v3.10.0.1
Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2018
Feb. 19, 2019
Jun. 30, 2018
Document and Entity Information      
Entity Registrant Name IMMUNOGEN INC    
Entity Central Index Key 0000855654    
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
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 Large Accelerated Filer    
Entity Public Float     $ 1,425,536,472
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   149,409,825  
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY