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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Aggregate number of common shares reserved for future issuance.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Number of shares authorized for issuance under share-based payment arrangement.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Percentage of vesting of award under share-based payment arrangement.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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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	9 Months Ended		12 Months Ended
	May 31, 2022 shares	Sep. 30, 2022 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) $ / shares	Jun. 30, 2022 $ / shares shares	Sep. 30, 2022 USD ($) plan $ / shares shares	Sep. 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 \| $		$ 5.3	$ 3.3		$ 14.3	$ 10.6
Estimated fair value that could be expensed \| $		$ 54.3			$ 54.3
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)		83.30%	82.70%		83.20%	85.20%
Risk-free interest rate (as a percent)		3.44%	0.95%		2.48%	0.68%
Expected life		5 years 7 months 6 days	6 years		5 years 10 months 24 days	6 years
Weighted-average grant date fair value (in dollars per share) \| $ / shares		$ 3.62	$ 3.99		$ 3.58	$ 5.34
Number of Stock Options
Outstanding at the beginning of the period (in shares)				21,219,000	21,219,000
Granted (in shares)					11,290,000
Exercised (in shares)					(280,000)
Forfeited/Canceled (in shares)				(825,000)
Outstanding at the end of the period (in shares)		31,404,000			31,404,000		21,219,000
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) \| $ / shares				$ 6.28	$ 6.28
Granted (in dollars per share) \| $ / shares					5.05
Exercised (in dollars per share) \| $ / shares					3.82
Forfeited/Canceled (in dollars per share) \| $ / shares				$ 9.46
Outstanding at the end of the period (in dollars per share) \| $ / shares		$ 5.78			$ 5.78		$ 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	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	$ 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