License and Collaboration Agreements - Additional Information (Detail) - USD ($)				1 Months Ended				3 Months Ended					9 Months Ended		22 Months Ended	36 Months Ended
	Nov. 25, 2013	May. 05, 2011	Nov. 10, 2009	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2015	Sep. 30, 2014	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Mar. 31, 2012	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Dec. 31, 2012	Dec. 31, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenues									$ 16,440,000		$ 28,002,000		$ 67,839,000	$ 68,851,000
Deferred revenue, current									55,072,000				$ 55,072,000		$ 55,072,000		$ 59,275,000
Agreement termination scheduled date													Dec. 17, 2014
Assets maintained related to Sanofi Agreement									102,684,000				$ 102,684,000		102,684,000		158,656,000
Liabilities maintained related to Sanofi Agreement									243,349,000				243,349,000		243,349,000		260,727,000
Research and development expenses									37,763,000		43,632,000		$ 116,248,000	107,751,000
Baxalta [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenues											0			0
Substantive milestone achieved										$ 20,000,000
Baxalta [Member] | License and Collaboration Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront license fee received							$ 100,000,000
Royalty for improved products													The Company is also entitled to tiered, escalating royalties ranging from sub-teen double-digits to low twenties percentages of net sales of MM-398 in the Licensed Territory.
Notice period of termination													180 days
Revenue recognized related to a substantive milestone payment										$ 20,000,000
Collaboration revenues									16,440,000				$ 67,839,000
Deferred revenue									65,136,000				65,136,000		65,136,000		91,156,000
Deferred revenue, current									55,100,000				55,100,000		55,100,000
Baxalta [Member] | License and Collaboration Agreements [Member] | Sales Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum amount of milestone payments that can be received													250,000,000
Baxalta [Member] | License and Collaboration Agreements [Member] | Research and Development Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum amount of milestone payments that can be received													100,000,000
Baxalta [Member] | License and Collaboration Agreements [Member] | Regulatory Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone license fee received													20,000,000
Maximum amount of milestone payments that can be received													520,000,000
Baxalta [Member] | License and Collaboration Agreements [Member] | Clinical Trials in Pancreatic Cancer [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone license fee received													15,000,000
Collaboration agreement costs													98,800,000
Baxalta [Member] | License and Collaboration Agreements [Member] | Scenario Forecast [Member] | Research and Development Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments expects to receive								$ 47,500,000
Baxalta [Member] | License and Collaboration Agreements [Member] | Subsequent Event [Member] | Phase 2 Clinical Trial in Pancreatic Cancer [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized related to a substantive milestone payment				$ 47,500,000
Sanofi [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront license fee received			$ 60,000,000
Milestone license fee received																$ 25,000,000
Collaboration revenues													0
Expected development period from the effective date of agreement			12 years
Assets maintained related to Sanofi Agreement									0				0		0
Liabilities maintained related to Sanofi Agreement									0				$ 0		0
Sanofi [Member] | License and Collaboration Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenues											28,002,000			68,851,000
Recognized revenue related to excess spending														5,800,000
PharmaEngine [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront license fees paid		$ 10,000,000
Milestone payment					$ 11,000,000
PharmaEngine [Member] | Development and Regulatory Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum milestone payment obligation		80,000,000
PharmaEngine [Member] | Sales Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum milestone payment obligation		$ 130,000,000
PharmaEngine [Member] | Phase 3 Clinical Trial in Pancreatic Cancer [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment												$ 5,000,000
PharmaEngine [Member] | License and Collaboration Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment							$ 7,000,000
Milestone payment due date													Apr. 30, 2015
Research and development expenses									100,000		$ 12,400,000		$ 11,400,000	$ 12,500,000
PharmaEngine [Member] | License and Collaboration Agreements [Member] | New Drug Application [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment						$ 5,000,000
Actavis [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum amount of milestone payments that can be received	$ 15,100,000
Notice period of termination													90 days
Collaborative arrangement gross payments received															3,800,000
Agreement expiration term respect to each product													10 years
Additional renewal term													2 years
Milestones and development expenses									$ 3,900,000				$ 3,900,000		$ 3,900,000		$ 3,800,000
Expected revenue recognition period begins after first sale of applicable product													10 years