Epizyme (EPZM) 10-Q 28 Apr 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 24, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	EPZM
Entity Registrant Name	EPIZYME, INC.
Entity Central Index Key	1571498
Current Fiscal Year End Date	-19
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		41,204,670

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Current Assets:
Cash and cash equivalents	$244,504	$190,095
Accounts receivable	365	2,075
Prepaid expenses and other current assets	2,535	2,840
Total current assets	247,404	195,010
Property and equipment, net	5,205	3,620
Restricted cash and other assets	657	573
Total Assets	253,266	199,203
Current Liabilities:
Accounts payable	5,078	8,300
Accrued expenses	5,510	7,043
Current portion of capital lease obligation	521
Current portion of deferred revenue	771	1,702
Total current liabilities	11,880	17,045
Capital lease obligation, net of current portion	1,156
Deferred revenue, net of current portion	21,449	21,449
Other long-term liabilities	409	427
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; 40,487,056 shares and 34,426,012 shares issued and outstanding, respectively	4	3
Additional paid-in capital	390,779	271,364
Accumulated deficit	-172,411	-111,085
Total stockholders' equity	218,372	160,282
Total Liabilities and Stockholders' Equity	$253,266	$199,203

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
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	40,487,056	34,426,012
Common stock, shares outstanding	40,487,056	34,426,012

Consolidated_Statements_of_Ope

Consolidated Statements of Operations and Comprehensive Loss (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Collaboration revenue	$911	$13,391
Operating expenses:
Research and development	57,051	15,347
General and administrative	5,237	4,956
Total operating expenses	62,288	20,303
Operating loss	-61,377	-6,912
Other income, net:
Interest income	31	16
Other income, net	20	12
Other income, net	51	28
Net loss	-61,326	-6,884
Loss per share allocable to common stockholders:
Basic	($1.75)	($0.22)
Diluted	($1.75)	($0.22)
Weighted average shares outstanding:
Basic	34,992	30,959
Diluted	34,992	30,959
Comprehensive loss	($61,326)	($6,884)

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	($61,326)	($6,884)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Acquired in-process research and development	40,000	0
Depreciation and amortization	260	169
Stock-based compensation	2,416	1,463
Loss on disposal of property and equipment	6
Changes in operating assets and liabilities:
Accounts receivable	1,710	25,516
Prepaid expenses and other current assets	305	343
Accounts payable	-3,201	-1,321
Accrued expenses	-1,848	-1,804
Deferred revenue	-931	-5,645
Restricted cash and other assets	-84	33
Other long-term liabilities	-18	5
Net cash (used in) provided by operating activities	-22,711	11,875
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of in-process research and development	-40,000	0
Purchases of property and equipment	-163	-316
Net cash used in investing activities	-40,163	-316
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment under capital lease obligation	-55
Proceeds from public offering, net of commissions	117,030	101,283
Proceeds from stock options exercised	99	650
Issuance of shares under employee stock purchase plan	239	201
Payment of public offering costs	-30	-468
Proceeds from reimbursement of public offering costs		269
Net cash provided by financing activities	117,283	101,935
Net increase in cash and cash equivalents	54,409	113,494
Cash and cash equivalents, beginning of period	190,095	123,564
Cash and cash equivalents, end of period	244,504	237,058
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	37	327
Equipment acquired under capital lease	1,732
Income taxes paid	2	241
Public offering costs incurred but unpaid at period end	$338	$183

