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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Restricted Stock Units, Stock Grants, and Stock Units      
Nonvested shares      
Balance at the beginning of the period (in shares) 291,288    
Granted (in shares) 132,997    
Vested (in shares) (147,938)    
Forfeited (in shares) (5,356)    
Balance at the end of the period (in shares) 270,991 291,288  
Weighted-Average Grant-Date Fair Value      
Balance at the beginning of the period (in dollars per share) $ 69.78    
Granted (in dollars per share) 77.51 $ 72.60 $ 67.34
Vested (in dollars per share) 67.12    
Forfeited (in dollars per share) 73.42    
Balance at the end of the period (in dollars per share) $ 74.39 $ 69.78  
Vested Awards Outstanding at December 31, 2017 (in shares) 73,144    
Vested Awards Outstanding at December 31, 2017 (in dollars per share)    
Number of nonvested awards to be settled in cash (in shares) 148,131    
Performance Shares      
Nonvested shares      
Balance at the beginning of the period (in shares) 309,502    
Granted (in shares) 171,708    
Vested (in shares) (159,284)    
Forfeited (in shares) (9,542)    
Balance at the end of the period (in shares) 312,384 309,502  
Weighted-Average Grant-Date Fair Value      
Balance at the beginning of the period (in dollars per share) $ 72.46    
Granted (in dollars per share) 76.56 $ 78.99 $ 66.60
Vested (in dollars per share) 66.61    
Forfeited (in dollars per share) 73.34    
Balance at the end of the period (in dollars per share) $ 77.67 $ 72.46  
Vested Awards Outstanding at December 31, 2017 (in shares) 159,284    
Vested Awards Outstanding at December 31, 2017 (in dollars per share)