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Share-Based Payments (Performance Share Awards (PSAs) Activity) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Accelerated vesting of outstanding stock options $ 0  
PSUs    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares, Issued 182,983 63.14
RSUs    
Nonvested, Beginning Balance (in shares) 408,742  
Granted (in shares) 109,574  
Vested (in shares) (110,158)  
Reinvested dividend equivalents (in shares) 2,579  
Forfeited (in shares) (19,896)  
Nonvested, Ending Balance (in shares) 390,841 408,742
Weighted Average Grant Date Fair Value Per Share    
Nonvested, Beginning Balance (in dollars per share) $ 61.53  
Granted (in dollars per share) 100.34  
Vested (in dollars per share) 63.01  
Reinvested dividend equivalents (in dollars per share) 68.41  
Forfeited (in dollars per share) 63.58  
Nonvested, Ending Balance (in dollars per share) $ 71.93 $ 61.53