Overview_and_Basis_of_Presenta

Overview and Basis of Presentation	3 Months Ended
	Mar. 31, 2015
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 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.
	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, 2014 (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 March 31, 2015 and 2014 are referred to as the first quarter of 2015 and 2014, 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 March 2015, the Company conducted a public offering of its common stock, selling 6,000,000 shares at a price of $20.75 per share. The Company received net proceeds before expenses from the sale of these 6,000,000 shares of $117.0 million after deducting underwriting discounts and commissions paid by the Company.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2	Summary of Significant Accounting Policies
	In the first quarter of 2015, the Company updated its accounting policy regarding property and equipment as a result of property and equipment acquired pursuant to a capital lease.
	Property and Equipment
	The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception.
	The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:
	Asset Category		Useful Lives
	Laboratory equipment		5 - 20 years
	Office furniture and equipment		3 - 10 years or term of respective lease, if shorter
	Leasehold improvements		3 - 10 years or term of respective lease, if shorter
	Amortization of capital lease assets is included in depreciation expense. The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation, from the related accounts, and the resulting gain or loss is reflected in the results of operations.
	Additionally, the Company updated its accounting policies as a result of the amended and restated collaboration and license agreement the Company executed with Eisai Co., Ltd. (“Eisai”), pursuant to which the Company recorded the reacquisition of worldwide rights, excluding Japan, to its EZH2 program, including EPZ-6438, as an acquisition of in-process research and development.
	Acquired In-Process Research and Development
	The Company records upfront payments that relate to the acquisition of a development-stage product candidate as research and development expense in the period in which they are incurred, provided that the acquired development-stage product candidate did not also include processes or activities that would constitute a business, the product candidate has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use.
	There have been no other 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 consolidated financial statements; however, it is disclosure-only in nature.
	In April 2015, the FASB issued ASU No. 2015-05, Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement. ASU 2015-05 amends ASC 350-40, Internal-Use Software, by providing customers with guidance on determining whether a cloud computing arrangement contains a software license that should be accounted for as internal-use software. ASU 2015-05 will be effective for the Company for annual periods beginning after December 15, 2015 and interim period within annual periods beginning after December 15, 2015. The Company is evaluating the impact that this ASU may have on its consolidated financial statements, if any.

Fair_Value_Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015 and December 31, 2014 consisted primarily of cash and cash equivalents, accounts receivable and accounts payable. As of March 31, 2015 and December 31, 2014, the Company’s financial assets recognized at fair value consisted of the following:
			Fair Value as of March 31, 2015
			Total		Level 1		Level 2		Level 3
			(In thousands)
	Cash equivalents		$	196,885	$	196,885	$	—	$	—
	Total		$	196,885	$	196,885	$	—	$	—
			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	$	—	$	—

Accrued_Expenses

Accrued Expenses	3 Months Ended
	Mar. 31, 2015
Payables and Accruals [Abstract]
Accrued Expenses	4	Accrued Expenses
	Accrued expenses consisted of the following:
			March 31,		December 31,
			2015		2014
			(In thousands)
	Employee compensation and benefits		$	1,490	$	2,623
	Research and development and professional expenses			4,020		4,420
	Accrued expenses		$	5,510	$	7,043

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	5	Income Taxes
	The Company did not record a federal or state income tax provision or benefit for the three months ended March 31, 2015 and 2014 due to the expected loss before income taxes to be incurred for the years ended December 31, 2015 and 2014, as well as the Company’s continued maintenance of a full valuation allowance against its net deferred tax assets.

Commitments_and_Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	6	Commitments and Contingencies
	Commitments
	In the first quarter of 2015, the Company acquired computer equipment pursuant to a capital lease. Future minimum lease payments by year and the related capital lease obligation as of March 31, 2015 are as follows:
			(In thousands)
	Future minimum lease payments in year ending December 31:
	2015		$	499
	2016			665
	2017			665
	2018			111
	Total future minimum lease payments			1,940
	Less: Amount representing imputed interest			(263	)
	Capital lease obligation		$	1,677
	In connection with the amended and restated collaboration and license agreement the Company executed with Eisai in March 2015, the Company and Eisai entered into an amended and restated letter agreement related to their December 2012 companion diagnostic agreement with Roche Molecular Systems (“Roche”). Upon the execution of the amended and restated letter agreement with Eisai, the Company assumed responsibility for up to $15.5 million of the remaining development costs under the agreement with Roche.
	Contingencies
	In connection with the execution of the amended and restated collaboration and license agreement with Eisai, the Company agreed to pay Eisai up to $20.0 million upon the achievement of specified clinical development milestones and up to $50.0 million upon the achievement of specified regulatory milestones. In addition, the Company may be required to pay Eisai royalties at a percentage in the mid-teens on worldwide net sales of any EZH2 product, excluding net sales in Japan.

