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Stock award plan - Restricted Stock Activity (Details) - Unvested restricted stock
12 Months Ended
Dec. 31, 2019
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 571,479
Granted (in shares) | shares 306,549
Vested (in shares) | shares (181,327)
Forfeited (in shares) | shares (54,282)
Balance at the end of the period (in shares) | shares 642,419
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 32.97
Vested (in dollars per share) | $ / shares 17.56
Forfeited (in dollars per share) | $ / shares 23.14
Balance at the end of the period (in dollars per share) | $ / shares $ 24.50