Document and Entity Information - USD ($)
12 Months Ended
Jun. 30, 2016
Aug. 18, 2016
Dec. 31, 2015
Document and Entity Information      
Entity Registrant Name IMMUNOGEN INC    
Entity Central Index Key 0000855654    
Document Type 10-K    
Document Period End Date Jun. 30, 2016    
Amendment Flag false    
Current Fiscal Year End Date --06-30    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Public Float     $ 1,176,154,266
Entity Common Stock, Shares Outstanding   87,326,441  
Document Fiscal Year Focus 2016    
Document Fiscal Period Focus FY