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Stock-based compensation - Summary of Additional Information Relating to Service-based Options (Details) - Service-based Stock Options - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation expense $ 3,219 $ 2,435 $ 4,286
Intrinsic value of options exercised $ 2,890 $ 12,841 $ 2,486
Weighted-average grant date fair value of options granted (in USD per share) $ 6.81 $ 9.51 $ 5.07