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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
3 Months Ended
Mar. 31, 2018
$ / shares
shares
Shares of restricted stock outstanding  
Beginning balance (in shares) | shares 1,248,824
Granted (in shares) | shares 1,012,990
Vested (in shares) | shares (172,065)
Forfeited (in shares) | shares (29,320)
Ending balance (in shares) | shares 2,060,429
Weighted-average grant date fair value  
Beginning balance (in USD per share) | $ / shares $ 23.37
Granted (in USD per share) | $ / shares 18.43
Vested (in USD per share) | $ / shares 26.80
Forfeited (in USD per share) | $ / shares 22.76
Ending balance (in USD per share) | $ / shares $ 20.66