Epizyme (EPZM) 10-Q 23 Oct 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Oct. 18, 2013
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	30-Sep-13
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q3
Entity Registrant Name	Epizyme, Inc.
Entity Central Index Key	1571498
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		28,419,288

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Current Assets:
Cash and cash equivalents	$139,575	$97,981
Accounts receivable	2,155	1,829
Prepaid expenses and other current assets	2,130	815
Total current assets	143,860	100,625
Property and equipment, net	2,028	2,140
Restricted cash and other assets	1,462	746
Total Assets	147,350	103,511
Current Liabilities:
Accounts payable	3,525	2,967
Accrued expenses	5,384	4,328
Current portion of deferred revenue	21,556	28,208
Total current liabilities	30,465	35,503
Deferred revenue, net of current portion	29,150	41,237
Other long-term liabilities	452	1,741
Commitments and contingencies
Redeemable convertible preferred stock, $0.0001 par value; 5,000,000 shares and 61,899,922 shares (Series A, B and C) authorized, respectively; 0 shares and 61,899,165 shares issued and outstanding, respectively; aggregate liquidation preference of $0 and $79,000, respectively		76,156
Common stock, $0.0001 par value; 125,000,000 shares and 90,000,000 shares authorized, respectively; 28,418,420 shares and 1,694,862 shares issued, respectively; 28,408,698 shares and 1,672,639 shares outstanding, respectively	3
Treasury stock, at cost; 0 shares and 11,544 shares, respectively
Additional paid-in capital	159,290	1,471
Accumulated deficit	-72,010	-52,597
Total stockholders' equity (deficit)	87,283	-51,126
Total Liabilities and Stockholders' Equity (Deficit)	$147,350	$103,511

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, except Share data, unless otherwise specified
Statement Of Financial Position [Abstract]
Preferred stock, par value	$0.00	$0.00
Preferred stock, shares authorized	5,000,000	61,899,922
Preferred stock, shares issued	0	61,899,165
Preferred stock, shares outstanding	0	61,899,165
Preferred stock, aggregate liquidation Preference	$0	$79,000
Common stock, par value	$0.00	$0.00
Common stock, shares authorized	125,000,000	90,000,000
Common stock, shares issued	28,418,420	1,694,862
Common stock, shares outstanding	28,408,698	1,672,639
Treasury stock, shares	0	11,544

Consolidated_Statements_of_Ope

Consolidated Statements of Operations and Comprehensive (Loss) Income (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Statement [Abstract]
Collaboration revenue	$8,444	$15,331	$32,165	$36,327
Operating expenses:
Research and development	14,584	9,258	41,882	27,385
General and administrative	3,587	1,630	9,664	5,175
Total operating expenses	18,171	10,888	51,546	32,560
Operating (loss) income	-9,727	4,443	-19,381	3,767
Other income (expense):
Interest income	20	44	54	108
Other income (expense), net	3	-39	-86	-39
Other income (expense), net	23	5	-32	69
Net (loss) income	-9,704	4,448	-19,413	3,836
Less: accretion of redeemable convertible preferred stock to redemption value		159	264	326
Less: income allocable to participating securities		3,972		3,239
(Loss) income allocable to common stockholders-basic	-9,704	317	-19,677	271
Undistributed income re-allocated to common stockholders		229		147
(Loss) income allocable to common stockholders-diluted	-9,704	546	-19,677	418
(Loss) earnings per share allocable to common stockholders:
Basic	($0.34)	$0.19	($1.49)	$0.17
Diluted	($0.34)	$0.18	($1.49)	$0.16
Weighted average shares outstanding:
Basic	28,406	1,651	13,212	1,637
Diluted	28,406	3,017	13,212	2,641
Comprehensive (loss) income	($9,704)	$4,448	($19,413)	$3,836

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
CASH FLOWS FROM OPERATING ACTIVITIES:
Net (loss) income	($19,413)	$3,836
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation and amortization	534	440
Stock-based compensation	1,819	210
Loss on disposal of property and equipment		20
Changes in operating assets and liabilities:
Accounts receivable	-326	-3,663
Prepaid expenses and other current assets	-1,315	-553
Accounts payable	364	-517
Accrued expenses	1,052	1,933
Deferred revenue	-18,739	45,952
Restricted cash and other assets	-716	-495
Other long-term liabilities	-1,289	1,190
Net cash (used in) provided by operating activities	-38,029	48,353
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	-230	-239
Net cash used in investing activities	-230	-239
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from sale of redeemable convertible preferred stock		21,961
Proceeds from initial public offering, net of commissions	82,491
Proceeds from stock options exercised	171	3
Payment of redeemable convertible preferred stock issuance costs		-38
Payment of initial public offering costs	-2,809
Net cash provided by financing activities	79,853	21,926
Net increase in cash and cash equivalents	41,594	70,040
Cash and cash equivalents, beginning of period	97,981	33,341
Cash and cash equivalents, end of period	139,575	103,381
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	192
Conversion of redeemable convertible preferred stock to common stock	76,420
Accretion of redeemable convertible preferred stock to redemption value	264	326
Vesting of restricted stock liability		9
Initial public offering costs incurred but unpaid at period end	$6

