Income Taxes (Details)	3 Months Ended
Mar. 31, 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

Amorization 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