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Stock award plan - Restricted Stock Activity (Details) - Unvested restricted stock
12 Months Ended
Dec. 31, 2020
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 642,419
Grants in period (in shares) | shares 663,045
Vested (in shares) | shares (232,975)
Forfeited (in shares) | shares (90,431)
Balance at the end of the period (in shares) | shares 982,058
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 51.32
Vested (in dollars per share) | $ / shares 23.87
Forfeited (in dollars per share) | $ / shares 36.44
Balance at the end of the period (in dollars per share) | $ / shares $ 41.78