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Stock-Based Employee Compensation (Other Equity) (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
Dec. 31, 2014
shares
Restricted Stock Units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based awards conversion ratio 1      
Number of Awards        
Non-vested share units, beginning balance (Share Units) | shares 748,516      
Granted (Share Units) | shares 425,170      
Vested (Share Units) | shares (376,992)      
Canceled (Share Units) | shares (58,795)      
Non-vested share units, ending balance (Share Units) | shares 737,899 748,516    
Weighted-Average Grant Date Fair Value        
Non-vested share units, beginning balance (Price per Share) $ 61.95      
Granted (Price per Share) 99.03 $ 64.51 $ 73.98  
Vested (Price per Share) 59.62      
Canceled (Price per Share) 71.47      
Non-vested share units expected to vest, ending balance (Price per Share) 83.78 61.95    
Weighted-average estimated fair value of restricted stock units granted (Price per Share) $ 99.03 $ 64.51 $ 73.98  
Fair value of shares released on vesting of restricted stock units | $ $ 38.7 $ 23.2 $ 27.6  
Total unrecognized compensation cost | $ $ 24.2      
Weighted-average period of unrecognized compensation cost to be recognized (Years) 1 year 8 months 23 days      
Performance Shares        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based awards conversion ratio 1      
Number of Awards        
Non-vested share units, beginning balance (Share Units) | shares 342,152      
Granted (Share Units) | shares 140,542     63,771
Vested (Share Units) | shares (105,615)      
Canceled (Share Units) | shares (23,929)      
Non-vested share units, ending balance (Share Units) | shares 353,150 342,152    
Weighted-Average Grant Date Fair Value        
Non-vested share units, beginning balance (Price per Share) $ 61.78      
Granted (Price per Share) 84.16 $ 65.83 $ 71.36  
Vested (Price per Share) 45.62      
Canceled (Price per Share) 71.37      
Non-vested share units expected to vest, ending balance (Price per Share) 74.87 61.78    
Weighted-average estimated fair value of restricted stock units granted (Price per Share) $ 84.16 $ 65.83 $ 71.36  
Fair value of shares released on vesting of restricted stock units | $ $ 10.0 $ 16.9 $ 18.3  
Total unrecognized compensation cost | $ $ 11.9      
Weighted-average period of unrecognized compensation cost to be recognized (Years) 1 year 29 days      
Maximum actual number of shares underlying each performance share award as a percentage of target performance shares 200.00%      
Restricted Stock        
Number of Awards        
Granted (Share Units) | shares 137,948      
Weighted-Average Grant Date Fair Value        
Maximum actual number of shares underlying each performance share award as a percentage of target performance shares 200.00%      
Senior Officers | Restricted Stock        
Number of Awards        
Non-vested share units, beginning balance (Share Units) | shares 132,228      
Granted (Share Units) | shares 137,948      
Vested (Share Units) | shares 0      
Canceled (Share Units) | shares 0      
Non-vested share units, ending balance (Share Units) | shares 270,176 132,228    
Weighted-Average Grant Date Fair Value        
Non-vested share units, beginning balance (Price per Share) $ 66.93      
Granted (Price per Share) 95.04 $ 66.93    
Vested (Price per Share) 0.00      
Canceled (Price per Share) 0.00      
Non-vested share units expected to vest, ending balance (Price per Share) 81.28 66.93    
Weighted-average estimated fair value of restricted stock units granted (Price per Share) $ 95.04 $ 66.93    
Total unrecognized compensation cost | $ $ 2.0      
Weighted-average period of unrecognized compensation cost to be recognized (Years) 1 year 7 months 13 days