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Income Taxes - Reconciliation of provision (benefit) for income taxes (Details)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Income Taxes      
Benefit for income taxes at statutory rate 19.00% 19.00% 19.00%
(Decreases) increases resulting from:      
Federal tax credits 0.60% 0.90% 1.30%
Change in valuation allowance (27.00%) (15.40%) (8.00%)
Net losses surrendered for research credit (5.90%) (6.20%) (5.30%)
Preferred share warrants     (3.30%)
Impact of statutory rate change 12.10% 1.80%  
Other 3.60% 1.30% (2.90%)
Effective income tax rate 2.40% 1.40% 0.80%