CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) $ in Thousands	12 Months Ended
Jun. 30, 2015 USD ($)
CONSOLIDATED STATEMENTS OF CASH FLOWS
Transaction costs for royalty agreements	$ 5,865

Represents transaction costs to conclude royalty agreements.

No definition available.

Name:	imgn_TransactionCostsForRoyaltyAgreements
Namespace Prefix:	imgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration