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Income taxes - Schedule of effective income tax rate reconciliation (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Percent [Abstract]      
Federal income tax expense at statutory rate 35.00% 35.00% 35.00%
State income tax, net of federal benefit 5.00% 5.00% 3.40%
Permanent differences (0.70%) 0.00% (0.20%)
Stock-based compensation (1.90%) (2.60%) (6.30%)
Research credits 2.20% 1.90% 1.80%
Other, net (1.30%) (0.10%) 0.40%
Payroll Tax Credit Election (0.70%) 0.00% 0.00%
Change in valuation allowance (1.10%) (39.20%) (34.10%)
Deferred rate change (36.50%) 0.00% 0.00%
Effective tax rate 0.00% 0.00% 0.00%