Document and Entity Information
6 Months Ended
Dec. 31, 2013
Jan. 27, 2014
Document and Entity Information    
Entity Registrant Name IMMUNOGEN INC  
Entity Central Index Key 0000855654  
Document Type 10-Q  
Document Period End Date Dec. 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   85,672,716
Document Fiscal Year Focus 2014  
Document Fiscal Period Focus Q2