License and Collaboration Agreements - Additional Information (Detail) - USD ($)				1 Months Ended		3 Months Ended			6 Months Ended
	Nov. 25, 2013	May. 05, 2011	Nov. 10, 2009	Jul. 31, 2015	Sep. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2012	Dec. 31, 2015	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenues						$ 36,558,000	$ 27,815,000			$ 51,399,000	$ 40,849,000
Deferred revenue, current						59,346,000				$ 59,346,000		$ 59,275,000
Agreement termination scheduled date										Dec. 17, 2014
Assets maintained related to Sanofi Agreement						105,043,000				$ 105,043,000		158,656,000
Liabilities maintained related to Sanofi Agreement						248,107,000				248,107,000		260,727,000
Research and development expenses						42,806,000	33,795,000			78,485,000	64,119,000
Baxalta [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenues							0				0
Substantive milestone achieved						20,000,000				$ 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						36,558,000				$ 51,399,000
Deferred revenue						64,611,000				64,611,000		91,156,000
Deferred revenue, current						59,200,000				59,200,000
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]
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									$ 62,500,000
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
Liabilities maintained related to Sanofi Agreement						0				$ 0
Sanofi [Member] | License and Collaboration Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenues							27,815,000				40,849,000
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
PharmaEngine [Member] | Subsequent Event [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
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						11,100,000	$ 100,000			$ 11,300,000	$ 200,000
PharmaEngine [Member] | License and Collaboration Agreements [Member] | New Drug Application [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration milestone obligation, accrued										5,000,000
Actavis [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone license fee received										$ 3,800,000
Maximum amount of milestone payments that can be received	$ 15,100,000
Notice period of termination										90 days
Agreement expiration term respect to each product										10 years
Additional renewal term										2 years
Milestones and development expenses						3,800,000				$ 3,800,000		$ 3,800,000
Milestones and development expenses - short term						$ 100,000				$ 100,000