Epizyme (EPZM) 10-Q6 Nov 142014 Q3 Quarterly reportFinancial data

Company Profile

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
Document and Entity Information	Sep. 30, 2014	Oct. 31, 2014
Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q3	'
Trading Symbol	'EPZM	'
Entity Registrant Name	'EPIZYME, INC.	'
Entity Central Index Key	'0001571498	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Non-accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	34,163,233

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Current Assets:	'	'
Cash and cash equivalents	$211,669	$123,564
Accounts receivable	2,109	33,667
Prepaid expenses and other current assets	3,273	2,421
Total current assets	217,051	159,652
Property and equipment, net	2,749	2,157
Restricted cash and other assets	953	1,179
Total Assets	220,753	162,988
Current Liabilities:	'	'
Accounts payable	5,715	4,698
Accrued expenses	8,652	6,632
Current portion of deferred revenue	9,415	23,243
Total current liabilities	23,782	34,573
Deferred revenue, net of current portion	23,735	23,629
Other long-term liabilities	482	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; 34,063,303 shares and 28,494,447 shares issued, respectively; 34,063,303 shares and 28,488,892 shares outstanding, respectively	3	3
Additional paid-in capital	268,801	160,390
Accumulated deficit	-96,050	-56,080
Total stockholders' equity	172,754	104,313
Total Liabilities and Stockholders' Equity	$220,753	$162,988

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Sep. 30, 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	34,063,303	28,494,447
Common stock, shares outstanding	34,063,303	28,488,892

Consolidated_Statements_of_Ope

Consolidated Statements of Operations and Comprehensive Loss (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]	'	'	'	'
Collaboration revenue	$8,177	$8,444	$31,062	$32,165
Operating expenses:	'	'	'	'
Research and development	22,244	14,584	55,090	41,882
General and administrative	5,669	3,587	15,931	9,664
Total operating expenses	27,913	18,171	71,021	51,546
Operating loss	-19,736	-9,727	-39,959	-19,381
Other income (expense):	'	'	'	'
Interest income	24	20	66	54
Other income (expense), net	17	3	41	-86
Other income (expense), net	41	23	107	-32
Loss before income taxes	-19,695	-9,704	-39,852	-19,413
Income tax expense	5	0	118	0
Net loss	-19,700	-9,704	-39,970	-19,413
Less: accretion of redeemable convertible preferred stock to redemption value	0	0	0	264
Loss allocable to common stockholders	-19,700	-9,704	-39,970	-19,677
Loss per share allocable to common stockholders:	'	'	'	'
Basic	($0.58)	($0.34)	($1.23)	($1.49)
Diluted	($0.58)	($0.34)	($1.23)	($1.49)
Weighted average shares outstanding:	'	'	'	'
Basic	33,676	28,406	32,607	13,212
Diluted	33,676	28,406	32,607	13,212
Comprehensive loss	($19,700)	($9,704)	($39,970)	($19,413)

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
CASH FLOWS FROM OPERATING ACTIVITIES:	'	'
Net loss	($39,970)	($19,413)
Adjustments to reconcile net loss to net cash used in operating activities:	'	'
Depreciation and amortization	548	534
Stock-based compensation	4,678	1,819
Changes in operating assets and liabilities:	'	'
Accounts receivable	31,558	-326
Prepaid expenses and other current assets	-852	-1,315
Accounts payable	1,107	364
Accrued expenses	2,020	1,052
Deferred revenue	-13,722	-18,739
Restricted cash and other assets	226	-716
Other long-term liabilities	9	-1,289
Net cash used in operating activities	-14,398	-38,029
CASH FLOWS FROM INVESTING ACTIVITIES:	'	'
Purchases of property and equipment	-1,230	-230
Net cash used in investing activities	-1,230	-230
CASH FLOWS FROM FINANCING ACTIVITIES:	'	'
Proceeds from public offering, net of commissions	101,283	82,491
Proceeds from stock options exercised	2,348	171
Excess tax benefit from stock option plan	28	0
Issuance of shares under employee stock purchase plan	454	0
Payment of public offering costs	-649	-2,809
Proceeds from reimbursement of public offering costs	269	0
Net cash provided by financing activities	103,733	79,853
Net increase in cash and cash equivalents	88,105	41,594
Cash and cash equivalents, beginning of period	123,564	97,981
Cash and cash equivalents, end of period	211,669	139,575
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:	'	'
Conversion of redeemable convertible preferred stock to common stock	0	76,420
Purchases of property and equipment unpaid at period end	0	192
Accretion of redeemable convertible preferred stock to redemption value	0	264
Public offering costs incurred but unpaid at period end	$0	$6

