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Stock award plan - Restricted Stock (Details) - Unvested restricted stock
 12 Months Ended
Dec. 31, 2021
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 982,058
Granted (in shares) | shares 1,018,228
Vested (in shares) | shares (335,114)
Forfeited (in shares) | shares (145,341)
Balance at the end of the period (in shares) | shares 1,519,831
Weighted Average Grant Date Fair Value  
Balance at the beginning of the period (in dollars per share) | $ / shares $ 41.78
Granted (in dollars per share) | $ / shares 62.54
Vested (in dollars per share) | $ / shares 36.09
Forfeited (in dollars per share) | $ / shares 57.71
Balance at the end of the period (in dollars per share) | $ / shares $ 55.43