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Stock-based compensation - Summary of Activity for Options Vest Based upon Performance or Market Conditions (Details) - Performance-based and Market-based Vesting Options
$ / shares in Units, $ in Thousands
6 Months Ended
Jun. 30, 2018
USD ($)
$ / shares
shares
Options outstanding  
Beginning balance (in shares) | shares 2,406,537
Exercised (in shares) | shares (37,260)
Canceled or forfeited (in shares) | shares (22,250)
Ending balance (in shares) | shares 2,347,027
Weighted-average exercise price  
Beginning balance (in USD per share) | $ / shares $ 6.58
Exercised (in USD per share) | $ / shares 2.24
Canceled or forfeited (in USD per share) | $ / shares 12.95
Ending balance (in USD per share) | $ / shares $ 6.59
Weighted-average remaining contractual life (in years), Outstanding 7 years
Aggregate intrinsic values, Outstanding | $ $ 25,117