Overview_and_Basis_of_Presenta

Overview and Basis of Presentation	9 Months Ended
Overview and Basis of Presentation	Sep. 30, 2014
Accounting Policies [Abstract]	'
Overview and Basis of Presentation	'

	1. Overview and Basis of Presentation

	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 innovative personalized therapeutics for patients with genetically defined cancers. 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 genetically defined cancers.

	The consolidated financial statements of the Company included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these consolidated financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2013 (the “Annual Report”).

	The unaudited consolidated financial statements include the accounts of Epizyme and its subsidiary. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation. In the opinion of management, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair presentation of the results for the reported interim periods. The Company considers events or transactions that occur after the balance sheet date but before the financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The three months ended September 30, 2014 and 2013 are referred to as the third quarter of 2014 and 2013, respectively. The results of operations for interim periods are not necessarily indicative of results to be expected for the full year or any other interim period.

	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.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	9 Months Ended
Summary of Significant Accounting Policies	Sep. 30, 2014
Accounting Policies [Abstract]	'
Summary of Significant Accounting Policies	'

	2. Summary of Significant Accounting Policies

	There have been no material changes to the significant accounting policies previously disclosed in the Company’s Annual Report.

	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.

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
Fair Value Measurements	Sep. 30, 2014
Fair Value Disclosures [Abstract]	'
Fair Value Measurements	'

	3. 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 September 30, 2014 and December 31, 2013 consisted primarily of cash and cash equivalents, accounts receivable and accounts payable. As of September 30, 2014 and December 31, 2013, the Company’s financial assets recognized at fair value consisted of the following:



		Fair Value as of September 30, 2014
		Total		Level 1		Level 2		Level 3
		(In thousands)
	Cash equivalents	$	199,532	$	199,532	$	—	$	—

	Total	$	199,532	$	199,532	$	—	$	—




		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	$	—	$	—

Accrued_Expenses

Accrued Expenses	9 Months Ended
Accrued Expenses	Sep. 30, 2014
Payables and Accruals [Abstract]	'
Accrued Expenses	'

	4. Accrued Expenses

	Accrued expenses consisted of the following:



		September 30,		December 31,
		2014		2013
		(In thousands)
	Employee compensation and benefits	$	2,870	$	2,607
	Contract termination obligation		94		355
	Research and development and professional expenses		5,688		3,670

	Accrued expenses	$	8,652	$	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 the nine months ended September 30, 2014, the Company made cash payments of $0.7 million and recorded sublease income of $0.4 million, resulting in total remaining contract termination obligations of $0.1 million as of September 30, 2014.

Income_Taxes

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2014
Income Tax Disclosure [Abstract]	'
Income Taxes	'

	5. Income Taxes

	The Company recorded $0.1 million of income tax expense in the nine months ended September 30, 2014 due to provision-to-return adjustments identified related to the year ended December 31, 2013. The Company did not record a federal or state income tax provision or benefit for the three or nine months ended September 30, 2014 related to the year ending December 31, 2014, due to the expected loss before income taxes to be incurred for the year ending December 31, 2014, as well as the Company’s continued maintenance of a full valuation allowance against its net deferred tax assets.

	The Company did not record a federal or state income tax provision or benefit for the three or nine months ended September 30, 2013 due to the expected loss before income taxes to be incurred for the year ended December 31, 2013, as well as the Company’s continued maintenance of a full valuation allowance against its net deferred tax assets.

Collaborations

Collaborations	9 Months Ended
Collaborations	Sep. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Collaborations	'

	6. Collaborations

	Celgene

	In April 2012, the Company entered into a collaboration and license agreement with Celgene Corporation and Celgene International Sàrl (collectively, “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 product platform for patients with genetically defined cancers, excluding targets already selected by the Company’s two other existing therapeutic collaborations (the “available targets”).