Collaborations

Collaborations	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborations	7	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, excluding the EZH2 HMT and targets covered by our GSK collaboration (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 $6.3 million of global development co-funding through March 31, 2015. 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 available target as to which Celgene exercises its option during an initial option period ending in July 2015 (each a “selected target”), and up to $100.0 million in substantive regulatory milestone payments for each selected target. Celgene has the right to extend the option period until July 2016 by making a significant option extension payment. As to DOT1L and each selected target, 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 remaining DOT1L program development costs, 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 selected 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.
	Collaboration Revenue
	Through March 31, 2015, 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 $99.3 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 March 31, 2015, the Company has recognized $71.4 million of collaboration revenue, including $0.1 million and $1.7 million in the three months ended March 31, 2015 and 2014, respectively, and $6.3 million of global development co-funding as a reduction to research and development expense, including $0.5 million and $0.4 million in the three months ended March 31, 2015 and 2014, respectively, in the consolidated statements of operations and comprehensive loss related to this agreement. As of March 31, 2015 and December 31, 2014, the Company had deferred revenue of $21.6 million and $21.7 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 during a research term that ended January 8, 2015, or earlier if selection of a development candidate occurred. In March 2014, the Company and GSK amended certain terms of this agreement for the third licensed target, revising the license terms with respect to candidate compounds and amending the corresponding financial terms, including reallocating milestone payments and increasing royalty rates as to the third target. As of the end of the first quarter of 2015, the Company has substantially completed its research obligations under this agreement and expects to complete the transfer of the remaining data and materials for these programs to GSK in the second quarter of 2015.
	Agreement Structure
	Under the agreement, the Company 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. 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 licensed target in the collaboration, the Company was primarily responsible for research until the earlier of selection of a development candidate for the target or January 8, 2015, 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 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 March 31, 2015, the Company recorded a total of $53.0 million in payments under the GSK agreement. Through March 31, 2015, the Company has recognized $52.4 million of collaboration revenue in the consolidated statements of operations and comprehensive loss related to this agreement, including $0.8 million and $10.1 million in the three months ended March 31, 2015 and 2014, respectively. As of March 31, 2015 and December 31, 2014, the Company had deferred revenue of $0.6 million and $1.4 million, respectively, related to this agreement.
	Eisai
	In April 2011, the Company entered into a collaboration and license agreement with Eisai under which the Company granted Eisai an exclusive worldwide license to its small molecule HMT inhibitors directed to the EZH2 HMT, including the Company’s product candidate 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 (the “original agreement”). Additionally, as part of the research collaboration, the Company provided research and development services related to the licensed compounds through December 31, 2014.
	The Company recognized $1.6 million of collaboration revenue in the three months ended March 31, 2014 under the original agreement. As of December 31, 2014, the Company had completed its performance obligations under the original agreement. Accordingly, the Company had no remaining deferred revenue as of December 31, 2014 related to the original agreement.
	In March 2015, the Company entered into an amended and restated collaboration and license agreement with Eisai, under which the Company reacquired worldwide rights, excluding Japan, to its EZH2 program, including EPZ-6438. Under the amended and restated collaboration and license agreement, the Company is 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 agreement, Eisai was solely responsible for funding all research, development and commercialization costs for EZH2 compounds. Under the amended and restated collaboration and license agreement, the Company is solely responsible for funding global development, manufacturing and commercialization costs for EZH2 compounds outside of Japan, including up to $15.5 million of the remaining development costs due under the Roche companion diagnostic agreement, and Eisai is solely responsible for funding Japan-specific development and commercialization costs for EZH2 compounds.
	The Company recorded the reacquisition of worldwide rights, excluding Japan, to the EZH2 program, including EPZ-6438, under the amended and restated collaboration and license agreement with Eisai as an acquisition of an in-process research and development asset. As this asset was acquired without corresponding processes or activities that would constitute a business, has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use, the Company has recorded the $40.0 million upfront payment made to Eisai in March 2015 as research and development expense in the three months ended March 31, 2015. The Company has 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.
	Companion Diagnostics
	Roche
	In December 2012, Eisai and the Company entered into an agreement with Roche under which Eisai and the Company agreed to fund Roche’s development of a companion diagnostic to identify patients who possess certain point mutations in EZH2. At the same time, Eisai and the Company entered into a letter agreement pursuant to which Eisai agreed to be responsible for costs the development costs under the Roche agreement. In October 2013, this agreement was amended to include additional point mutations in EZH2. Under the terms of the amended agreement, Roche is to be paid up to a total of $21.5 million to develop and to make commercially available the companion diagnostic.
	In connection with the March 2015 execution of the amended and restated collaboration and license agreement with Eisai, the Company and Eisai entered into an amended and restated letter agreement, pursuant to which the Company agreed to be responsible for up to $15.5 million of the remaining development costs under the agreement with Roche as of March 31, 2015.

