Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Jul. 31, 2019
Document and Entity Information    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2019  
Document Transition Report false  
Entity File Number 0-17999  
Entity Registrant Name ImmunoGen, Inc.  
Entity Tax Identification Number 04-2726691  
Entity Incorporation, State or Country Code MA  
Entity Address, Address Line One 830 Winter Street  
Entity Address, City or Town Waltham  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 02451  
City Area Code 781  
Local Phone Number 895-0600  
Title of 12(b) Security Common Stock  
Trading Symbol IMGN  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   149,884,816
Entity Central Index Key 0000855654  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Amendment Flag false