Document and Entity Information
6 Months Ended
Dec. 31, 2012
Jan. 22, 2013
Document and Entity Information    
Entity Registrant Name IMMUNOGEN INC  
Entity Central Index Key 0000855654  
Document Type 10-Q  
Document Period End Date Dec. 31, 2012  
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   84,222,024
Document Fiscal Year Focus 2013  
Document Fiscal Period Focus Q2