StockBased_Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	8	Stock-Based Compensation
	Total stock-based compensation expense related to stock options, restricted stock units and the employee stock purchase plan was $2.4 million and $1.5 million for the three months ended March 31, 2015 and 2014, respectively.
	Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows:
			Three Months Ended March 31,
			2015			2014
			(In thousands)
	Research and development		$	1,358		$	681
	General and administrative			1,058			782
	Total		$	2,416		$	1,463
	Stock Options
	The weighted-average fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $15.01 and $23.58 per option for those options granted during the three months ended March 31, 2015 and 2014, respectively. Key assumptions used to apply this pricing model were as follows:
			Three Months Ended March 31
			2015			2014
	Risk-free interest rate			1.5	%		1.6	%
	Expected life of options			6.0 years			6.0 years
	Expected volatility of underlying stock			84.4	%		93.6	%
	Expected dividend yield			0	%		0	%
	The following is a summary of stock option activity for the three months ended March 31, 2015:
			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, 2014			2,959,506		$	10.66
	Granted			666,917			21.06
	Exercised			(49,097	)		2.01
	Forfeited or expired			(81,202	)		22.97
	Outstanding at March 31, 2015			3,496,124		$	12.48			7.8	$	32,362
	Exercisable at March 31, 2015			1,538,292		$	4.46			6.4	$	23,786
	As of March 31, 2015, there was $22.9 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 2.7 years.
	Restricted Stock Units
	The following is a summary of restricted stock unit activity for the three months ended March 31, 2015:
			Number of			Weighted
			Units			Average Grant
						Date Fair Value
						per Unit
	Outstanding at December 31, 2014			—		$	—
	Granted			37,313			18.49
	Outstanding at March 31, 2015			37,313		$	18.49
	The Company did not grant any restricted stock units during the three months ended March 31, 2014. As of March 31, 2015, there was $1.2 million of unrecognized compensation cost related to restricted stock units that are expected to vest, including $0.7 million of unrecognized compensation cost related to restricted stock units to be granted in the first quarter of 2016, pursuant to a February 2015 employment agreement with our chief financial officer, for which service is currently being provided. These costs are expected to be recognized over a weighted average remaining vesting period of 3.9 years.

