Agreements - Debiopharm (Details) - USD ($)	1 Months Ended		3 Months Ended												12 Months Ended
	Dec. 31, 2017	May 31, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Agreements disclosures
Revenue from contract with customer			$ 44,864,000	$ 13,281,000	$ 15,542,000	$ 8,584,000	$ 13,416,000	$ 10,928,000	$ 9,287,000	$ 19,815,000	$ 39,448,000	$ 8,480,000	$ 39,020,000	$ 28,499,000	$ 82,271,000	$ 53,446,000	$ 115,447,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer			29,551,000	$ 79,000	5,079,000	79,000	1,747,000	672,000	1,321,000	11,540,000	29,580,000	79,000	31,080,000	18,730,000	34,788,000	15,280,000	79,469,000
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer					$ 51,000	$ 17,000	$ 218,000	$ 388,000	$ 388,000	$ 383,000	$ 452,000	$ 650,000	$ 902,000	$ 1,478,000	68,000	1,377,000	3,482,000
Debiopharm | IMGN529 program
Collaborative Agreements disclosures
Revenue from contract with customer	$ 4,500,000
Debiopharm | Phase 3 Clinical Trial
Collaborative Agreements disclosures
Potential milestone payment			$ 25,000,000												$ 25,000,000
Debiopharm | Transfer of ImmunoGen technologies
Collaborative Agreements disclosures
Potential milestone payment		$ 5,000,000
Debiopharm | Upfront payment | IMGN529 program
Collaborative Agreements disclosures
Revenue from contract with customer		$ 25,000,000
Debiopharm | License and milestone fees | IMGN529 program
Collaborative Agreements disclosures
Revenue from contract with customer																$ 500,000	$ 29,500,000