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Stock-based compensation - Summary of Activities for Restricted Stock Awards ("RSAs") and Restricted Stock Units ("RSUs") (Details) - Restricted Stock Awards and Restricted Stock Units
6 Months Ended
Jun. 30, 2018
$ / shares
shares
Shares of restricted stock outstanding  
Beginning balance (in shares) | shares 1,248,824
Granted (in shares) | shares 1,129,355
Vested (in shares) | shares (221,181)
Canceled or forfeited (in shares) | shares (60,564)
Ending balance (in shares) | shares 2,096,434
Weighted-average grant date fair value  
Beginning balance (in USD per share) | $ / shares $ 23.37
Granted (in USD per share) | $ / shares 18.49
Vested (in USD per share) | $ / shares 26.19
Canceled or forfeited (in USD per share) | $ / shares 21.09
Ending balance (in USD per share) | $ / shares $ 20.50