Document and Entity Information (USD $)
12 Months Ended
Jun. 30, 2013
Aug. 20, 2013
Dec. 31, 2012
Document and Entity Information      
Entity Registrant Name IMMUNOGEN INC    
Entity Central Index Key 0000855654    
Document Type 10-K    
Document Period End Date Jun. 30, 2013    
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     $ 1,070,664,433
Entity Common Stock, Shares Outstanding   85,109,108  
Document Fiscal Year Focus 2013    
Document Fiscal Period Focus FY