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Stock award plan - Restricted Stock (Details) - Unvested restricted stock
6 Months Ended
Jun. 30, 2020
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 642,419
Granted (in shares) | shares 589,070
Vested (in shares) | shares (229,076)
Forfeited (in shares) | shares (65,491)
Balance at the end of the period (in shares) | shares 936,922
Weighted Average Grant Date Fair Value  
Balance at the beginning of the period (in dollars per share) | $ / shares $ 24.50
Granted (in dollars per share) | $ / shares 49.67
Vested (in dollars per share) | $ / shares 23.53
Forfeited (in dollars per share) | $ / shares 36.11
Balance at the end of the period (in dollars per share) | $ / shares $ 39.97