Income Taxes (Details)	6 Months Ended
Jun. 30, 2022
Tax Cuts and Jobs Act (TCJA)
Capitalize research and development expenses, amortization period	5 years
Capitalize research and experimentation expenses, amortization period	15 years

Amortization period for capitalize research and experimentation expenses.

No definition available.

Name:	imgn_CapitalizeResearchAndExperimentationExpensesAmortizationPeriod
Namespace Prefix:	imgn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration