic	$	(4.27	)	$	(0.25	)	$	(0.34	)	$	0.56
	Diluted	$	(4.27	)	$	(0.25	)	$	(0.34	)	$	0.52
	Weighted average shares outstanding:
	Basic		1,791			9,146			28,406			28,434
	Diluted		1,791			9,146			28,406			30,901

The_Company_Additional_Informa

The Company - Additional Information (Detail) (USD $)	0 Months Ended		1 Months Ended		12 Months Ended
In Thousands, except Share data, unless otherwise specified	Jun. 05, 2013	13-May-13	Feb. 28, 2014	Jun. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of public offering of common stock	5,913,300		3,673,901	5,913,300
Amount of per share for all additional shares	$15		$29.25
Net proceeds from public offering	$82,500		$101,300		$101,283	$82,491	$0
Reverse stock split of common stock		0.3333
Redeemable convertible preferred stock conversion ratio	One-for-three
Issuance of shares of common stock	20,633,046			20,633,046
Common stock issued					34,426,012	28,494,447
Accumulated deficit					($111,085)	($56,080)

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Summary of Company's Financial Assets Recognized at Fair Value (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$184,257	$121,424
Total	184,257	121,424
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	184,257	121,424
Total	184,257	121,424
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total	0	0
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total	$0	$0

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	Segment
Accounting Policies [Abstract]
Allowance for doubtful accounts	$0	$0
Impairment of long-lived assets	$0	$0	$0
Largest amount of tax benefit	50.00%
Reportable business segment	1

Summary_of_Significant_Account5

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets Acquired (Detail)	12 Months Ended
	Dec. 31, 2014
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	20 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_Sch

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property, Plant and Equipment [Line Items]
Property and equipment	$6,900	$4,704
Less: accumulated depreciation and amortization	-3,280	-2,547
Property and equipment, net	3,620	2,157
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	3,456	2,981
Office Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	2,971	1,293
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$473	$430

Property_and_Equipment_Net_Add

Property and Equipment, Net - Additional Information (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$742	$703	$847

Accrued_Expenses_Schedule_of_A

Accrued Expenses - Schedule of Accrued Expenses (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Accrued Liabilities, Current [Abstract]
Employee compensation and benefits	$2,623	$2,607
Contract termination obligation	0	355
Research and development and professional expenses	4,420	3,670
Accrued expenses	$7,043	$6,632

Accrued_Expenses_Additional_In

Accrued Expenses - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	31-May-12
Restructuring Reserve [Abstract]
Contract termination obligations	$0	$400,000	$400,000
Cash payments for contract termination obligations	900,000
Sublease income	$500,000

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Deferred income tax expense	$0	$0	$0

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of Federal Statutory Income Tax Rate and Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	34.00%	34.00%	34.00%
State income taxes	4.90%	5.20%	5.10%
Research and development and other tax credits	13.60%	115.80%	68.60%
Permanent items	-5.20%	-18.30%	-41.70%
Change in valuation allowance	-44.30%	-140.70%	-60.90%
Return-to-provision adjustments	-3.10%	1.00%	3.60%
Change in deferred taxes		-9.60%	-9.00%
Other	0.10%	1.50%	0.20%
Effective income tax rate	-0.20%	-11.10%	-0.10%

Income_Taxes_Deferred_Tax_Asse

Income Taxes - Deferred Tax Assets Liabilities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Deferred tax assets:
Net operating loss carryforwards	$24,166	$0
Research and development and other credit carryforwards	11,776	4,499
Capitalized start-up costs	2,317	2,518
Capitalized research and development costs	361	465
Deferred revenue	8,869	16,748
Accruals and allowances	1,095	1,111
Other	2,336	1,247
Gross deferred tax assets	50,920	26,588
Deferred tax asset valuation allowance	-50,608	-26,289
Total deferred tax assets	312	299
Deferred tax liabilities:
Depreciation and other	-312	-299
Total deferred tax liabilities	-312	-299
Net deferred tax asset (liability)	$0	$0

Income_Taxes_Valuation_Allowan

Income Taxes (Valuation Allowance) - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Valuation allowance increase	$24.30

Income_Taxes_Operating_Loss_Ca

Income Taxes (Operating Loss Carryforwards) - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
United States, Federal [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$112.30
Operating loss carryforwards, expiration beginning year	2029
United States, State [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$115.70
Operating loss carryforwards, expiration beginning year	2031

Income_Taxes_Research_and_Deve

Income Taxes (Research and Development Tax Credit) - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Tax Credit Carryforward [Line Items]
Research and development tax credit carryforward expiration beginning year	2024
Research and development tax credit carryforward expiration ending year	2034
Federal orphan drug tax credit carryforwards expiration beginning year	2033
Expiry period of federal orphan drug tax credit carryforwards	2034
Federal orphan drug tax credit carryforwards	$9,100,000
Federal alternative minimum tax credit	400,000
Excess tax benefits	400,000
Unrecognized tax benefits	3,062,000	1,829,000	0
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	4,600,000
Research Tax Credit Carryforward [Member] | United States, State [Member]
Tax Credit Carryforward [Line Items]
Research and development credit carryforwards	$1,400,000

Income_Taxes_Summary_of_Unreco

Income Taxes - Summary of Unrecognized Tax Benefits (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Unrecognized tax benefits - as of beginning of year	$1,829	$0
Gross increases - tax positions of prior periods	-66	1,217
Gross increases - current period tax positions	1,299	612
Unrecognized tax benefits - as of end of year	$3,062	$1,829

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Detail)	12 Months Ended				3 Months Ended		1 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2014	Oct. 31, 2013	Jan. 31, 2008	Dec. 31, 2013	Dec. 31, 2014
	USD ($)	USD ($)	USD ($)	USD ($)	Equipment Leases [Member]	Formulation Patent License Agreement [Member]	University Patent License Agreement [Member]	University Patent License Agreement [Member]	University Patent License Agreement [Member]
					Lease	EUR (€)	USD ($)	USD ($)	USD ($)
Other Commitments [Line Items]
Letter of credit as a security deposit				$0.50
Lease expiration date	30-Nov-17
Lease term extension	5 years
Number of new leases					3
Lease term					3 years
Lease commencing year					2015
Rental expense	2.5	2	1.2
License agreement cost							1.9
Milestone payments paid								0.1
Milestone payment payable						€ 0.30			$0.20
Milestone payment due period							60 days

Commitments_and_Contingencies_2

Commitments and Contingencies - Contractual Commitments (Detail) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Commitment And Contingencies [Line Items]
Total	$10,057
2015	3,352
2016	3,431
2017	3,274
Real Estate [Member]
Commitment And Contingencies [Line Items]
Total	8,061
2015	2,687
2016	2,765
2017	2,609
Equipment Leases [Member]
Commitment And Contingencies [Line Items]
Total	1,996
2015	665
2016	666
2017	$665

Redeemable_Convertible_Preferr2

Redeemable Convertible Preferred Stock - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended		1 Months Ended
	Jun. 05, 2013	Dec. 31, 2013	Dec. 31, 2012	Apr. 30, 2012
Temporary Equity [Line Items]
Exchange ratio of convertible preferred stock to common stock	One-for-three
Preferred stock outstanding		0	61,899,165
Accretion of redeemable convertible preferred stock, value		$264,000	$486,000
Series C [Member] | Celgene [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, issued				9,803,922
Preferred stock, par value				$0.00
Redeemable convertible preferred stock issued, price per share				$2.55
Redeemable convertible preferred stock issued, gross proceeds				25,000,000
Redeemable convertible preferred stock issued, premium price per share				$0.31
Redeemable convertible preferred stock issued, premium value				3,000,000
Redeemable convertible preferred stock issued, value				$22,000,000

Redeemable_Convertible_Preferr3

Redeemable Convertible Preferred Stock - Summary of Preferred Stock (Detail) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2013
Temporary Equity [Line Items]
Preferred Shares Authorized	61,899,922
Preferred Shares Issued and Outstanding	61,899,165	0
Redemption Value / Liquidation Preference	$79,000
Carrying Value	76,156
Common Stock Issuable Upon Conversion	20,633,046
Series A Preferred Stock Issued 2/28/2008 [Member]
Temporary Equity [Line Items]
Issuance Date(s)	28-Feb-08
Preferred Shares Issued and Outstanding	3,756,248
Redemption Value / Liquidation Preference	3,756
Carrying Value	3,697
Common Stock Issuable Upon Conversion	1,252,081
Series A Preferred Stock Issued 5/16/2008 [Member]
Temporary Equity [Line Items]
Issuance Date(s)	16-May-08
Preferred Shares Issued and Outstanding	10,243,752
Redemption Value / Liquidation Preference	10,244
Carrying Value	10,226
Common Stock Issuable Upon Conversion	3,414,581
Series A [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	14,000,000
Preferred Shares Issued and Outstanding	14,000,000
Redemption Value / Liquidation Preference	14,000
Carrying Value	13,923
Common Stock Issuable Upon Conversion	4,666,662
Series B Preferred Stock Issued 9/18/2009 [Member]
Temporary Equity [Line Items]
Issuance Date(s)	18-Sep-09
Preferred Shares Issued and Outstanding	14,285,716
Redemption Value / Liquidation Preference	15,000
Carrying Value	14,928
Common Stock Issuable Upon Conversion	4,761,902
Series B Preferred Stock Issued 12/4/2009 [Member]
Temporary Equity [Line Items]
Issuance Date(s)	4-Dec-09
Preferred Shares Issued and Outstanding	5,714,286
Redemption Value / Liquidation Preference	6,000
Carrying Value	5,961
Common Stock Issuable Upon Conversion	1,904,762
Series B Preferred Stock Issued 9/30/2011 [Member]
Temporary Equity [Line Items]
Issuance Date(s)	30-Sep-11
Preferred Shares Issued and Outstanding	18,095,241
Redemption Value / Liquidation Preference	19,000
Carrying Value	18,983
Common Stock Issuable Upon Conversion	6,031,746
Series B [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	38,096,000
Preferred Shares Issued and Outstanding	38,095,243
Redemption Value / Liquidation Preference	40,000
Carrying Value	39,872
Common Stock Issuable Upon Conversion	12,698,410
Series C Preferred Stock Issued 4/2/2012 [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	9,803,922
Issuance Date(s)	2-Apr-12
Preferred Shares Issued and Outstanding	9,803,922
Redemption Value / Liquidation Preference	25,000
Carrying Value	$22,361
Common Stock Issuable Upon Conversion	3,267,974

Stockholders_Deficit_Equity_Ad

Stockholders' (Deficit) Equity - Additional Information (Detail)	Dec. 31, 2014
	Vote
Equity [Abstract]
Number of votes for each share of common stock	1
Common stock reserved for future issuance	4,698,476

Collaborations_Additional_Info

Collaborations - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended			33 Months Ended	1 Months Ended			3 Months Ended	45 Months Ended	1 Months Ended	18 Months Ended	36 Months Ended	48 Months Ended	1 Months Ended		12 Months Ended	1 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Apr. 30, 2012	Jun. 30, 2013	Apr. 30, 2011	Mar. 31, 2012	Dec. 31, 2014	Mar. 31, 2014	Jun. 30, 2012	Dec. 31, 2013	Dec. 31, 2014	Jun. 30, 2012	Mar. 31, 2012	31-May-12	Oct. 31, 2013	Feb. 28, 2013	Jun. 30, 2011
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue	$10,349,000	$8,177,000	$9,494,000	$13,391,000	$36,317,000	$8,444,000	$14,839,000	$8,882,000	$41,411,000	$68,482,000	$45,222,000
Accrued termination value of collaborative agreement	0				400,000				0	400,000		0					0			400,000	0			400,000
Celgene [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Company received upfront payment									65,000,000			65,000,000
Proceeds from redeemable convertible preferred stock												25,000,000
Upfront payment recorded												68,000,000
Clinical development milestone achieved										25,000,000		25,000,000
Global development co-funding									3,900,000	1,900,000		5,800,000
Option period ending												Jul-15
Right to extend option exercise period date												2016-07
Cash and accounts receivable												98,800,000
convertible preferred stock issued, premium value												3,000,000
Collaboration revenue									9,600,000	37,800,000	23,900,000
Deferred revenue	21,700,000								21,700,000			21,700,000					21,700,000				21,700,000
Celgene [Member] | Minimum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Notice period in days												60 days
Celgene [Member] | Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Notice period in days												120 days
Celgene [Member] | DOT1L [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Clinical development milestone payments	35,000,000								35,000,000			35,000,000					35,000,000				35,000,000
Additional milestone payments	100,000,000								100,000,000			100,000,000					100,000,000				100,000,000
Potential milestone payments receivable	35,000,000								35,000,000			35,000,000					35,000,000				35,000,000
Applicable profit margin												10.00%
Maximum milestone receivable	135,000,000								135,000,000			135,000,000					135,000,000				135,000,000
Celgene [Member] | DOT1L [Member] | License and Pre-IND Services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
License and research services related cost												15,700,000
Celgene [Member] | DOT1L [Member] | Post-IND Services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Deferred revenue	300,000								300,000			300,000					300,000				300,000
License and research services related cost												12,100,000
Estimated time to complete the phase													17 months
Celgene [Member] | DOT1L [Member] | Alternative Compound [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
License and research services related cost												18,800,000
Estimated time to complete the phase													37 months
Celgene [Member] | Available Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments	100,000,000								100,000,000			100,000,000					100,000,000				100,000,000
Additional payments	65,000,000								65,000,000			65,000,000					65,000,000				65,000,000
License and research services related cost												81,400,000
Maximum milestone receivable	165,000,000								165,000,000			165,000,000					165,000,000				165,000,000
Celgene [Member] | Other Potential Dotil Product Candidates [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Recognized remaining balance of arrangement consideration												4,600,000
Eisai [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment recorded																	3,000,000
Clinical development milestone achieved										6,000,000				6,000,000			6,000,000
Clinical development milestone payments	25,000,000								25,000,000			25,000,000					25,000,000				25,000,000
Additional milestone payments	55,000,000								55,000,000			55,000,000					55,000,000				55,000,000
Potential milestone payments receivable	10,000,000								10,000,000			10,000,000					10,000,000				10,000,000
Cash and accounts receivable																	38,700,000
Collaboration revenue									6,300,000	14,300,000	11,500,000
Deferred revenue	0								0			0					0				0
Notice period in days									90 days
Milestone payments received															3,000,000	4,000,000	7,000,000
Research and development services compensation earned																	22,700,000
Sales-based milestone payments	115,000,000								115,000,000			115,000,000					115,000,000				115,000,000
Entitled offset funding percentage									25.00%
Decreased rate of certain circumstances									50.00%
GSK [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment recorded																		3,000,000		20,000,000
Clinical development milestone payments	109,000,000								109,000,000			109,000,000					109,000,000				109,000,000
Additional milestone payments	275,000,000								275,000,000			275,000,000					275,000,000				275,000,000
Cash and accounts receivable																					53,000,000
Collaboration revenue				3,000,000					25,500,000	16,400,000	9,700,000
Deferred revenue	1,400,000								1,400,000			1,400,000					1,400,000				1,400,000
License and research services related cost																					30,000,000
Notice period in days									90 days
Milestone payments received					4,000,000				3,000,000		4,000,000								4,000,000		15,000,000
Sales-based milestone payments	218,000,000								218,000,000			218,000,000					218,000,000				218,000,000
Fixed research funding received																					6,000,000
Research and development services																					9,000,000
Additional substantive preclinical research and development milestone payments	18,000,000								18,000,000			18,000,000					18,000,000				18,000,000
Percentage rate of research and development costs									100.00%
Upfront payment recorded	1,200,000								1,200,000			1,200,000					1,200,000				1,200,000
Percentage of securities issued									10.00%
LLS [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment recorded																								1,100,000
Cash and accounts receivable																								2,600,000
Maximum milestone receivable																											7,500,000
Milestone payments received																							1,500,000
Transfer payment relating to company's entry into collaboration																								400,000
Repayment of upfront payment on termination of collaborative arrangement																						800,000
Collaborative agreement termination, interest rate											10.00%
Accrued termination value of collaborative agreement											2,000,000
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Notice period in days									90 days
Entitled offset funding percentage																									25.00%
Development milestone payments																									21,500,000
Maximum Potential Future Funding Obligation At Opt-in																									50.00%
Abbott [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment paid by the Company																										900,000
Aggregate milestone-based development payments																										6,000,000
Maximum reimbursable costs																										$900,000
Collaboration agreement termination date																										2014-10