	Agreement Structure

	Under the terms of the agreement, the Company recorded a $65.0 million upfront payment and $25.0 million from the sale of its series C redeemable convertible preferred stock to an affiliate of Celgene, of which $3.0 million was considered a premium and included as collaboration arrangement consideration for a total upfront payment of $68.0 million. In addition, the Company has recorded a $25.0 million clinical development milestone payment and $4.4 million of global development co-funding through September 30, 2014. The Company is also eligible to receive 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 additional 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.

	The Company is obligated to conduct and solely fund research and development costs of the Phase 1 clinical trials for EPZ-5676. For all development costs other than the development costs of the Phase 1 clinical trials for EPZ-5676, Celgene and the Company will equally co-fund global development and each party will solely fund territory-specific development costs for its territory. The Company is obligated to conduct and solely fund research and development costs through the effectiveness of the first investigational new drug application (“IND”) for an HMT inhibitor directed to each available target selected by Celgene, 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. As of September 30, 2014, the Company had recorded accounts receivable of $1.1 million related to non-Phase 1 global development costs subject to the co-funding provisions of the agreement. Co-funded amounts received or receivable from Celgene are recorded as a reduction to research and development expense.



	Collaboration Revenue

	Through September 30, 2014, in addition to amounts allocated to Celgene’s purchase of shares of the Company’s series C redeemable convertible preferred stock, the Company had recorded a total of $97.4 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 redeemable convertible preferred stock. Through September 30, 2014, the Company has recognized $65.8 million of collaboration revenue, including $1.2 million and $4.1 million in the three and nine months ended September 30, 2014, respectively, and $3.5 million and $10.8 million in the three and nine months ended September 30, 2013, respectively, and $4.4 million of global development co-funding as a reduction to research and development expense, including $1.2 million and $2.5 million in the three and nine months ended September 30, 2014, respectively, and $0.7 million in the three and nine months ended September 30, 2013 in the consolidated statements of operations and comprehensive loss related to this agreement. Revenue recognized in the three and nine months ended September 30, 2014 reflects the Company’s current plan to reach IND effectiveness for a licensed compound from the other potential DOT1L product candidates by mid-2016. As a result, the remaining deferred revenue of approximately $5.3 million attributed to this deliverable as of September 30, 2014 will be recognized ratably through June 30, 2016. As of September 30, 2014 and December 31, 2013, the Company had deferred revenue of $27.2 million and $31.3 million, respectively, related to this agreement.

	Eisai

	In April 2011, the Company entered into a collaboration and license agreement with Eisai Co. Ltd. (“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.

	Agreement Structure

	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 payment and $21.8 million for research and development services through September 30, 2014. The Company is eligible to receive 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 ongoing Phase 1/2 clinical trial of EPZ-6438.

	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. If the Company exercises its opt-in right to a licensed compound, the licensed compound would become 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 milestones or royalties due to the Company, subject to certain limitations specified in the agreement; (iv) all subsequent milestone payments 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.



	Collaboration Revenue

	Through September 30, 2014, the Company recorded a total of $37.8 million in cash and accounts receivable under the Eisai agreement. Through September 30, 2014, the Company has recognized $37.4 million of collaboration revenue in the consolidated statements of operations and comprehensive loss related to this agreement, including $1.4 million and $5.0 million in the three and nine months ended September 30, 2014, respectively, and $1.9 million and $12.4 million in the three and nine months ended September 30, 2013, respectively, with a $6.0 million clinical development milestone achieved and recognized as collaboration revenue in the nine months ended September 30, 2013. As of September 30, 2014 and December 31, 2013, the Company had deferred revenue of $0.4 million and $1.6 million, respectively, related to this agreement.

	GSK

	In January 2011, the Company entered into a collaboration and license agreement with Glaxo Group Limited, an affiliate of GlaxoSmithKline (“GSK”), to discover, develop and commercialize novel small molecule HMT inhibitors directed to available targets from the Company’s platform. Under the terms of the agreement, the Company granted GSK exclusive worldwide license rights to HMT inhibitors directed to three targets. Additionally, as part of the research collaboration, the Company agreed to provide research and development services related to the licensed targets pursuant to agreed upon research plans until the earlier of the achievement of development candidate selection for a target or the end of the research term on January 8, 2015. During 2013, development candidate selection was achieved for the first target under the collaboration and license agreement, and, accordingly, the Company is no longer providing research and development services related to this target.