Overview_and_Basis_of_Presenta

Overview and Basis of Presentation	9 Months Ended
	Sep. 30, 2013
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 Prospectus filed with the SEC pursuant to Rule 424(b)(4) on May 31, 2013 (the “Prospectus”).
	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, 2013 and 2012 are referred to as the third quarter of 2013 and 2012, 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.
	On June 5, 2013, the Company completed an initial public offering (“IPO”) of its common stock, which resulted in the sale of 5,913,300 shares, including all additional shares available to cover over-allotments, at a price of $15.00 per share. The Company received net proceeds before expenses from the IPO of $82.5 million after deducting underwriting discounts and commissions paid by the Company. In preparation for the IPO, the Company’s Board of Directors and stockholders approved a one-for-three reverse stock split of the Company’s common stock effective May 13, 2013. All share and per share amounts in the consolidated financial statements and notes thereto have been retroactively adjusted, where necessary, to give effect to this reverse stock split. In connection with the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted to common stock at a one-for-three ratio as of June 5, 2013, resulting in an additional 20,633,046 shares of common stock of the Company becoming outstanding. Following these transactions, the Company’s total issued common stock as of September 30, 2013 was 28,418,420 shares. The significant increase in shares outstanding in June 2013 is expected to impact the year-over-year comparability of the Company’s (loss) earnings per share calculations through 2014.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2013
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 Prospectus.
	Recent Accounting Pronouncements
	In July 2013, the Financial Accounting Standards Board issued Accounting Standards Update (“ASU”) No. 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists. ASU 2013-11 amends Accounting Standards Codification (“ASC”) 740, Income Taxes, by providing guidance on the financial statement presentation of an unrecognized benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. The ASU does not affect the recognition or measurement of uncertain tax positions under ASC 740. ASU 2013-11 will be effective for the Company for interim and annual periods beginning after December 15, 2013, with early adoption permitted. The Company does not expect the adoption of this ASU to have any impact on its consolidated financial statements.

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2013
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, 2013 and December 31, 2012 consisted primarily of cash and cash equivalents, accounts receivable and accounts payable. The Company believes the carrying value of its cash, accounts receivable and accounts payable approximates the fair value due to the short-term nature of these instruments. As of September 30, 2013 and December 31, 2012, the Company’s financial assets recognized at fair value consisted of the following:
			Fair Value as of September 30, 2013
			Total		Level 1		Level 2		Level 3
			(In thousands)
	Cash equivalents		$	121,409	$	121,409	$	—	$	—
	Total		$	121,409	$	121,409	$	—	$	—
			Fair Value as of December 31, 2012
			Total		Level 1		Level 2		Level 3
			(In thousands)
	Cash equivalents		$	97,375	$	97,375	$	—	$	—
	Total		$	97,375	$	97,375	$	—	$	—

Accrued_Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2013
Payables And Accruals [Abstract]
Accrued Expenses	4	Accrued Expenses
	Accrued expenses consisted of the following:
			September 30,		December 31,
			2013		2012
			(In thousands)
	Employee compensation and benefits		$	1,882	$	1,880
	Current portion of contract termination obligations			538		1,274
	Research and development and professional expenses			2,964		1,174
	Accrued expenses		$	5,384	$	4,328
	Contract termination obligations include estimated repayments related to the termination of a research agreement in June 2012 and estimated lease exit charges related to the Company’s former facility at 325 Vassar Street in Cambridge, Massachusetts. The Company’s obligation related to its termination of a research agreement was accelerated as a result of the closing of the Company’s IPO, and, as a result, this termination obligation was paid in full in June 2013. As of December 31, 2012, the Company had recorded contract termination obligations of $3.0 million. During the nine months ended September 30, 2013, the Company recorded a net non-cash liability reduction of $0.1 million and made cash payments of $2.3 million, resulting in total remaining contract termination obligations of $0.6 million as of September 30, 2013. The non-current portion of contract termination obligations is included in other long-term liabilities.

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2013
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 or nine months ended September 30, 2013 and 2012 due to the expected loss before income taxes to be incurred and the actual loss before income taxes incurred for the years ended December 31, 2013 and 2012, respectively, as well as the Company’s continued maintenance of a full valuation allowance against its net deferred tax assets.

