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Income taxes - Reconciliation of Company's Actual Tax Rate to U.S. Federal Statutory Rate (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Income tax rates      
- Statutory U.S. federal rate 21.00% 21.00% 35.00%
- State income taxes, net of federal benefit 0.00% 0.00%  
- Excess tax benefits related to stock compensation 0.00% 0.00%  
- AMT credit 1.00% 0.00%  
- Effect of 2017 Tax Act 0.00% 2.00%  
- Change in valuation allowance (25.00%) (20.00%)  
Total (3.00%) 3.00%