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Employee Benefit Plans (Tables)
 12 Months Ended
Dec. 31, 2021
Employee Benefit Plans [Abstract]  
Schedule of Share-based Compensation, Employee Stock Purchase Plan, Activity [Table Text Block]
The following table provides information about the Company’s ESPP activity during 2021, 2020, and 2019:
Total number of shares purchased in yearAverage price paid per share
Weighted-average fair value of discount under the ESPP (1)
As of December 31, 201938,464 $133.54 $23.51 
As of December 31, 202056,866 $90.63 $14.10 
As of December 31, 202139,760 $174.68 $30.00 
(1) The weighted-average fair value of the discount under the ESPP granted is equal to a percentage discount from the market value of the common stock at the end of each semi-annual purchase period. 15 percent is the maximum allowable discount under the ESPP.
Schedule of Nonvested Restricted Stock Units Activity A summary of the status of non-vested restricted stock grants during the years ended December 31, 2021, 2020 and 2019 is presented below:
SharesWeighted Average Grant Date Fair Value Per Share
Non-vested at December 31, 2018196,194 $153.88 
Granted218,477 143.72 
Vested(104,816)152.07 
Forfeited(15,047)148.97 
Non-vested at December 31, 2019294,808 $147.25 
Granted267,169 137.80 
Vested(291,303)147.58 
Forfeited(5,147)145.82 
Non-vested at December 31, 2020265,527 $142.25 
Granted120,456 194.66 
Vested(197,530)136.71 
Forfeited(6,900)147.05 
Non-vested at December 31, 2021181,553 $183.63