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Summary of Significant Accounting Policies - Stock-Based Compensation (Details)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2019
plan
shares
Mar. 31, 2023
USD ($)
plan
$ / shares
shares
Dec. 31, 2022
USD ($)
$ / shares
shares
Sep. 30, 2022
USD ($)
Jun. 30, 2022
USD ($)
Mar. 31, 2022
USD ($)
$ / shares
Dec. 31, 2022
$ / 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%                  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]                      
Directors' deferred share unit compensation | $   $ 151 $ 146 $ 146 $ 213 $ 211          
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                      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]                      
Stock compensation expense | $   6,900       $ 4,200          
Estimated fair value that could be expensed | $   $ 72,400                  
Stock options                      
Stock-Based Compensation                      
Common stock authorized for issuance (in shares)   0                  
Weighted-average assumptions used to estimate the fair value of each stock option                      
Dividend (as a percent)   0.00%       0.00%          
Volatility (as a percent)   82.30%       83.00%          
Risk-free interest rate (as a percent)   3.65%       1.81%          
Expected life   5 years 7 months 6 days       6 years          
Weighted-average grant date fair value (in dollars per share) | $ / shares   $ 3.26       $ 3.78          
Number of Stock Options                      
Outstanding at the beginning of the period (in shares)   33,126,000                  
Granted (in shares)   4,757,000                  
Exercised (in shares)   (16,000)                  
Forfeited/Canceled (in shares)   (763,000)                  
Outstanding at the end of the period (in shares)   37,104,000 33,126,000       33,126,000        
Weighted-Average Exercise Price                      
Outstanding at the beginning of the period (in dollars per share) | $ / shares   $ 5.76                  
Granted (in dollars per share) | $ / shares   4.62                  
Exercised (in dollars per share) | $ / shares   2.31                  
Forfeited/Canceled (in dollars per share) | $ / shares   5.60                  
Outstanding at the end of the period (in dollars per share) | $ / shares   $ 5.61 $ 5.76       $ 5.76        
Performance shares                      
Stock-Based Compensation                      
Vesting percentage             75.00%        
Forfeited percentage             12.50%        
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]                      
Estimated fair value that could be expensed | $   $ 1,300                  
Restricted stock                      
Number of Restricted Stock Shares                      
Unvested at the beginning of the period (in shares)   138,000                  
Granted (in shares)   1,824,000                  
Forfeited (in shares)   (2,000)                  
Unvested at the end of the period (in shares)   1,960,000 138,000       138,000        
Weighted-Average Grant Date Fair Value                      
Unvested at the beginning of the period (in dollars per share) | $ / shares   $ 5.45                  
Granted (in dollars per share) | $ / shares   4.65                  
Forfeited (in dollars per share) | $ / shares   4.66                  
Unvested at the end of the period (in dollars per share) | $ / shares   $ 4.71 $ 5.45       $ 5.45        
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) 13,500,000                    
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