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Stock-based compensation - Summary of Activity for Options Vest Solely Based upon Satisfaction of Service Condition (Details) - Service-based Vesting Stock Options
$ / shares in Units, $ in Thousands
 6 Months Ended
Jun. 30, 2018
USD ($)
$ / shares
shares
Options outstanding  
Beginning balance (in shares) | shares 2,597,294
Granted (in shares) | shares 585,250
Exercised (in shares) | shares (122,090)
Canceled or forfeited (in shares | shares (115,934)
Ending balance (in shares) | shares 2,944,520
Options outstanding, Exercisable (in shares) | shares 1,237,480
Weighted-average exercise price  
Beginning balance (in USD per share) | $ / shares $ 10.66
Granted (in USD per share) | $ / shares 18.43
Exercised (in USD per share) | $ / shares 3.38
Canceled or forfeited (in USD per share) | $ / shares 7.63
Ending balance (in USD per share) | $ / shares 12.62
Weighted-average exercise price, Exercisable (in USD per share) | $ / shares $ 7.26
Weighted-average remaining contractual life (in years), Outstanding 8 years
Weighted-average remaining contractual life (in years), Exercisable 7 years 2 months
Aggregate intrinsic values, Outstanding | $ $ 13,101
Aggregate intrinsic values, Exercisable | $ $ 10,755