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Stock-based compensation - Narratives (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended
Jan. 01, 2024
Mar. 31, 2024
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award          
Issuance price (in usd per share)   $ 5.68 $ 5.68    
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)   20,266,875 20,266,875   17,104,505
Share-based compensation arrangement by share-based payment award, options, outstanding, weighted average exercise price (in usd per share)   $ 3.31 $ 3.31   $ 3.03
2021 Plan          
Share-based Compensation Arrangement by Share-based Payment Award          
Additional shares authorized for issuance (in shares) 4,654,384        
Shares available for grant (in shares)   3,667,088 3,667,088    
2023 Inducement Plan          
Share-based Compensation Arrangement by Share-based Payment Award          
Shares available for employee (in shares)   2,500,000 2,500,000    
Stock options | 2020 Plan          
Share-based Compensation Arrangement by Share-based Payment Award          
Weighted average grant date fair value of shares granted (in usd per share)     $ 3.15 $ 1.46  
Share based compensation expense not yet recognized, options   $ 29.6 $ 29.6    
Share based compensation expense not yet recognized, recognition period (in years)     2 years 7 months 6 days    
Stock options | 2020 Plan | Tranche One          
Share-based Compensation Arrangement by Share-based Payment Award          
Vesting percentage (as a percent)     25.00%    
Stock options | 2020 Plan | Tranche Two          
Share-based Compensation Arrangement by Share-based Payment Award          
Vesting percentage (as a percent)     25.00%    
Stock options | 2020 Plan | Tranche Three          
Share-based Compensation Arrangement by Share-based Payment Award          
Vesting percentage (as a percent)     25.00%    
Stock options | 2020 Plan | Tranche Four          
Share-based Compensation Arrangement by Share-based Payment Award          
Vesting percentage (as a percent)     25.00%    
Unvested restricted stock          
Share-based Compensation Arrangement by Share-based Payment Award          
Shares outstanding (in shares)   268,872 268,872   374,208
Unvested restricted stock | 2020 Plan          
Share-based Compensation Arrangement by Share-based Payment Award          
Share based compensation expense not yet recognized, recognition period (in years)     10 months 24 days    
Share based compensation expense not yet recognized other than options   $ 0.6 $ 0.6    
Restricted stock units          
Share-based Compensation Arrangement by Share-based Payment Award          
Weighted average grant date fair value of shares granted (in usd per share)     $ 3.62    
Share based compensation expense not yet recognized, recognition period (in years)     2 years    
Share based compensation expense not yet recognized other than options   $ 11.7 $ 11.7    
Fair value of vested options     $ 0.0    
Granted (in shares)     2,426,855    
Shares outstanding (in shares)   4,548,955 4,548,955   2,198,334
Restricted stock units | Minimum          
Share-based Compensation Arrangement by Share-based Payment Award          
Vesting period (in years)     1 year    
Restricted stock units | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award          
Vesting period (in years)     4 years    
Restricted Stock Award with Market Conditions          
Share-based Compensation Arrangement by Share-based Payment Award          
Share based compensation expense not yet recognized, recognition period (in years)     1 year 3 months 18 days    
Granted (in shares)   1,500,000      
Grant-date fair value     $ 5.5    
Volatility rate   97.00%      
Risk-free interest rate   4.50%      
SARs | 2020 Plan          
Share-based Compensation Arrangement by Share-based Payment Award          
Shares outstanding (in shares)   394,736 394,736    
Intrinsic value, shares outstanding   $ 2.2 $ 2.2    
SARs | 2020 And 2021 Plan          
Share-based Compensation Arrangement by Share-based Payment Award          
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)   197,150 197,150    
Share-based compensation arrangement by share-based payment award, options, outstanding, weighted average exercise price (in usd per share)   $ 4.65 $ 4.65    
Deferred compensation expense   $ 0.1 $ 0.1