Employee_Benefit_Plans_Additio

Employee Benefit Plans - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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 and restricted stock, vesting period	4 years
Incentive and non-qualified stock options, expiration period	10 years
Stock-based compensation expense related to stock options and restricted stock	$6,864,000	$2,819,000	$689,000
Non-employee stock option awards granted	0		0
Expected dividend yield	0.00%
Aggregate stock options granted to employees, including a non-employee	876,385	1,514,828	766,297
Weighted-average fair value of options granted	$21.73	$7.85	$1.83
Grant date fair value of options vested	4,900,000	600,000	600,000
Aggregate intrinsic value of stock option exercised	59,400,000	2,100,000
Unrecognized compensation expense related to stock options	16,500,000
Expected weighted average period for recognition of compensation cost	2 years 6 months
Percentage of employer's matching contribution	50.00%
Maximum percentage of employee contribution	3.00%
Contribution to 401 (k) plan	200,000
Non Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield		0.00%
Aggregate stock options granted to employees, including a non-employee		10,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Granted	0	0	33,333
Weighted Average Grant Date Fair Value per Share, Granted			$0.60
Intrinsic value of restricted stock vested during period	$100,000	$400,000	$100,000
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_Schedul

Employee Benefit Plans - Schedule of Stock-Based Compensation Expense (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$6,864	$2,819	$689
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	3,299	1,072	448
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$3,565	$1,747	$241

Employee_Benefit_Plans_Assumpt

Employee Benefit Plans - Assumptions Used in Applying Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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	1.60%	1.00%	0.70%
Expected life of options	6 years	6 years	6 years
Expected volatility of underlying stock	92.10%	94.70%	98.90%
Expected dividend yield	0.00%	0.00%	0.00%

Employee_Benefit_Plans_Assumpt1

Employee Benefit Plans - Assumptions Used in Applying Pricing Model, Non Employees (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%
Non Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate		3.00%
Expected life of options		10 years
Expected volatility of underlying stock		86.30%
Expected dividend yield		0.00%

Employee_Benefit_Plans_Summary

Employee Benefit Plans - Summary of Stock Option Activity (Detail) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding, Beginning balance	4,728,503
Number of Options, Granted	876,385	1,514,828	766,297
Number of Options, Exercised	-2,239,643
Number of Options, Forfeited or expired	-405,739
Number of Options, Outstanding, Ending balance	2,959,506	4,728,503
Number of Options, Exercisable	1,393,210
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$3.13
Weighted Average Exercise Price per Share, Granted	$28.88
Weighted Average Exercise Price per Share, Exercised	$1.22
Weighted Average Exercise Price per Share, Forfeited or expired	$14.32
Weighted Average Exercise Price per Share, Outstanding, Ending balance	$10.66	$3.13
Weighted Average Exercise Price per Share, Exercisable	$2.70
Weighted Average Remaining Contractual Term (In Years), Outstanding	6 years 9 months 18 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	5 years 6 months
Aggregate Intrinsic Value, Outstanding	$33,538
Aggregate Intrinsic Value, Exercisable	$23,112

Employee_Benefit_Plans_Summary1

Employee Benefit Plans - Summary of Restricted Stock Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Number of Shares, Outstanding, Beginning balance	5,555
Number of Shares, Vested	-5,555
Weighted Average Grant Date Fair Value per Share, Beginning balance	$0.60
Weighted Average Grant Date Fair Value per Share, Vested	$0.60

Loss_per_Share_Schedule_of_Bas

Loss per Share - Schedule of Basic and Diluted Loss Per Share (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net loss	($15,035)	($19,700)	($13,386)	($6,884)	$15,930	($9,704)	($2,212)	($7,497)	($55,005)	($3,483)	($702)
Less: accretion of redeemable convertible preferred stock to redemption value					0	0	107	157	0	264	486
Loss allocable to common stockholders					$15,926	($9,704)	($2,319)	($7,654)	($55,005)	($3,747)	($1,188)
Weighted average shares outstanding	34,273	33,676	33,156	30,959	28,434	28,406	9,146	1,791	33,027	17,049	1,645
Basic and diluted loss per share allocable to common stockholders									($1.67)	($0.22)	($0.72)

Loss_per_Share_Additional_Info

Loss per Share - Additional Information (Detail)	0 Months Ended	1 Months Ended
	Jun. 05, 2013	Feb. 28, 2014	Jun. 30, 2013	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Issuance of common stock	5,913,300	3,673,901	5,913,300
Common stock in connection with the automatic conversion of its preferred stock	20,633,046		20,633,046
Shares outstanding				34,426,012	28,488,892

Loss_per_Share_Common_Stock_Eq

Loss per Share - Common Stock Equivalents from Calculation of Diluted Loss Per Share Attributable to Common Stockholders (Detail)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	2,966	4,783	24,148
Redeemable Convertible Preferred 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		0	20,633
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	2,960	4,729	3,493
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		6	22
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	6	48