Commitments

Commitments	9 Months Ended
	Sep. 30, 2013
Commitments And Contingencies Disclosure [Abstract]
Commitments	6	Commitments
	In September 2013, the Company entered into an amendment to the operating lease agreement for its current office and laboratory facility in Cambridge, Massachusetts, under which the Company leased additional office space for aggregate rent payments of $2.5 million. These additional rent payments are expected to be paid from the commencement of the lease for this additional space, which is projected to be in the first quarter of 2014, through the end of the existing lease agreement, November 30, 2017.

Collaborations

Collaborations	9 Months Ended
	Sep. 30, 2013
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 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 received a $65.0 million upfront payment and $25.0 million from the sale of its series C redeemable convertible preferred stock to an affiliate of Celgene, of which $3.0 million was considered a premium and included as collaboration arrangement consideration for a total upfront payment of $68.0 million. In addition, the Company is eligible to earn up to $60.0 million in substantive clinical development milestone payments and up to $100.0 million in substantive regulatory milestone payments related to DOT1L, where milestones are considered to be substantive if (a) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone; (b) the consideration relates solely to past performance; and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company is also eligible to earn up to $65.0 million in payments, including a combination of substantive clinical development milestone payments and an option exercise fee, 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 pharmaceutical development, the Company may not receive any milestone or royalty payments from Celgene. The first potential milestone payment that the Company might be entitled to receive under this agreement is a $25.0 million substantive milestone for achieving proof-of-concept, 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 I clinical trials for the DOT1L target and 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 Celgene and the Company will equally co-fund global development and each party will solely fund territory-specific development costs for its territory. In the third quarter of 2013, the Company recorded accounts receivable of $0.7 million related to non-Phase I global development costs subject to the co-funding provisions of the agreement. Co-funded amounts receivable from Celgene are recorded net in research and development expense.
	Collaboration Revenue
	Through September 30, 2013, in addition to amounts allocated to Celgene’s purchase of shares of the Company’s series C redeemable convertible preferred stock, the Company had received a total of $68.0 million in upfront payments under the Celgene agreement, including a $3.0 million implied premium on Celgene’s purchase of shares of the Company’s series C redeemable convertible preferred stock. Through September 30, 2013, the Company has recognized $34.7 million of collaboration revenue in the consolidated statements of operations and comprehensive (loss) income related to this agreement, including $3.5 million and $10.8 million in the three and nine months ended September 30, 2013, respectively, and $6.7 million and $20.3 million in the three and nine months ended September 30, 2012, respectively. Revenue recognized in the three and nine months ended September 30, 2013 reflects the Company’s current plan to complete the ongoing Phase I study of EPZ-5676 in 2014. As a result, the remaining deferred revenue of approximately $2.1 million attributed to this deliverable as of September 30, 2013 will be recognized ratably through December 31, 2014. As of September 30, 2013 and December 31, 2012, the Company had deferred revenue of $33.3 million and $44.1 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 has 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 cash payments and accounts receivable totaling $16.5 million for research and development services through September 30, 2013. The Company is eligible to earn up to $25.0 million in additional clinical development milestone payments, including substantive milestone payments of up to $10.0 million, up to $55.0 million in regulatory milestone payments and up to $115.0 million in sales-based milestone payments. The Company is also eligible to receive royalties at a percentage in the mid-single digits on any net product sales outside of the United States and at a percentage from the mid-single digits to low double-digits on any product sales in the United States, subject to reduction in specified circumstances. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with pharmaceutical 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 II portion of the Phase I/II clinical trial.
	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 Eisai’s obligation to pay royalties to the Company as to such shared product in the United States will terminate; Eisai and the Company will share in net profits or losses with respect to such shared product in the United States; 25.0% of specified past development costs will become creditable by Eisai against future milestones or royalties due to the Company, subject to specified limitations; Eisai and the Company will share equally in subsequent development costs allocated to the United States; and all subsequent milestone payments that become payable by Eisai to the Company based on the shared product will be decreased by 50.0%.
	Collaboration Revenue
	Through September 30, 2013, the Company has recorded a total of $32.5 million in cash and accounts receivable and has recognized $30.5 million of collaboration revenue in the consolidated statements of operations and comprehensive (loss) income related to this agreement, including $1.9 million and $12.4 million in the three and nine months ended September 30, 2013, respectively, and $1.9 million and $9.3 million in the three and nine months ended September 30, 2012, respectively, with a $6.0 million clinical development milestone achieved and recognized as collaboration revenue in the nine months ended September 30, 2013 and a $4.0 million research milestone achieved and recognized as collaboration revenue in the nine months ended September 30, 2012. As of September 30, 2013 and December 31, 2012, the Company had deferred revenue of $2.0 million and $3.2 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 product platform. Under the terms of the agreement, the Company has granted GSK exclusive worldwide license rights to HMT inhibitors directed to three targets. Additionally, as part of the research collaboration provided for in the agreement, 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 ends January 8, 2015.
	Agreement Structure
	Under the agreement, the Company has received a $20.0 million upfront payment, $8.0 million in preclinical research and development milestone payments and $6.0 million of fixed research funding. The Company is eligible to receive up to $21.0 million in additional substantive preclinical research and development milestone payments, up to $99.0 million in clinical development milestone payments, up to $240.0 million in regulatory milestone payments and up to $270.0 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 pharmaceutical development, the Company may not receive any additional milestone payments or royalty payments from GSK. The next potential milestone payment that the Company might be entitled to receive under this agreement is a substantive research milestone. However, 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, 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 will be 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 any research activities conducted by the Company in the fourth year of the research term.
	Collaboration Revenue
	Through September 30, 2013, the Company has received a total of $34.0 million in payments under the GSK agreement and has recognized $18.7 million of collaboration revenue in the consolidated statements of operations and comprehensive (loss) income related to this agreement, including $3.0 million and $9.0 million in the three and nine months ended September 30, 2013, respectively, and $6.7 million in both the three and nine months ended September 30, 2012, with $4.0 million in preclinical research and development milestones achieved and recognized as collaboration revenue in the three and nine months ended September 30, 2012. As of September 30, 2013 and December 31, 2012, the Company had deferred revenue of $15.3 million and $22.0 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. 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 agreement, Eisai has agreed to pay Roche defined milestone payments of up to $21.0 million to develop and 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) income. 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, is obligated to make aggregate milestone-based development payments of up to $6.0 million and is obligated to reimburse Abbott for specified costs expected to be incurred in connection with Abbott conducting clinical trials to obtain the necessary regulatory approvals for the companion diagnostic (the “reimbursable costs”). The reimbursable costs are not to exceed $0.9 million unless any excess costs are agreed to in advance by both the Company and Abbott. In addition to the upfront payment, the Company expects to pay an aggregate of approximately $1.5 million in milestone-base development payments under this agreement during 2013.

