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Stock-based compensation - Additional Information Related to RSAs and RSUs (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
RSAs and RSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation expense $ 12,434 $ 7,550 $ 700
RSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Intrinsic value of RSUs released $ 6,280 $ 3,398 $ 0