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Stock award plan - Restricted Stock (Details) - Unvested restricted stock
 3 Months Ended
Mar. 31, 2022
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 1,519,831
Granted (in shares) | shares 1,490,190
Vested (in shares) | shares (426,470)
Forfeited (in shares) | shares (68,570)
Balance at the end of the period (in shares) | shares 2,514,981
Weighted Average Grant Date Fair Value  
Balance at the beginning of the period (in dollars per share) | $ / shares $ 55.43
Granted (in dollars per share) | $ / shares 38.10
Vested (in dollars per share) | $ / shares 51.26
Forfeited (in dollars per share) | $ / shares 48.45
Balance at the end of the period (in dollars per share) | $ / shares $ 46.06