StockBased_Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2013
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	8	Stock-Based Compensation
	Total stock-based compensation expense related to stock options, restricted stock and the employee stock purchase plan was $0.9 million and $0.1 million for the three months ended September 30, 2013 and 2012, respectively, and $1.8 million and $0.2 million for the nine months ended September 30, 2013 and 2012, respectively.
	Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive (loss) income as follows:
			Three Months Ended September 30,						Nine Months Ended September 30,
			2013			2012			2013		2012
			(In thousands)						(In thousands)
	Research and development		$	294		$	55		$	695	$	104
	General and administrative			575			37			1,124		106
	Total		$	869		$	92		$	1,819	$	210
	Stock Options
	The weighted-average fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $21.81 per option for those options granted during the three months ended September 30, 2013 and $7.57 and $1.70 per option for those options granted during the nine months ended September 30, 2013 and 2012, respectively. There were no stock options granted during the three months ended September 31, 2012. Key assumptions used to apply this pricing model were as follows:
			Nine Months Ended			Nine Months Ended
			September 30, 2013			September 30, 2012
	Risk-free interest rate			0.9	%		0.7	%
	Expected life of options			6.0 years			6.0 years
	Expected volatility of underlying stock			94.7	%		98.8	%
	Expected dividend yield			0	%		0	%
	The following is a summary of stock option activity for the nine months ended September 30, 2013:
			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, 2012			3,492,694		$	0.9
	Granted			1,483,328			7.51
	Exercised			(177,212	)		0.97
	Forfeited or expired			(25,780	)		2.74
	Outstanding at September 30, 2013			4,773,030		$	2.94			7.7	$	176,875
	Exercisable at September 30, 2013			2,557,179		$	0.7			6.7	$	100,489
	As of September 30, 2013, there was $10.4 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 2.8 years.
	Restricted Stock
	The following is a summary of restricted stock activity for the nine months ended September 30, 2013:
			Number of			Weighted
			Shares			Average Grant
						Date Fair Value
						per Share
	Outstanding at December 31, 2012			22,223		$	0.6
	Vested			(12,501	)		0.6
	Outstanding at September 30, 2013			9,722		$	0.6
	As of September 30, 2013, there was an insignificant amount of unrecognized compensation cost related to restricted stock that is expected to vest.

