Document and Entity Information - USD ($)	6 Months Ended
	Dec. 31, 2016	Feb. 16, 2017
Document and Entity Information
Entity Registrant Name	IMMUNOGEN INC
Entity Central Index Key	0000855654
Document Type	10-KT
Document Period End Date	Dec. 31, 2016
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Public Float	$ 176,818,406
Entity Common Stock, Shares Outstanding		87,354,589
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY