Document and Entity Information - shares
6 Months Ended
Dec. 31, 2015
Jan. 29, 2016
Document and Entity Information    
Entity Registrant Name IMMUNOGEN INC  
Entity Central Index Key 0000855654  
Document Type 10-Q  
Document Period End Date Dec. 31, 2015  
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   87,060,869
Document Fiscal Year Focus 2016  
Document Fiscal Period Focus Q2