	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. This $3.0 million upfront payment has been allocated equally to the remaining two targets for which the Company is actively providing research and development services, as these remaining two targets are at similar stages of development, have equal probabilities of success and the remaining research services are 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, or earlier if a target reaches development candidate selection.

	Agreement Structure

	Under 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 $9.5 million for research and development services through September 30, 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 from the mid-single digits to the low double-digits, on a licensed product-by-licensed product basis, on worldwide net product sales, subject to reduction in specified circumstances. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone payments or royalty payments from GSK. 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.

	For each selected target in the collaboration, the Company is primarily responsible for research until the selection of the development candidate, and GSK is solely responsible for subsequent development and commercialization. GSK provided a fixed amount of research funding during the second and third years of the research term and is obligated to provide research funding equal to 100.0% of research and development costs, subject to specified limitations, for research activities conducted by the Company in the fourth year of the research term.

	Collaboration Revenue

	Through September 30, 2014, the Company recorded a total of $53.5 million in payments under the GSK agreement. Through September 30, 2014, the Company has recognized $48.1 million of collaboration revenue in the consolidated statements of operations and comprehensive loss related to this agreement, including $5.6 million and $22.0 million in the three and nine months ended September 30, 2014, respectively, and $3.0 million and $9.0 million in the three and nine months ended September 30, 2013, respectively, with $3.0 million in preclinical research and development milestones achieved and recognized as collaboration revenue in the nine months ended September 30, 2014. As of September 30, 2014 and December 31, 2013, the Company had deferred revenue of $5.4 million and $13.9 million, respectively, related to this agreement.



	Companion Diagnostics

	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 are the responsibility of Eisai until such time, if any, as the Company exercises its opt-in right under the 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.

	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 agreed to 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 is not expected to have a material impact on the Company’s financial statements.

StockBased_Compensation

Stock-Based Compensation	9 Months Ended
Stock-Based Compensation	Sep. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Stock-Based Compensation	'

	7. Stock-Based Compensation

	Total stock-based compensation expense related to stock options, restricted stock and the employee stock purchase plan was $1.6 million and $0.9 million for the three months ended September 30, 2014 and 2013, respectively, and $4.7 million and $1.8 million for the nine months ended September 30, 2014 and 2013, respectively.

	Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows:



		Three Months Ended September 30,							Nine Months Ended September 30,
		2014				2013			2014		2013
		(In thousands)
	Research and development	$	749			$	294		$	2,245	$	695
	General and administrative		834				575			2,433		1,124

	Total	$	1,583			$	869		$	4,678	$	1,819


	Stock Options

	The weighted-average fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $21.32 and $21.81 per option for those options granted during the three months ended September 30, 2014 and 2013, respectively, and $22.21 and $7.57 per option for those options granted during the nine months ended September 30, 2014 and 2013, respectively. Key assumptions used to apply this pricing model were as follows:



		Nine Months Ended				Nine Months Ended
		September 30, 2014				September 30, 2013
	Risk-free interest rate		1.6	%			0.9	%
	Expected life of options		6.0 years				6.0 years
	Expected volatility of underlying stock		92.4	%			94.7	%
	Expected dividend yield		0	%			0	%

	The following is a summary of stock option activity for the nine months ended September 30, 2014:



						Weighted			Weighted
						Average			Average
						Exercise			Remaining		Aggregate
		Number of				Price per			Contractual		Intrinsic
		Options				Share			Term		Value
									(In years)		(In thousands)
	Outstanding at December 31, 2013		4,728,503			$	3.13
	Granted		782,619				29.48
	Exercised		(1,876,934	)			1.25
	Forfeited or expired		(81,757	)			18.24

	Outstanding at September 30, 2014		3,552,431			$	9.58			7.7	$	64,943

	Exercisable at September 30, 2014		1,606,793			$	2.04			6.4	$	40,379


	As of September 30, 2014, there was $19.6 million of unrecognized compensation cost related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.6 years.

