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Stock award plan - Restricted Stock (Details) - Unvested restricted stock
6 Months Ended
Jun. 30, 2019
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 571,479
Granted (in shares) | shares 304,549
Vested (in shares) | shares (179,577)
Forfeited (in shares) | shares (31,449)
Balance at the end of the period (in shares) | shares 665,002
Weighted Average Grant Date Fair Value  
Balance at the beginning of the period (in dollars per share) | $ / shares $ 17.61
Granted (in dollars per share) | $ / shares 32.94
Vested (in dollars per share) | $ / shares 17.54
Forfeited (in dollars per share) | $ / shares 21.47
Balance at the end of the period (in dollars per share) | $ / shares $ 24.45