Loss_Earnings_Per_Share

(Loss) Earnings Per Share	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]
(Loss) Earnings Per Share	9	(Loss) Earnings 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) earnings per share allocable to common stockholders are computed as follows:
			Three Months Ended September 30,					Nine Months Ended September 30,
			2013			2012		2013			2012
			(In thousands except per share data)
	Net (loss) income		$	(9,704	)	$	4,448	$	(19,413	)	$	3,836
	Less: accretion of redeemable convertible preferred stock to redemption value			—			159		264			326
	Less: income allocable to participating securities			—			3,972		—			3,239
	(Loss) income allocable to common stockholders		$	(9,704	)	$	317	$	(19,677	)	$	271
	Weighted average shares outstanding			28,406			1,651		13,212			1,637
	Basic (loss) earnings per share allocable to common stockholders		$	(0.34	)	$	0.19	$	(1.49	)	$	0.17
			Three Months Ended September 30,					Nine Months Ended September 30,
			2013			2012		2013			2012
			(In thousands except per share data)
	Net (loss) income		$	(9,704	)	$	4,448	$	(19,413	)	$	3,836
	Less: accretion of redeemable convertible preferred stock to redemption value			—			159		264			326
	Less: income allocable to participating securities			—			3,743		—			3,092
	(Loss) income allocable to common stockholders		$	(9,704	)	$	546	$	(19,677	)	$	418
	Weighted average shares outstanding			28,406			1,651		13,212			1,637
	Effect of dilutive securities			—			1,366		—			1,004
	Diluted weighted average shares outstanding			28,406			3,017		13,212			2,641
	Diluted (loss) earnings per share allocable to common stockholders		$	(0.34	)	$	0.18	$	(1.49	)	$	0.16
	In June 2013, the Company issued an additional 5,913,300 shares of common stock in connection with its IPO and 20,633,046 shares of common stock in connection with the automatic conversion of its redeemable convertible preferred stock upon the closing of the IPO. The issuance of these shares resulted in a significant increase in the Company’s shares outstanding, to 28,408,698 shares as of September 30, 2013, and weighted average shares outstanding for the three and nine months ended September 30, 2013 when compared to the comparable prior year periods and is expected to continue to impact the year-over-year comparability of the Company’s (loss) earnings per share calculations through 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 September 30,					Nine Months Ended September 30,
			2013			2012		2013			2012
			(In thousands)
	Stock options			4,773			—		4,773			—
	Unvested restricted stock			10			—		10			—
	Shares issuable under employee stock purchase plan			2			—		2			—
				4,785			—		4,785			—

Related_Party_Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2013
Related Party Transactions [Abstract]
Related Party Transactions	10	Related Party Transactions
	In connection with its entry into the collaboration and license agreement with Celgene, on April 2, 2012, the Company sold Celgene 9,803,922 shares of its series C redeemable convertible preferred stock. As a result of this transaction, Celgene owned 12.5% of the Company’s fully diluted equity as of December 31, 2012. Refer to Note 6, Collaborations, for additional information regarding this collaboration agreement. In the second quarter of 2013, in connection with the IPO, Celgene made an additional investment in the Company, acquiring an additional 66,666 shares of the Company’s common stock. Additionally, as a result of the IPO, Celgene’s shares of series C redeemable convertible preferred stock automatically converted to common stock of the Company at a one-for-three ratio, collectively resulting in Celgene owning 3,334,640 shares of the Company’s common stock as of September 30, 2013, representing 10.0% of the Company’s fully diluted equity and 11.7% of the voting interests of the Company as of September 30, 2013.
	Under the Celgene collaboration agreement, the Company recognized $3.5 million and $10.8 million of collaboration revenue in the three and nine months ended September 30, 2013, respectively, and $6.7 million and $20.3 million of collaboration revenue in the three and nine months ended September 30, 2012, respectively. Additionally, in the three and nine months ended September 30, 2013, the Company recorded $0.7 million in co-funded accounts receivable from Celgene net in research and development expense. Accordingly, as of September 30, 2013 and December 31, 2012, the Company had recorded $33.3 million and $44.1 million of deferred revenue, respectively, and, as of September 30, 2013, the Company had accounts receivable of $0.7 million related to this collaboration arrangement.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2013
Accounting Policies [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	In July 2013, the Financial Accounting Standards Board issued Accounting Standards Update (“ASU”) No. 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists. ASU 2013-11 amends Accounting Standards Codification (“ASC”) 740, Income Taxes, by providing guidance on the financial statement presentation of an unrecognized benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. The ASU does not affect the recognition or measurement of uncertain tax positions under ASC 740. ASU 2013-11 will be effective for the Company for interim and annual periods beginning after December 15, 2013, with early adoption permitted. The Company does not expect the adoption of this ASU to have any impact on its consolidated financial statements.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2013
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of September 30, 2013 and December 31, 2012, the Company’s financial assets recognized at fair value consisted of the following:
		Fair Value as of September 30, 2013
		Total		Level 1		Level 2		Level 3
		(In thousands)
	Cash equivalents	$	121,409	$	121,409	$	—	$	—
	Total	$	121,409	$	121,409	$	—	$	—
		Fair Value as of December 31, 2012
		Total		Level 1		Level 2		Level 3
		(In thousands)
	Cash equivalents	$	97,375	$	97,375	$	—	$	—
	Total	$	97,375	$	97,375	$	—	$	—