	Restricted Stock

	The following is a summary of restricted stock activity for the nine months ended September 30, 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 September 30, 2014		—			$	—

Loss_Per_Share

Loss Per Share	9 Months Ended
Loss Per Share	Sep. 30, 2014
Earnings Per Share [Abstract]	'
Loss Per Share	'

	8. Loss Per Share

	Basic (loss) earnings per share is computed by dividing (loss) income allocable to common stockholders by the weighted average number of common shares outstanding. During periods of income, participating securities are allocated 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, no loss is allocated to participating securities since they have no contractual obligation to share in the losses of the Company. Diluted (loss) earnings per share is computed 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.



	Basic and diluted loss per share allocable to common stockholders are computed as follows:



		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2014			2013			2014			2013
		(In thousands except per share data)
	Net loss	$	(19,700	)	$	(9,704	)	$	(39,970	)	$	(19,413	)
	Less: accretion of redeemable convertible preferred stock to redemption value		—			—			—			264

	Loss allocable to common stockholders	$	(19,700	)	$	(9,704	)	$	(39,970	)	$	(19,677	)

	Weighted average shares outstanding		33,676			28,406			32,607			13,212

	Basic and diluted loss per share allocable to common stockholders	$	(0.58	)	$	(0.34	)	$	(1.23	)	$	(1.49	)


	In June 2013, the Company issued 5,913,300 shares of common stock in connection with its initial public offering (“IPO”) and 20,633,046 shares of common stock in connection with the automatic conversion of its redeemable convertible 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 34,063,303 shares as of September 30, 2014, and in the weighted average shares outstanding for the three and nine months ended September 30, 2014 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 2014.

	The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive:



		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2014			2013			2014			2013
		(In thousands)
	Stock options		3,552			4,773			3,552			4,773
	Unvested restricted stock		—			10			—			10
	Shares issuable under employee stock purchase plan		2			2			2			2

			3,554			4,785			3,554			4,785

Related_Party_Transactions

Related Party Transactions	9 Months Ended
Related Party Transactions	Sep. 30, 2014
Related Party Transactions [Abstract]	'
Related Party Transactions	'

	9. Related Party Transactions

	The Company’s collaboration partner Celgene has made a series of equity investments in the Company, owning 3,334,640 shares of 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 10.8% of the voting interests of the Company as of September 30, 2014. Refer to Note 6, Collaborations, for additional information regarding this collaboration agreement.

	Under the Celgene collaboration agreement, the Company recognized $1.2 million and $4.1 million of collaboration revenue in the three and nine months ended September 30, 2014 and $3.5 million and $10.8 million of collaboration revenue in the three and nine months ended September 30, 2013, respectively, and as of September 30, 2014 and December 31, 2013, had recorded $27.2 million and $31.3 million of deferred revenue related to the Celgene collaboration arrangement, respectively. Additionally, in the three and nine months ended September 30, 2014, the Company recorded $1.2 million and $2.5 million, respectively, in global development co-funding from Celgene. As of September 30, 2014 and December 31, 2013, the Company had accounts receivable of $1.1 million and $26.2 million, respectively, related to this collaboration arrangement.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2014
Accounting Policies [Abstract]	'
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.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	9 Months Ended
Fair Value Measurements (Tables)	Sep. 30, 2014
Fair Value Disclosures [Abstract]	'
Summary of Company's Financial Assets Recognized at Fair Value	'

	As of September 30, 2014 and December 31, 2013, the Company’s financial assets recognized at fair value consisted of the following:



		Fair Value as of September 30, 2014
		Total		Level 1		Level 2		Level 3
		(In thousands)
	Cash equivalents	$	199,532	$	199,532	$	—	$	—

	Total	$	199,532	$	199,532	$	—	$	—




		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	$	—	$	—

Accrued_Expenses_Tables

Accrued Expenses (Tables)	9 Months Ended
Accrued Expenses (Tables)	Sep. 30, 2014
Payables and Accruals [Abstract]	'
Schedule of Accrued Expenses	'
	Accrued expenses consisted of the following:


		September 30,		December 31,
		2014		2013
		(In thousands)
	Employee compensation and benefits	$	2,870	$	2,607
	Contract termination obligation		94		355
	Research and development and professional expenses		5,688		3,670