Loss_Per_Share

Loss Per Share	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Loss Per Share	9	Loss Per Share
	Basic and diluted loss per share allocable to common stockholders are computed as follows:
			Three Months Ended March 31,
			2015			2014
			(In thousands except per share data)
	Net loss		$	(61,326	)	$	(6,884	)
	Weighted average shares outstanding			34,992			30,959
	Basic and diluted loss per share allocable to common stockholders		$	(1.75	)	$	(0.22	)
	In February 2014, the Company issued an additional 3,673,901 shares of common stock in connection with a public offering. In March 2015, the Company issued an additional 6,000,000 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 40,487,056 shares as of March 31, 2015, and in the weighted average shares outstanding for the three months ended March 31, 2015 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 anti-dilutive:
			Three Months Ended March 31,
			2015			2014
			(In thousands)
	Stock options			3,496			4,502
	Unvested restricted stock units			37			—
	Unvested restricted stock			—			1
	Shares issuable under employee stock purchase plan			2			2
				3,535			4,505

Related_Party_Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Related Party Transactions	10	Related Party Transactions
	The Company’s collaboration partner Celgene has made a series of equity investments in the Company, owning 3,674,640 shares of common stock representing 8.3% of the Company’s fully diluted equity and 9.1% of the voting interests of the Company as of March 31, 2015. Refer to Note 7, Collaborations, for additional information regarding this collaboration agreement.
	Under the Celgene collaboration agreement, the Company recognized $0.1 million and $1.7 million of collaboration revenue in the three months ended March 31, 2015 and 2014, respectively, and as of March 31, 2015 and December 31, 2014, had recorded $21.6 million and $21.7 million of deferred revenue related to the Celgene collaboration arrangement, respectively. Additionally, in the three months ended March 31, 2015 and 2014, the Company recorded $0.5 million and $0.4 million, respectively, in global development co-funding from Celgene. As of March 31, 2015 and December 31, 2014, the Company had accounts receivable of $0.4 million and $1.1 million, respectively, related to this collaboration arrangement.

Subsequent_Event

Subsequent Event	3 Months Ended
	Mar. 31, 2015
Subsequent Events [Abstract]
Subsequent Event	11	Subsequent Event
	In April 2015, the Company issued and sold an additional 701,448 shares at a price of $20.75 per share pursuant to the underwriters’ option to purchase additional shares that the Company granted in connection with the March 2015 public offering. The Company received net proceeds before expenses from the sale of these 701,448 shares of $13.7 million after deducting underwriting discounts and commissions paid by the Company.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Acquired In-Process Research and Development	Acquired In-Process Research and Development
	The Company records upfront payments that relate to the acquisition of a development-stage product candidate as research and development expense in the period in which they are incurred, provided that the acquired development-stage product candidate did not also include processes or activities that would constitute a business, the product candidate has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use.
	There have been no other material changes to the significant accounting policies previously disclosed in the Company’s Annual Report.
Property and Equipment	Property and Equipment
	The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception.
	The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:
	Asset Category	Useful Lives
	Laboratory equipment	5 - 20 years
	Office furniture and equipment	3 - 10 years or term of respective lease, if shorter
	Leasehold improvements	3 - 10 years or term of respective lease, if shorter
	Amortization of capital lease assets is included in depreciation expense. The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation, from the related accounts, and the resulting gain or loss is reflected in the results of operations.
	Additionally, the Company updated its accounting policies as a result of the amended and restated collaboration and license agreement the Company executed with Eisai Co., Ltd. (“Eisai”), pursuant to which the Company recorded the reacquisition of worldwide rights, excluding Japan, to its EZH2 program, including EPZ-6438, as an acquisition of in-process research and development.
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 consolidated financial statements; however, it is disclosure-only in nature.
	In April 2015, the FASB issued ASU No. 2015-05, Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement. ASU 2015-05 amends ASC 350-40, Internal-Use Software, by providing customers with guidance on determining whether a cloud computing arrangement contains a software license that should be accounted for as internal-use software. ASU 2015-05 will be effective for the Company for annual periods beginning after December 15, 2015 and interim period within annual periods beginning after December 15, 2015. The Company is evaluating the impact that this ASU may have on its consolidated financial statements, if any.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Useful Lives for Property, Plant and Equipment	The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception.
	The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:
	Asset Category	Useful Lives
	Laboratory equipment	5 - 20 years
	Office furniture and equipment	3 - 10 years or term of respective lease, if shorter
	Leasehold improvements	3 - 10 years or term of respective lease, if shorter

