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Stock-Based Compensation - Status of Nonvested Restricted Stock, Stock Grants, Stock Units and Performance Shares (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Restricted Stock Units, Stock Grants, and Stock Units      
Nonvested shares      
Balance at the beginning of the period (in shares) 242,612    
Granted (in shares) 118,403    
Vested (in shares) (136,893)    
Forfeited (in shares) (3,565)    
Balance at the end of the period (in shares) 220,557 242,612  
Weighted-Average Grant-Date Fair Value      
Balance at the beginning of the period (in dollars per share) $ 81.38    
Granted (in dollars per share) 71.70 $ 89.15 $ 77.51
Vested (in dollars per share) 73.80    
Forfeited (in dollars per share) 82.61    
Balance at the end of the period (in dollars per share) $ 77.93 $ 81.38  
Vested awards outstanding at end of year (in shares) 82,921    
Vested awards outstanding at end of year (in dollars per share)    
Number of nonvested awards to be settled in cash (in shares) 126,996    
Performance Shares      
Nonvested shares      
Balance at the beginning of the period (in shares) 306,970    
Granted (in shares) 122,830    
Vested (in shares) (161,906)    
Forfeited (in shares) (7,890)    
Balance at the end of the period (in shares) 260,004 306,970  
Weighted-Average Grant-Date Fair Value      
Balance at the beginning of the period (in dollars per share) $ 83.65    
Granted (in dollars per share) 104.74 $ 92.16 $ 76.56
Vested (in dollars per share) 76.53    
Forfeited (in dollars per share) 85.06    
Balance at the end of the period (in dollars per share) $ 98.28 $ 83.65  
Vested awards outstanding at end of year (in shares) 161,906    
Vested awards outstanding at end of year (in dollars per share)