	Accrued expenses	$	8,652	$	6,632

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	9 Months Ended
Stock-Based Compensation (Tables)	Sep. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Schedule of Stock-Based Compensation Expense	'
	Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows:


		Three Months Ended September 30,						Nine Months Ended September 30,
		2014			2013			2014		2013
		(In thousands)
	Research and development	$	749		$	294		$	2,245	$	695
	General and administrative		834			575			2,433		1,124

	Total	$	1,583		$	869		$	4,678	$	1,819

Assumptions Used to Applying Pricing Model	'
	Key assumptions used to apply this pricing model were as follows:


		Nine Months Ended			Nine Months Ended
		September 30, 2014			September 30, 2013
	Risk-free interest rate		1.6	%		0.9	%
	Expected life of options		6.0 years			6.0 years
	Expected volatility of underlying stock		92.4	%		94.7	%
	Expected dividend yield		0	%		0	%
Summary of Stock Option Activity	'
	The following is a summary of stock option activity for the nine months ended September 30, 2014:


					Weighted			Weighted
					Average			Average
					Exercise			Remaining		Aggregate
		Number of			Price per			Contractual		Intrinsic
		Options			Share			Term		Value
								(In years)		(In thousands)
	Outstanding at December 31, 2013		4,728,503		$	3.13
	Granted		782,619			29.48
	Exercised		(1,876,934	)		1.25
	Forfeited or expired		(81,757	)		18.24

	Outstanding at September 30, 2014		3,552,431		$	9.58			7.7	$	64,943

	Exercisable at September 30, 2014		1,606,793		$	2.04			6.4	$	40,379

Summary of Restricted Stock Activity	'

	The following is a summary of restricted stock activity for the nine months ended September 30, 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 September 30, 2014		—		$	—

Loss_Per_Share_Tables

Loss Per Share (Tables)	9 Months Ended
Loss Per Share (Tables)	Sep. 30, 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:



		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2014			2013			2014			2013
		(In thousands except per share data)
	Net loss	$	(19,700	)	$	(9,704	)	$	(39,970	)	$	(19,413	)
	Less: accretion of redeemable convertible preferred stock to redemption value		—			—			—			264

	Loss allocable to common stockholders	$	(19,700	)	$	(9,704	)	$	(39,970	)	$	(19,677	)

	Weighted average shares outstanding		33,676			28,406			32,607			13,212

	Basic and diluted loss per share allocable to common stockholders	$	(0.58	)	$	(0.34	)	$	(1.23	)	$	(1.49	)

Common Stock Equivalents Excluded from Calculation of Diluted Loss Per Share Attributable to Common Stockholders	'

	The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive:



		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2014			2013			2014			2013
		(In thousands)
	Stock options		3,552			4,773			3,552			4,773
	Unvested restricted stock		—			10			—			10
	Shares issuable under employee stock purchase plan		2			2			2			2

			3,554			4,785			3,554			4,785

Overview_and_Basis_of_Presenta1

Overview and Basis of Presentation - Additional Information (Detail) (USD $)	1 Months Ended		9 Months Ended
In Thousands, except Share data, unless otherwise specified	Feb. 28, 2014	Jun. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'	'	'	'
Number of initial public offering of common stock	3,673,901	5,913,300	'	'
Common stock price per share	$29.25	'	'	'
Net proceeds from public offering	$101,283	'	$101,283	$82,491

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]	'	'
Cash equivalents	$199,532	$121,424
Total	199,532	121,424
Level 1 [Member]	'	'
Fair Value Inputs, Assets, Quantitative Information [Line Items]	'	'
Cash equivalents	199,532	121,424
Total	199,532	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

Accrued_Expenses_Schedule_of_A

Accrued Expenses - Schedule of Accrued Expenses (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Accrued Liabilities, Current [Abstract]	'	'
Employee compensation and benefits	$2,870	$2,607
Contract termination obligation	94	355
Research and development and professional expenses	5,688	3,670
Accrued expenses	$8,652	$6,632

