Document and Entity Information
Feb. 12, 2020
Document and Entity Information [Abstract]  
Document Type 8-K
Document Period End Date Feb. 12, 2020
Entity File Number 0-17999
Entity Registrant Name ImmunoGen, Inc.
Entity Incorporation, State or Country Code MA
Entity Tax Identificatoin Number 04-2726691
Entity Address, Address Line One 830 Winter Street
Entity Address, State or Province MA
Entity Address, City or Town Waltham
Entity Address, Postal Zip Code 02451
City Area Code 781
Local Phone Number 895-0600
Written Communications false
Soliciting Material false
Pre-commencement Tender Offer false
Pre-commencement Issuer Tender Offer false
Title of 12(b) Security Common Stock, $.01 par value
Trading Symbol IMGN
Security Exchange Name NASDAQ
Entity Emerging Growth Company false
Entity Central Index Key 0000855654
Amendment Flag false