Document and Entity Information
9 Months Ended
Mar. 31, 2013
Apr. 29, 2013
Document and Entity Information    
Entity Registrant Name IMMUNOGEN INC  
Entity Central Index Key 0000855654  
Document Type 10-Q  
Document Period End Date Mar. 31, 2013  
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   84,533,809
Document Fiscal Year Focus 2013  
Document Fiscal Period Focus Q3