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of March 31, 2015 and December 31, 2014, the Company’s financial assets recognized at fair value consisted of the following:
		Fair Value as of March 31, 2015
		Total		Level 1		Level 2		Level 3
		(In thousands)
	Cash equivalents	$	196,885	$	196,885	$	—	$	—
	Total	$	196,885	$	196,885	$	—	$	—
		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	$	—	$	—

Accrued_Expenses_Tables

Accrued Expenses (Tables)	3 Months Ended
	Mar. 31, 2015
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following:
		March 31,		December 31,
		2015		2014
		(In thousands)
	Employee compensation and benefits	$	1,490	$	2,623
	Research and development and professional expenses		4,020		4,420
	Accrued expenses	$	5,510	$	7,043

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments	Future minimum lease payments by year and the related capital lease obligation as of March 31, 2015 are as follows:
		(In thousands)
	Future minimum lease payments in year ending December 31:
	2015	$	499
	2016		665
	2017		665
	2018		111
	Total future minimum lease payments		1,940
	Less: Amount representing imputed interest		(263	)
	Capital lease obligation	$	1,677

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31,
		2015			2014
		(In thousands)
	Research and development	$	1,358		$	681
	General and administrative		1,058			782
	Total	$	2,416		$	1,463
Assumptions Used to Applying Pricing Model	Key assumptions used to apply this pricing model were as follows:
		Three Months Ended March 31
		2015			2014
	Risk-free interest rate		1.5	%		1.6	%
	Expected life of options		6.0 years			6.0 years
	Expected volatility of underlying stock		84.4	%		93.6	%
	Expected dividend yield		0	%		0	%
Summary of Stock Option Activity	The following is a summary of stock option activity for the three months ended March 31, 2015:
		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, 2014		2,959,506		$	10.66
	Granted		666,917			21.06
	Exercised		(49,097	)		2.01
	Forfeited or expired		(81,202	)		22.97
	Outstanding at March 31, 2015		3,496,124		$	12.48			7.8	$	32,362
	Exercisable at March 31, 2015		1,538,292		$	4.46			6.4	$	23,786
Summary of Restricted Stock Activity	The following is a summary of restricted stock unit activity for the three months ended March 31, 2015:
		Number of			Weighted
		Units			Average Grant
					Date Fair Value
					per Unit
	Outstanding at December 31, 2014		—		$	—
	Granted		37,313			18.49
	Outstanding at March 31, 2015		37,313		$	18.49

Loss_Per_Share_Tables

Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Basic and Diluted Loss Per Share	Basic and diluted loss per share allocable to common stockholders are computed as follows:
		Three Months Ended March 31,
		2015			2014
		(In thousands except per share data)
	Net loss	$	(61,326	)	$	(6,884	)
	Weighted average shares outstanding		34,992			30,959
	Basic and diluted loss per share allocable to common stockholders	$	(1.75	)	$	(0.22	)
Common Stock Equivalents Excluded from Calculation of Diluted Loss Per Share Attributable to Common Stockholders	The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive:
		Three Months Ended March 31,
		2015			2014
		(In thousands)
	Stock options		3,496			4,502
	Unvested restricted stock units		37			—
	Unvested restricted stock		—			1
	Shares issuable under employee stock purchase plan		2			2
			3,535			4,505