Accrued_Expenses_Tables

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2013
Payables And Accruals [Abstract]
Summary of Accrued Expenses	Accrued expenses consisted of the following:
		September 30,		December 31,
		2013		2012
		(In thousands)
	Employee compensation and benefits	$	1,882	$	1,880
	Current portion of contract termination obligations		538		1,274
	Research and development and professional expenses		2,964		1,174
	Accrued expenses	$	5,384	$	4,328

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2013
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense	Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive (loss) income as follows:
		Three Months Ended September 30,						Nine Months Ended September 30,
		2013			2012			2013		2012
		(In thousands)						(In thousands)
	Research and development	$	294		$	55		$	695	$	104
	General and administrative		575			37			1,124		106
	Total	$	869		$	92		$	1,819	$	210
Assumptions Used in Applying Pricing Model	Key assumptions used to apply this pricing model were as follows:
		Nine Months Ended			Nine Months Ended
		September 30, 2013			September 30, 2012
	Risk-free interest rate		0.9	%		0.7	%
	Expected life of options		6.0 years			6.0 years
	Expected volatility of underlying stock		94.7	%		98.8	%
	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, 2013:
		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, 2012		3,492,694		$	0.9
	Granted		1,483,328			7.51
	Exercised		(177,212	)		0.97
	Forfeited or expired		(25,780	)		2.74
	Outstanding at September 30, 2013		4,773,030		$	2.94			7.7	$	176,875
	Exercisable at September 30, 2013		2,557,179		$	0.7			6.7	$	100,489
Summary of Restricted Stock Activity	The following is a summary of restricted stock activity for the nine months ended September 30, 2013:
		Number of			Weighted
		Shares			Average Grant
					Date Fair Value
					per Share
	Outstanding at December 31, 2012		22,223		$	0.6
	Vested		(12,501	)		0.6
	Outstanding at September 30, 2013		9,722		$	0.6

Loss_Earnings_Per_Share_Tables

(Loss) Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Loss Per Share	Basic and diluted (loss) earnings per share allocable to common stockholders are computed as follows:
		Three Months Ended September 30,					Nine Months Ended September 30,
		2013			2012		2013			2012
		(In thousands except per share data)
	Net (loss) income	$	(9,704	)	$	4,448	$	(19,413	)	$	3,836
	Less: accretion of redeemable convertible preferred stock to redemption value		—			159		264			326
	Less: income allocable to participating securities		—			3,972		—			3,239
	(Loss) income allocable to common stockholders	$	(9,704	)	$	317	$	(19,677	)	$	271
	Weighted average shares outstanding		28,406			1,651		13,212			1,637
	Basic (loss) earnings per share allocable to common stockholders	$	(0.34	)	$	0.19	$	(1.49	)	$	0.17
		Three Months Ended September 30,					Nine Months Ended September 30,
		2013			2012		2013			2012
		(In thousands except per share data)
	Net (loss) income	$	(9,704	)	$	4,448	$	(19,413	)	$	3,836
	Less: accretion of redeemable convertible preferred stock to redemption value		—			159		264			326
	Less: income allocable to participating securities		—			3,743		—			3,092
	(Loss) income allocable to common stockholders	$	(9,704	)	$	546	$	(19,677	)	$	418
	Weighted average shares outstanding		28,406			1,651		13,212			1,637
	Effect of dilutive securities		—			1,366		—			1,004
	Diluted weighted average shares outstanding		28,406			3,017		13,212			2,641
	Diluted (loss) earnings per share allocable to common stockholders	$	(0.34	)	$	0.18	$	(1.49	)	$	0.16
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 September 30,					Nine Months Ended September 30,
		2013			2012		2013			2012
		(In thousands)
	Stock options		4,773			—		4,773			—
	Unvested restricted stock		10			—		10			—
	Shares issuable under employee stock purchase plan		2			—		2			—
			4,785			—		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	Jun. 05, 2013	Sep. 30, 2013	Dec. 31, 2012
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of initial public offering of common stock	5,913,300
Amount of per share for all additional shares	$15
Net proceeds from IPO	$82,500	$82,491
Ratio of common stock	one-for-three
Issuance of shares of common stock	20,633,046
Common stock issued		28,418,420	1,694,862

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$121,409	$97,375
Total	121,409	97,375
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	121,409	97,375
Total	121,409	97,375
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents
Total
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents
Total

