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Accounting for Stock-based Compensation - Summary of Performance Shares Activity (Details) - Performance Shares - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Number of Shares        
Number of Shares, Non-vested Beginning Balance 148,414 169,486 187,022  
Number of Shares, Granted 87,314 85,928 45,136 88,038
Number of Shares, Vested (88,038) (107,000) (30,576)  
Number of Shares, Cancelled (5,569)   (32,096)  
Number of Shares, Non-vested Ending Balance 142,121 148,414 169,486 187,022
Number of Shares, expected to vest in the future 94,178      
Weighted-Average Grant Date Fair Value        
Weighted-Average Grant Date Fair Value, Non-vested Beginning Balance $ 60.67 $ 45.15 $ 39.95  
Weighted-Average Grant Date Fair Value, Granted 51.44 62.07 65.05  
Weighted-Average Grant Date Fair Value, Vested 38.81 37.21 44.21  
Weighted-Average Grant Date Fair Value, Cancelled 69.66   43.72  
Weighted-Average Grant Date Fair Value, Non-vested Ending Balance 68.19 $ 60.67 $ 45.15 $ 39.95
Weighted-Average Grant Date Fair Value, expected to vest in the future $ 69.76      
Aggregate Intrinsic Value        
Aggregate Intrinsic Value, Non-vested $ 10,563,854      
Aggregate Intrinsic Value, expected to vest in the future $ 7,000,251