Basis of Presentation and Significant Accounting Policies - Stock-Based Compensation (Details) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	May 31, 2022 shares	Jun. 30, 2022 USD ($) $ / shares shares	Jun. 30, 2021 USD ($) $ / shares	Jun. 30, 2022 USD ($) plan $ / shares shares	Jun. 30, 2021 USD ($) $ / shares	Dec. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2020 shares	Jun. 15, 2022 shares	Apr. 01, 2022 shares	Jun. 30, 2018 shares
Stock-Based Compensation
Number of employee share-based compensation plans | plan				3
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)		0.00%	0.00%	0.00%	0.00%
ESPP
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Aggregate number of common shares reserved for future issuance										2,000,000
Stock options and restricted stock awards
Stock-Based Compensation
Vesting period				3 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Stock compensation expense | $		$ 4.8	$ 3.6	$ 9.0	$ 7.3
Estimated fair value that could be expensed | $		$ 50.1		$ 50.1
Stock options
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)		0.00%	0.00%	0.00%	0.00%
Volatility (as a percent)		83.40%	84.70%	83.10%	85.30%
Risk-free interest rate (as a percent)		3.00%	1.01%	2.18%	0.67%
Expected life		6 years	6 years	6 years	6 years
Weighted-average grant date fair value (in dollars per share) | $ / shares		$ 3.12	$ 4.92	$ 3.57	$ 5.40
Number of Stock Options
Outstanding at the beginning of the period (in shares)				21,219,000
Granted (in shares)				8,670,000
Exercised (in shares)				(173,000)
Forfeited/Canceled (in shares)				(263,000)
Outstanding at the end of the period (in shares)		29,453,000		29,453,000		21,219,000
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares				$ 6.28
Granted (in dollars per share) | $ / shares				5.03
Exercised (in dollars per share) | $ / shares				3.59
Forfeited/Canceled (in dollars per share) | $ / shares				6.72
Outstanding at the end of the period (in dollars per share) | $ / shares		$ 5.92		$ 5.92		$ 6.28
Performance shares
Stock-Based Compensation
Vesting percentage	25.00%
Number of Stock Options
Granted (in shares)	2,600,000						2,600,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Stock compensation expense | $						$ 2.6
Estimated fair value that could be expensed | $		$ 7.8		$ 7.8
Restricted stock
Number of Restricted Stock Shares
Unvested at the beginning of the period (in shares)				77,000
Vested (in shares)				(2,000)
Unvested at the end of the period (in shares)		75,000		75,000		77,000
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares				$ 5.59
Vested (in dollars per share) | $ / shares				2.53
Unvested at the end of the period (in dollars per share) | $ / shares		$ 5.68		$ 5.68		$ 5.59
Immunogen Inc Restated Stock Option Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)									28,742,013
2018 Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)								13,000,000
Inducement Plan
Stock-Based Compensation
Number of employee share-based compensation plans | plan				2
Common stock authorized for issuance (in shares)		10,500,000		10,500,000
2018 Plan and Inducement Plan
Stock-Based Compensation
Exercise period				10 years
2018 Plan and Inducement Plan | Maximum
Stock-Based Compensation
Vesting period				4 years