Accrued_Expenses_Additional_In

Accrued Expenses - Additional Information (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Restructuring Reserve [Abstract]	'	'
Contract termination obligations	$0.10	$0.40
Cash payments for contract termination obligations	0.7	'
Sublease income	$0.40	'

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
Income Taxes - Additional Information (Detail) (USD $)	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014
Income Tax Disclosure [Abstract]	'	'	'
Income tax expense	$100,000	'	$100,000
Federal income tax provision or benefit	0	0	0
State income tax provision or benefit	$0	$0	$0

Collaborations_Additional_Info

Collaborations - Additional Information (Detail) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

30 Months Ended

3 Months Ended

9 Months Ended

42 Months Ended

3 Months Ended

9 Months Ended

36 Months Ended

45 Months Ended

1 Months Ended

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Mar. 31, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Sep. 30, 2014

Oct. 31, 2013

Feb. 28, 2013

Celgene [Member]

Eisai [Member]

GSK [Member]

Roche [Member]

Abbott [Member]

DOT1L [Member]

Available Targets [Member]

Alternative Compound [Member]

Revenue Recognition, Multiple-deliverable Arrangements [Line Items]

Company received upfront payment

$65,000,000

Proceeds from redeemable convertible preferred stock

25,000,000

Upfront payment recorded

68,000,000

3,000,000

20,000,000

Clinical development milestone achieved

25,000,000

6,000,000

Global development co-funding

1,200,000

700,000

2,500,000

700,000

4,400,000

Clinical development milestone payments

35,000,000

25,000,000

109,000,000

Additional milestone payments

100,000,000

55,000,000

275,000,000

Additional payments

65,000,000

Option period ending

'July 2015

Right to extend option exercise period date

'2016-07

Potential milestone payments receivable

35,000,000

10,000,000

Accounts receivable related to non-phase 1 global development costs

1,100,000

Cash and accounts receivable

97,400,000

37,800,000

53,500,000

Premium received on purchase of shares

3,000,000

Collaboration revenue

8,177,000

8,444,000

31,062,000

32,165,000

1,200,000

3,500,000

4,100,000

10,800,000

65,800,000

1,400,000

1,900,000

5,000,000

12,400,000

37,400,000

5,600,000

3,000,000

22,000,000

9,000,000

48,100,000

Deferred revenue

27,200,000

31,300,000

5,300,000

400,000

1,600,000

5,400,000

13,900,000

5,400,000

Milestone payments received

7,000,000

3,000,000

15,000,000

Research and development services compensation earned

21,800,000

Sales-based milestone payments

115,000,000

218,000,000

Entitled offset funding percentage

25.00%

Decreased rate of certain circumstances

50.00%

Fixed research funding received

6,000,000

Research and development services

9,500,000

Additional substantive preclinical research and development milestone payments

18,000,000

Percentage rate of research and development costs

100.00%

Development milestone payments

21,500,000

Maximum Potential Future Funding Obligation At Opt-in

50.00%

Upfront payment paid by the Company

900,000

Development milestone payments

6,000,000

Maximum reimbursable costs

$900,000

Collaboration agreement termination date

'2014-10

StockBased_Compensation_Additi

Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'	'	'	'
Stock-based compensation expense related to stock options and restricted stock	$1,583,000	$869,000	$4,678,000	$1,819,000
Weighted-average fair value of options granted	$21.32	$21.81	$22.21	$7.57
Unrecognized compensation cost related to stock options	$19,600,000	'	$19,600,000	'
Expected weighted average remaining vesting period for recognized cost	'	'	'2 years 7 months 6 days	'

StockBased_Compensation_Schedu

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]	'	'	'	'
Share-based compensation expense	$1,583	$869	$4,678	$1,819
Research and Development [Member]	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]	'	'	'	'
Share-based compensation expense	749	294	2,245	695
General and Administrative [Member]	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]	'	'	'	'
Share-based compensation expense	$834	$575	$2,433	$1,124

StockBased_Compensation_Assump

Stock-Based Compensation - Assumptions Used to Applying Pricing Model (Detail)	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]	'	'
Risk-free interest rate	1.60%	0.90%
Expected life of options	'6 years	'6 years
Expected volatility of underlying stock	92.40%	94.70%
Expected dividend yield	0.00%	0.00%

