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Stock-Based Compensation Plans (Summary Of Restricted Stock Activity) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of shares vested $ 5.6 $ 4.4 $ 2.6
Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding, Nonvested as of December 31 168,546    
Granted 8,543    
Vested (17,338)    
Forfeited (451)    
Outstanding, Nonvested as of December 31 0 168,546  
Weighted Average Grant-Date Fair Value Per Share, Non-vested as of December 31 $ 111.91    
Weighted Average Grant-Date Fair Value Per Share, Nonvested, Granted 200.79 $ 196.96 $ 183.30
Weighted Average Grant-Date Fair Value Per Share, Nonvested, Vested 127.83    
Weighted Average Grant-Date Fair Value Per Share, Nonvested, Forfeited 198.81    
Weighted Average Grant-Date Fair Value Per Share, Non-vested as of December 31 $ 114.70 $ 111.91