Document and Entity Information
3 Months Ended
Sep. 30, 2014
Oct. 23, 2014
Document and Entity Information    
Entity Registrant Name IMMUNOGEN INC  
Entity Central Index Key 0000855654  
Document Type 10-Q  
Document Period End Date Sep. 30, 2014  
Amendment Flag false  
Current Fiscal Year End Date --06-30  
Entity Current Reporting Status Yes  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   85,935,634
Document Fiscal Year Focus 2015  
Document Fiscal Period Focus Q1