StockBased_Compensation_Summar

Stock-Based Compensation - Summary of Stock Option Activity (Detail) (USD $)	9 Months Ended
In Thousands, except Share data, unless otherwise specified	Sep. 30, 2014
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	782,619
Number of Options, Exercised	-1,876,934
Number of Options, Forfeited or expired	-81,757
Number of Options, Outstanding, Ending balance	3,552,431
Number of Options, Exercisable	1,606,793
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$3.13
Weighted Average Exercise Price per Share, Granted	$29.48
Weighted Average Exercise Price per Share, Exercised	$1.25
Weighted Average Exercise Price per Share, Forfeited or expired	$18.24
Weighted Average Exercise Price per Share, Outstanding, Ending balance	$9.58
Weighted Average Exercise Price per Share, Exercisable	$2.04
Weighted Average Remaining Contractual Term (In Years), Outstanding	'7 years 8 months 12 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	'6 years 4 months 24 days
Aggregate Intrinsic Value, Outstanding	$64,943
Aggregate Intrinsic Value, Exercisable	$40,379

StockBased_Compensation_Summar1

Stock-Based Compensation - Summary of Restricted Stock Activity (Detail) (USD $)	9 Months Ended
	Sep. 30, 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
Number of Shares, Outstanding, Ending balance	0
Weighted Average Grant Date Fair Value per Share, Beginning balance	$0.60
Weighted Average Grant Date Fair Value per Share, Vested	$0.60
Weighted Average Grant Date Fair Value per Share, Ending balance	$0

Loss_Per_Share_Schedule_of_Bas

Loss Per Share - Schedule of Basic and Diluted Loss Per Share (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share [Abstract]	'	'	'	'
Net loss	($19,700)	($9,704)	($39,970)	($19,413)
Less: accretion of redeemable convertible preferred stock to redemption value	0	0	0	264
Loss allocable to common stockholders	($19,700)	($9,704)	($39,970)	($19,677)
Weighted average shares outstanding	33,676	28,406	32,607	13,212
Basic and diluted loss per share allocable to common stockholders	($0.58)	($0.34)	($1.23)	($1.49)

Loss_Per_Share_Additional_Info

Loss Per Share - Additional Information (Detail)	1 Months Ended
Loss Per Share - Additional Information (Detail)	Feb. 28, 2014	Jun. 30, 2013	Sep. 30, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]	'	'	'	'
Additional shares of common stock in connection with IPO	3,673,901	5,913,300	'	'
Common stock in connection with the automatic conversion of its redeemable convertible preferred stock	'	20,633,046	'	'
Shares outstanding	'	'	34,063,303	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)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Common stock equivalents excluded from the calculation of diluted loss per share	3,554	4,785	3,554	4,785
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	3,552	4,773	3,552	4,773
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	0	10	0	10
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	2	2	2	2

Related_Party_Transactions_Add

Related Party Transactions - Additional Information (Detail) (USD $)	1 Months Ended				3 Months Ended		9 Months Ended
In Millions, except Share data, unless otherwise specified	Feb. 28, 2014	Jun. 30, 2013	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
	Feb. 28, 2014	Jun. 30, 2013	Sep. 30, 2014	Dec. 31, 2013	Beneficial Owner [Member]	Beneficial Owner [Member]	Beneficial Owner [Member]	Beneficial Owner [Member]	Beneficial Owner [Member]
Related Party Transaction [Line Items]	'	'	'	'	'	'	'	'	'
Diluted equity	'	'	'	'	9.80%	'	9.80%	'	'
Additional investment in common stock	3,673,901	5,913,300	'	'	'	'	340,000	'	'
Equity investments in common stock	'	'	34,063,303	28,488,892	'	'	'	'	3,334,640
Percentage of voting interests	'	'	'	'	10.80%	'	10.80%	'	'
Collaboration revenue related to agreement	'	'	'	'	$1.20	$3.50	$4.10	$10.80	'
Deferred revenue related to agreement	'	'	'	'	27.2	'	27.2	'	31.3
Global development co-funding from Celgene	'	'	'	'	1.2	'	2.5	'	'
Accounts receivable related to collaboration arrangement	'	'	'	'	$1.10	'	$1.10	'	$26.20