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Stock-based compensation - Summary of Restricted Stock Unit Activity and Related Information (Details) - RSAs and RSUs
12 Months Ended
Dec. 31, 2018
$ / shares
shares
Shares of restricted stock outstanding  
Beginning balance (in shares) | shares 1,248,824
Granted (in shares) | shares 1,331,745
Vested (in shares) | shares (376,130)
Canceled forfeited (in shares) | shares (168,315)
Ending balance (in shares) | shares 2,036,124
Weighted-average grant date fair value  
Beginning balance (in USD per share) | $ / shares $ 23.37
Granted (in USD per share) | $ / shares 17.73
Vested (in USD per share) | $ / shares 22.96
Canceled forfeited (in USD per share) | $ / shares 20.24
Ending balance (in USD per share) | $ / shares $ 20.01