v3.8.0.1
Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2017
Feb. 28, 2018
Jun. 30, 2017
Document and Entity Information      
Entity Registrant Name IMMUNOGEN INC    
Entity Central Index Key 0000855654    
Document Type 10-K    
Document Period End Date Dec. 31, 2017    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Accelerated Filer    
Entity Public Float     $ 617,703,981
Entity Common Stock, Shares Outstanding   132,846,535  
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY