Document and Entity Information (USD $)
12 Months Ended
Jun. 30, 2015
Aug. 20, 2015
Dec. 31, 2014
Document and Entity Information      
Entity Registrant Name IMMUNOGEN INC    
Entity Central Index Key 0000855654    
Document Type 10-K    
Document Period End Date Jun. 30, 2015    
Amendment Flag false    
Current Fiscal Year End Date --06-30    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Public Float     $ 526,298,576
Entity Common Stock, Shares Outstanding   86,961,537  
Document Fiscal Year Focus 2015    
Document Fiscal Period Focus FY