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Stock-based compensation - Summary of Additional Information Relating to Performance-based and Market-based Stock Options (Details) - Performance-based and Market-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 $ 1,168 $ 3,489 $ 1,813
Intrinsic value of options exercised $ 8,669 $ 42,874 $ 0
Weighted-average grant date fair value of options granted (in USD per share) $ 0.00 $ 10.65 $ 1.52