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Stock-Based Employee Compensation (Narrative) (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
plan
$ / shares
shares
Dec. 31, 2017
$ / shares
shares
Dec. 31, 2016
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of stock-based compensation plans | plan 1    
Maximum number of award to be granted per individual (in shares) 500,000    
Maximum aggregate number of shares available under the employee stock purchase plan (in shares) 1,300,000    
Purchase price for each share of common stock as percentage of the average of the market price 85.00%    
Shares of common stock issued under the ESPP plan (in shares) 74,100 51,989 42,347
Weighted-average price of shares of common stock issued under the ESPP plan (in dollars per share) | $ / shares $ 97.50 $ 93.15 $ 65.48
Director      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum value of awards that can be granted to participant in any fiscal year | $ $ 500,000    
Stock Option | Lower Limit      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 4 years    
Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Maximum actual number of shares underlying each performance share award as a percentage of target performance shares 200.00%    
Number of target performance shares issued (in shares) 184,550    
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Maximum actual number of shares underlying each performance share award as a percentage of target performance shares 200.00%    
Number of target performance shares issued (in shares) 120,022    
2008 Equity Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum number of shares authorized for issuance under stock-based compensation plans (in shares) 14,000,000