Accrued_Expenses_Summary_of_Ac

Accrued Expenses - Summary of Accrued Expenses (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Accrued Liabilities [Abstract]
Employee compensation and benefits	$1,882	$1,880
Current portion of contract termination obligations	538	1,274
Research and development and professional expenses	2,964	1,174
Accrued expenses	$5,384	$4,328

Accrued_Expenses_Additional_In

Accrued Expenses - Additional Information (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Restructuring Reserve [Abstract]
Contract termination obligations	$0.60	$3
Net non-cash liability reduction	0.1
Cash payments for contract termination obligations	$2.30

Commitments_Additional_Informa

Commitments - Additional Information (Detail) (Lease Amendment [Member], USD $)	Sep. 30, 2013
In Millions, unless otherwise specified
Lease Amendment [Member]
Other Commitments [Line Items]
Aggregate rent payments	$2.50

Collaborations_Additional_Info

Collaborations - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended	21 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended		33 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended	21 Months Ended	33 Months Ended		1 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012	Feb. 28, 2013
						Celgene [Member]	Celgene [Member]	Celgene [Member]	Celgene [Member]	Celgene [Member]	Celgene [Member]	Celgene [Member]	Celgene [Member]	Celgene [Member]	Eisai [Member]	Eisai [Member]	Eisai [Member]	Eisai [Member]	Eisai [Member]	Eisai [Member]	Eisai [Member]	GSK [Member]	GSK [Member]	GSK [Member]	GSK [Member]	GSK [Member]	GSK [Member]	GSK [Member]	GSK [Member]	Abbott [Member]
												DOT1L [Member]	DOT1L [Member]	Available Targets [Member]
													Post-IND Services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Company received upfront payment										$65,000,000										$3,000,000						$20,000,000
Proceeds from redeemable convertible preferred stock				21,961,000						25,000,000
Additional milestone payments												60,000,000			25,000,000		25,000,000				25,000,000	99,000,000		99,000,000			99,000,000	99,000,000
Additional milestone payments												100,000,000		100,000,000	55,000,000		55,000,000				55,000,000	240,000,000		240,000,000			240,000,000	240,000,000
Option period ending			Jul-15
Right to extend option exercise period date			2016-07
Company received upfront payment														65,000,000
Potential milestone payments receivable						25,000,000		25,000,000			25,000,000				10,000,000		10,000,000				10,000,000
Accounts receivable related to non-phase I global development costs.	2,155,000		2,155,000		1,829,000	700,000		700,000			700,000
Cash and accounts receivable											68,000,000										32,500,000							34,000,000
Premium received on purchase of shares										3,000,000
Collaboration revenue	8,444,000	15,331,000	32,165,000	36,327,000		3,500,000	6,700,000	10,800,000	20,300,000		34,700,000				1,900,000	1,900,000	12,400,000	9,300,000			30,500,000	3,000,000	6,700,000	9,000,000	6,700,000		18,700,000
Deferred revenue						33,300,000		33,300,000		44,100,000	33,300,000		2,100,000		2,000,000		2,000,000		3,200,000		2,000,000	15,300,000		15,300,000			15,300,000	15,300,000	22,000,000
Milestone payments received																					7,000,000							8,000,000
Clinical development milestone achieved																					6,000,000
Research and development services compensation earned																					16,500,000
Sales-based milestone payments															115,000,000		115,000,000				115,000,000	270,000,000		270,000,000			270,000,000	270,000,000
Entitled offset funding percentage																	25.00%		25.00%
Decreased rate of share product																	50.00%
Collaboration revenue recognized																	6,000,000	4,000,000					4,000,000		4,000,000
End of research term agreed upon research plan																								8-Jan-15
Fixed research funding received																												6,000,000
Additional milestone payments																						21,000,000		21,000,000			21,000,000	21,000,000
Percentage rate of research and development costs																								100.00%
Development milestone payments																			21,000,000
Upfront payment paid by the Company																														900,000
Development milestone payments																														6,000,000
Maximum reimbursable costs																														900,000
Expected development milestone payments																														$1,500,000

StockBased_Compensation_Additi

Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based compensation expense related to stock options, restricted stock and the employee stock purchase plan	$869,000	$92,000	$1,819,000	$210,000
Weighted-average fair value of options granted	$21.81		$7.57	$1.70
Stock options granted		0	1,483,328
Unrecognized compensation expense related to stock options	$10,400,000		$10,400,000
Expected weighted average period for recognition of compensation cost			2 years 9 months 18 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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$869	$92	$1,819	$210
Research and Development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	294	55	695	104
General and Administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$575	$37	$1,124	$106

