Agreements - Debiopharm (Details) - USD ($)	1 Months Ended			3 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	May 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017
Collaborative Agreements disclosures
Revenue from contract with customer				$ 8,584,000	$ 19,815,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer				$ 79,000	11,540,000
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
Debiopharm | Transfer of ImmunoGen technologies
Collaborative Agreements disclosures
Revenue from contract with customer	$ 500,000					$ 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