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Stock award plan - Restricted Stock (Details) - Restricted stock
 9 Months Ended
Sep. 30, 2021
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 982,058
Granted (in shares) | shares 994,603
Vested (in shares) | shares (326,519)
Forfeited (in shares) | shares (113,877)
Balance at the end of the period (in shares) | shares 1,536,265
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 63.15
Vested (in dollars per share) | $ / shares 35.49
Forfeited (in dollars per share) | $ / shares 57.95
Balance at the end of the period (in dollars per share) | $ / shares $ 55.76