Overview_and_Basis_of_Presenta1

Overview and Basis of Presentation - Additional Information (Detail) (USD $)	1 Months Ended		3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Feb. 28, 2014	Mar. 31, 2015	Mar. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of public offering of common stock	6,000,000	3,673,901
Common stock price per share	$20.75		$20.75
Net proceeds from public offering	$117,030		$117,030	$101,283

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets Acquired (Detail)	3 Months Ended
	Mar. 31, 2015
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

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$196,885	$184,257
Total	196,885	184,257
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	196,885	184,257
Total	196,885	184,257
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 $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Accrued Liabilities, Current [Abstract]
Employee compensation and benefits	$1,490	$2,623
Research and development and professional expenses	4,020	4,420
Accrued expenses	$5,510	$7,043

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Tax Disclosure [Abstract]
Federal income tax provision or benefit	$0	$0
State income tax provision or benefit	$0	$0

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Detail) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified
Roche [Member]
Other Commitments [Line Items]
Remaining development costs	$15.50
Eisai [Member]
Other Commitments [Line Items]
Clinical development milestone payments	20
Regulatory milestone payments	$50

Commitments_and_Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Detail) (USD $)	Mar. 31, 2015
In Thousands, unless otherwise specified
Contractual Obligation, Fiscal Year Maturity Schedule [Abstract]
2015	$499
2016	665
2017	665
2018	111
Total future minimum lease payments	1,940
Less: Amount representing imputed interest	-263
Capital lease obligation	$1,677

Collaborations_Additional_Info

Collaborations - Additional Information (Detail) (USD $)	3 Months Ended		36 Months Ended	1 Months Ended	36 Months Ended	51 Months Ended	1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue	$911,000	$13,391,000
Upfront payment made	40,000,000	0
Celgene [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Company received upfront payment			65,000,000
Proceeds from redeemable convertible preferred stock			25,000,000
Upfront payment recorded			68,000,000
Clinical development milestone achieved			25,000,000
Global development co-funding	500,000	400,000	6,300,000
Option period ending			Jul-15
Right to extend option exercise period date			2016-07
Cash and accounts receivable			99,300,000
Convertible preferred stock issued, premium value			3,000,000
Collaboration revenue	100,000	1,700,000	71,400,000
Deferred revenue	21,600,000		21,600,000			21,600,000	21,600,000	21,700,000
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
Additional milestone payments	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
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
Additional payments	65,000,000		65,000,000			65,000,000	65,000,000
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
Additional milestone payments	275,000,000		275,000,000			275,000,000	275,000,000
Cash and accounts receivable						53,000,000
Collaboration revenue	800,000	10,100,000				52,400,000
Deferred revenue	600,000		600,000			600,000	600,000	1,400,000
Fixed research funding received						6,000,000
Milestone payments received						15,000,000
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
Sales-based milestone payments	218,000,000		218,000,000			218,000,000	218,000,000
Percentage rate of research and development costs	100.00%
Eisai [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue		1,600,000
Deferred revenue								0
Upfront payment made							40,000,000
Clinical Development Milestone Payments Obligation	20,000,000		20,000,000			20,000,000	20,000,000
Regulatory Milestone Payments Obligation	50,000,000		50,000,000			50,000,000	50,000,000
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments	15,500,000		15,500,000			15,500,000	15,500,000
Total development milestone payments	$21,500,000		$21,500,000			$21,500,000	$21,500,000

StockBased_Compensation_Additi

Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options and restricted stock	$2,416,000	$1,463,000
Weighted-average fair value of options granted	$15.01	$23.58
Unrecognized compensation cost related to stock options	22,900,000
Expected weighted average period for recognition of compensation cost	2 years 8 months 12 days
Unvested Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected weighted average period for recognition of compensation cost	3 years 10 months 24 days
Unrecognized compensation cost related to restricted stock units	1,200,000
Number of Shares, Granted	37,313	0
Unrecognized compensation cost related to restricted stock units to be granted	$700,000