StockBased_Compensation_Assump

Stock-Based Compensation - Assumptions Used in Applying Pricing Model (Detail)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Abstract]
Risk-free interest rate	0.90%	0.70%
Expected life of options	6 years	6 years
Expected volatility of underlying stock	94.70%	98.80%
Expected dividend yield	0.00%	0.00%

StockBased_Compensation_Summar

Stock-Based Compensation - Summary of Stock Option Activity (Detail) (USD $)	3 Months Ended	9 Months Ended
In Thousands, except Share data, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2013
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding [Roll Forward]
Number of Options, Outstanding, Beginning balance		3,492,694
Number of Options, Granted	0	1,483,328
Number of Options, Exercised		-177,212
Number of Options, Forfeited or expired		-25,780
Number of Options, Outstanding, Ending balance		4,773,030
Number of Options, Exercisable		2,557,179
Weighted Average Exercise Price per Share, Outstanding, Beginning balance		$0.90
Weighted Average Exercise Price per Share, Granted		$7.51
Weighted Average Exercise Price per Share, Exercised		$0.97
Weighted Average Exercise Price per Share, Forfeited or expired		$2.74
Weighted Average Exercise Price per Share, Outstanding, Ending balance		$2.94
Weighted Average Exercise Price per Share, Exercisable		$0.70
Weighted Average Remaining Contractual Term (In Years), Outstanding		7 years 8 months 12 days
Weighted Average Remaining Contractual Term (In Years), Exercisable		6 years 8 months 12 days
Aggregate Intrinsic Value, Outstanding		$176,875
Aggregate Intrinsic Value, Exercisable		$100,489

StockBased_Compensation_Summar1

Stock-Based Compensation - Summary of Restricted Stock Activity (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013
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	22,223
Number of Shares, Vested	-12,501
Number of Shares, Outstanding, Ending balance	9,722
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	$0.60

Loss_Earnings_Per_Share_Schedu

(Loss) Earnings 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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Net (loss) income	($9,704)	$4,448	($19,413)	$3,836
Less: accretion of redeemable convertible preferred stock to redemption value		159	264	326
Less: income allocable to participating securities		3,972		3,239
(Loss) income allocable to common stockholders-basic	-9,704	317	-19,677	271
Weighted average shares outstanding	28,406	1,651	13,212	1,637
Basic (loss) earnings per share allocable to common stockholders	($0.34)	$0.19	($1.49)	$0.17
Net (loss) income	-9,704	4,448	-19,413	3,836
Less: accretion of redeemable convertible preferred stock to redemption value		159	264	326
Less: income allocable to participating securities		3,743		3,092
(Loss) income allocable to common stockholders-diluted	($9,704)	$546	($19,677)	$418
Weighted average shares outstanding	28,406	1,651	13,212	1,637
Effect of dilutive securities		1,366		1,004
Diluted weighted average shares outstanding	28,406	3,017	13,212	2,641
Diluted (loss) earnings per share allocable to common stockholders	($0.34)	$0.18	($1.49)	$0.16

Loss_Earnings_Per_Share_Additi

(Loss) Earnings Per Share - Additional Information (Detail)	1 Months Ended
	Jun. 05, 2013	Sep. 30, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Additional shares of common stock in connection with IPO	5,913,300
Common stock in connection with the automatic conversion of its redeemable convertible preferred stock	20,633,046
Common stock, shares outstanding		28,408,698	1,672,639

Loss_Earnings_Per_Share_Common

(Loss) Earnings Per Share - Common Stock Equivalents Excluded 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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	4,785		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	4,773		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	10		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

Related_Party_Transactions_Add

Related Party Transactions - Additional Information (Detail) (USD $)	1 Months Ended			3 Months Ended		9 Months Ended				1 Months Ended	9 Months Ended
In Millions, except Share data, unless otherwise specified	Jun. 05, 2013	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Apr. 02, 2012	Jun. 05, 2013	Sep. 30, 2013
				Beneficial Owner [Member]	Beneficial Owner [Member]	Beneficial Owner [Member]	Beneficial Owner [Member]	Beneficial Owner [Member]	Beneficial Owner [Member]	Beneficial Owner [Member]	Beneficial Owner [Member]
										Celgene [Member]	Celgene [Member]
Related Party Transaction [Line Items]
Redeemable convertible preferred stock		0	61,899,165						9,803,922
Diluted equity								12.50%			10.00%
Issuance of common stock	5,913,300									66,666
Conversion of Preferred stock to Common Stock		28,408,698	1,672,639								3,334,640
Percentage of Voting Interests											11.70%
Ratio of common stock	one-for-three										One-for-three
Collaboration revenue related to agreement				$3.50	$6.70	$10.80	$20.30
Deferred revenue related to agreement				33.3		33.3		44.1
Research and development expense				0.7		0.7
Accounts receivable related to collaboration arrangement				$0.70		$0.70