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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
 3 Months Ended
Mar. 31, 2018
USD ($)
$ / shares
shares
Options outstanding  
Beginning balance (in shares) | shares 2,406,537
Exercised (in shares) | shares (23,460)
Forfeited (in shares) | shares (9,000)
Canceled (in shares) | shares (13,250)
Ending balance (in shares) | shares 2,360,827
Weighted-average exercise price  
Beginning balance (in USD per share) | $ / shares $ 6.58
Exercised (in USD per share) | $ / shares 2.16
Forfeited (in USD per share) | $ / shares 26.84
Ending balance (in USD per share) | $ / shares $ 6.56
Weighted-average remaining contractual life (in years), Outstanding 7 years 2 months 12 days
Aggregate intrinsic values, Outstanding | $ $ 33,334
Share-based Compensation Arrangements by Share-based Payment Award, Options, Expirations in Period, Weighted Average Exercise Price | $ / shares $ 3.51