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Accounting for Stock-based Compensation - Outstanding Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of Shares, Outstanding Ending Balance 38,227    
Number of Shares, Exercisable at December 31, 2020 38,227    
Weighted Average Exercise Price, Exercisable at December 31, 2020 $ 31.93    
Employee Stock Option      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of Shares, Outstanding Beginning Balance 108,128 201,810 411,498
Number of Shares, Exercised (69,901) (93,682) (209,688)
Number of Shares, Granted 0 0 0
Number of Shares, Outstanding Ending Balance 38,227 108,128 201,810
Number of Shares, Vested plus expected to vest at December 31, 2020 38,227    
Number of Shares, Exercisable at December 31, 2020 38,227    
Weighted Average Exercise Price, Outstanding Beginning Balance $ 35.82 $ 33.68 $ 30.71
Weighted Average Exercise Price, Exercised 37.94 31.21 27.86
Weighted Average Exercise Price, Outstanding Ending Balance 31.93 $ 35.82 $ 33.68
Weighted Average Exercise Price, Vested plus expected to vest at December 31, 2020 31.93    
Weighted Average Exercise Price, Exercisable at December 31, 2020 $ 31.93    
Aggregate Intrinsic Value, Outstanding at December 31,2020 $ 1,620,688    
Aggregate Intrinsic Value, Vested plus expected to vest at December 31, 2020 1,620,688    
Aggregate Intrinsic Value, Exercisable at December 31, 2020 $ 1,620,688