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Income taxes - Tax Rate Reconciliation (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reconciliation of the U.S. statutory income tax rate to the entity's effective tax rate      
Federal income tax provision at statutory rate 21.00% 21.00% 21.00%
State income tax provision, net of federal benefit (3.31%) 1.08% 0.05%
Permanent differences (6.66%) (6.17%) (6.41%)
Research and development 4.93% 4.38% 6.49%
Change in valuation allowances (26.40%) (35.49%) 2.20%
Change in deferred tax assets 0.0293 0.1589 (0.1422)
Foreign tax rate differential 0.72% (1.88%) (9.10%)
Tax rate change (1.46%) (3.67%)  
(Accrual) release of uncertain tax positions (0.61%)    
Other 0.12%   0.01%
Effective income tax rate (8.74%) (4.86%) 0.02%