Agreements - Debiopharm (Details) - USD ($)	1 Months Ended			3 Months Ended			9 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	May 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018
Collaborative Agreements disclosures
Revenue from contract with customer				$ 13,281,000	$ 10,928,000		$ 37,407,000	$ 40,030,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer				$ 79,000	672,000		$ 5,237,000	13,533,000
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
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