Related_Party_Transactions_Add

Related Party Transactions - Additional Information (Detail) (USD $)	0 Months Ended	1 Months Ended		3 Months Ended		12 Months Ended
	Jun. 05, 2013	Feb. 28, 2014	Jun. 30, 2013	Mar. 31, 2014	Jun. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 02, 2012
Related Party Transaction [Line Items]
Issuance of common stock	5,913,300	3,673,901	5,913,300
Redeemable convertible preferred stock conversion ratio	One-for-three
Equity investments in common stock						34,426,012	28,488,892
Beneficial Owner [Member]
Related Party Transaction [Line Items]
Diluted equity						9.80%		12.50%
Issuance of common stock				340,000	66,666
Redeemable convertible preferred stock conversion ratio					One-for-three
Collaboration revenue related to agreement						$9,600,000	$37,800,000	$23,900,000
Deferred revenue related to agreement						21,700,000	31,300,000
Global development co-funding from Celgene						3,900,000	1,900,000
Accounts receivable related to collaboration arrangement						$1,100,000	$26,200,000
Equity investments in common stock							3,334,640
Percentage of voting interests						10.70%
Beneficial Owner [Member] | Series C [Member]
Related Party Transaction [Line Items]
Redeemable convertible preferred stock, issued									9,803,922

Unaudited_Quarterly_Results_Su

Unaudited Quarterly Results - Summary of Results of Operations on a Quarterly Basis (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$10,349	$8,177	$9,494	$13,391	$36,317	$8,444	$14,839	$8,882	$41,411	$68,482	$45,222
Operating expenses:
Research and development	20,505	22,244	17,499	15,347	15,685	14,584	13,937	13,361	75,595	57,567	38,482
General and administrative	4,935	5,669	5,306	4,956	4,378	3,587	3,079	2,998	20,866	14,042	7,508
Total operating expenses	25,440	27,913	22,805	20,303	20,063	18,171	17,016	16,359	96,461	71,609	45,990
Operating (loss) income	-15,091	-19,736	-13,311	-6,912	16,254	-9,727	-2,177	-7,477	-55,050	-3,127	-768
Other (expense) income, net	47	41	38	28	25	23	-35	-20	154	-7	67
(Loss) before income taxes	-15,044	-19,695	-13,273	-6,884	16,279	-9,704	-2,212	-7,497	-54,896	-3,134	-701
Income tax expense (benefit)	-9	5	113		349	0	0	0	109	349	1
Net (loss) income	-15,035	-19,700	-13,386	-6,884	15,930	-9,704	-2,212	-7,497	-55,005	-3,483	-702
Less: accretion of redeemable convertible preferred stock to redemption value					0	0	107	157	0	264	486
Less: income allocable to participating securities					4	0	0	0
Loss allocable to common stockholders					$15,926	($9,704)	($2,319)	($7,654)	($55,005)	($3,747)	($1,188)
(Loss) earnings per share allocable to common stockholders:
Basic	($0.44)	($0.58)	($0.40)	($0.22)	$0.56	($0.34)	($0.25)	($4.27)	($1.67)	($0.22)	($0.72)
Diluted	($0.44)	($0.58)	($0.40)	($0.22)	$0.52	($0.34)	($0.25)	($4.27)	($1.67)	($0.22)	($0.72)
Weighted average shares outstanding:
Basic	34,273	33,676	33,156	30,959	28,434	28,406	9,146	1,791	33,027	17,049	1,645
Diluted	34,273	33,676	33,156	30,959	30,901	28,406	9,146	1,791	33,027	17,049	1,645

Subsequent_Event_Additional_In

Subsequent Event - Additional Information (Detail) (USD $)	12 Months Ended	45 Months Ended	1 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2014	Mar. 31, 2015	Oct. 31, 2013
Eisai [Member]
Subsequent Event [Line Items]
Upfront payment		$3
Notice period in days	90 days
Roche [Member]
Subsequent Event [Line Items]
Development milestone payments				21.5
Notice period in days	90 days
Subsequent Event [Member] | Eisai [Member]
Subsequent Event [Line Items]
Upfront payment			40
Clinical development milestone payments			20
Regulatory milestone payments			50
Notice period in days			90 days
Subsequent Event [Member] | Roche [Member]
Subsequent Event [Line Items]
Development milestone payments			$8.50