StockBased_Compensation_Schedu

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$2,416	$1,463
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	1,358	681
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$1,058	$782

StockBased_Compensation_Assump

Stock-Based Compensation - Assumptions Used to Applying Pricing Model (Detail) (Employee Stock Option [Member])	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.50%	1.60%
Expected life of options	6 years	6 years
Expected volatility of underlying stock	84.40%	93.60%
Expected dividend yield	0.00%	0.00%

StockBased_Compensation_Summar

Stock-Based Compensation - Summary of Stock Option Activity (Detail) (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding, Beginning balance	2,959,506
Number of Options, Granted	666,917
Number of Options, Exercised	-49,097
Number of Options, Forfeited or expired	-81,202
Number of Options, Outstanding, Ending balance	3,496,124
Number of Options, Exercisable	1,538,292
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$10.66
Weighted Average Exercise Price per Share, Granted	$21.06
Weighted Average Exercise Price per Share, Exercised	$2.01
Weighted Average Exercise Price per Share, Forfeited or expired	$22.97
Weighted Average Exercise Price per Share, Outstanding, Ending balance	$12.48
Weighted Average Exercise Price per Share, Exercisable	$4.46
Weighted Average Remaining Contractual Term (In Years), Outstanding	7 years 9 months 18 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	6 years 4 months 24 days
Aggregate Intrinsic Value, Outstanding	$32,362
Aggregate Intrinsic Value, Exercisable	$23,786

StockBased_Compensation_Summar1

Stock-Based Compensation - Summary of Restricted Stock Activity (Detail) (Unvested Restricted Stock Units [Member], USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Unvested Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Outstanding, Beginning balance	0
Number of Units, Granted	37,313	0
Number of Units, Outstanding, Ending balance	37,313
Weighted Average Grant Date Fair Value per Units, Beginning balance	$0
Weighted Average Grant Date Fair Value per Units, Granted	$18.49
Weighted Average Grant Date Fair Value per Units, Ending balance	$18.49

Loss_Per_Share_Schedule_of_Bas

Loss Per Share - Schedule of Basic and Diluted Loss Per Share (Detail) (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Net loss	($61,326)	($6,884)
Weighted average shares outstanding	34,992	30,959
Basic and diluted loss per share allocable to common stockholders	($1.75)	($0.22)

Loss_Per_Share_Additional_Info

Loss Per Share - Additional Information (Detail)	1 Months Ended
	Mar. 31, 2015	Feb. 28, 2014	Dec. 31, 2014
Earnings Per Share [Abstract]
Issuance of common stock	6,000,000	3,673,901
Common stock, shares outstanding	40,487,056		34,426,012

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
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	3,535	4,505
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	3,496	4,502
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	37	0
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	1
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

Related_Party_Transactions_Add

Related Party Transactions - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Related Party Transaction [Line Items]
Equity investments in common stock	40,487,056		34,426,012
Beneficial Owner [Member]
Related Party Transaction [Line Items]
Equity investments in common stock	3,674,640
Diluted equity	8.30%
Percentage of voting interests	9.10%
Collaboration revenue related to agreement	$100,000	$1,700,000
Deferred revenue	21,600,000		21,700,000
Global development co-funding from Celgene	500,000	400,000
Accounts receivable related to collaboration arrangement	$400,000		$1,100,000

Subsequent_Event_Additional_In

Subsequent Event - Additional Information (Detail) (USD $)	1 Months Ended		3 Months Ended		1 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Feb. 28, 2014	Mar. 31, 2015	Mar. 31, 2014	Apr. 30, 2015
Subsequent Event [Line Items]
Number of shares issued	6,000,000	3,673,901
Common stock price per share	$20.75		$20.75
Net proceeds from public offering	$117,030		$117,030	$101,283
Subsequent Event [Member]
Subsequent Event [Line Items]
Number of shares issued					701,448
Common stock price per share					$20.75
Net proceeds from public offering					$13,700