License and Collaboration Agreements - Additional Information (Detail) - USD ($)					1 Months Ended						3 Months Ended								12 Months Ended			25 Months Ended	36 Months Ended
	Sep. 23, 2014	Nov. 25, 2013	May. 05, 2011	Nov. 10, 2009	Oct. 31, 2015	Aug. 31, 2015	Jul. 31, 2015	Apr. 30, 2015	Jan. 31, 2015	Sep. 30, 2014	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue, current											$ 50,137,000				$ 59,275,000				$ 50,137,000	$ 59,275,000		$ 50,137,000
Agreement termination scheduled date																			Dec. 17, 2014
License and collaboration revenues											17,090,000	$ 16,440,000	$ 36,558,000	$ 14,842,000	33,905,000	$ 28,002,000	$ 27,815,000	$ 13,034,000	$ 84,930,000	102,756,000	$ 47,786,000
Assets maintained related to Sanofi Agreement											234,880,000				158,506,000				234,880,000	158,506,000		234,880,000
Liabilities maintained related to Sanofi Agreement											418,569,000				260,577,000				418,569,000	260,577,000		418,569,000
Research and development expenses											44,740,000	$ 37,763,000	42,806,000	$ 35,679,000	30,744,000	$ 43,632,000	$ 33,795,000	$ 30,324,000	$ 160,988,000	138,495,000	147,139,000
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					$ 47,500,000	$ 15,000,000							$ 20,000,000
Deferred revenue											97,365,000				91,156,000				$ 97,365,000	91,156,000		97,365,000
Deferred revenue, current											50,100,000								50,100,000			50,100,000
License and collaboration revenues																			84,930,000	10,460,000
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																			62,500,000
Collaboration agreement costs																			98,800,000
Baxalta [Member] | Non-Substantive Collaborative Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone license fee received																			62,500,000
Milestone license fee	90,000,000
Baxalta [Member] | Substantive Collaborative Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone license fee received																			20,000,000
Milestone license fee	$ 530,000,000
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
Expected development period from the effective date of agreement				12 years
License and collaboration revenues																			0
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]
License and collaboration revenues																				92,296,000	47,233,000
Recognized revenue related to excess spending																				5,800,000
Spending in excess of budget																					10,100,000
PharmaEngine [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront license fees paid			$ 10,000,000
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] | License and Collaboration Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment							$ 11,000,000			$ 7,000,000
Research and development expenses																			$ 11,400,000	12,600,000	$ 1,500,000
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																						3,900,000
Aggregate milestone payments eligible to receive, decrease									$ 400,000
Agreement expiration term respect to each product																			10 years
Additional renewal term																			2 years
Milestones and development expenses											$ 4,000,000				$ 3,800,000				$ 4,000,000	$ 3,800,000		$ 4,000,000
Expected revenue recognition period begins after first sale of applicable product																			10 years