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Income taxes - Income Taxes Rate Reconciliation (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reconciliation of effect of applying the federal statutory rate to the net loss and the effective income tax rate      
Statutory federal income tax rate 21.00% 21.00% 21.00%
Earnings in jurisdictions taxed at rates different from the statutory U.S. federal tax rate 0.20% 0.40% 1.00%
Permanent differences 1.70% 3.70% 0.40%
Changes in valuation allowance (22.50%) (25.10%) (16.70%)
Rate change due to Swedish tax reform     (5.70%)
Other 0.40%