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Share-Based Payments (Performance Share Awards (PSAs) 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]      
Accelerated vesting of outstanding stock options $ 0 $ 1 $ 2
PSUs      
Shares      
Nonvested, Beginning Balance (in shares) 289,889    
Granted (in shares) 136,964    
Vested (in shares) (8,434)    
Reinvested dividend equivalents (in shares) 2,812    
Forfeited (in shares) (12,489)    
Nonvested, Ending Balance (in shares) 408,742 289,889  
Weighted Average Grant Date Fair Value Per Share      
Nonvested, Beginning Balance (in dollars per share) $ 55.29    
Granted (in dollars per share) 74.28    
Vested (in dollars per share) 58.15    
Reinvested dividend equivalents (in dollars per share) 60.26    
Forfeited (in dollars per share) 58.53    
Nonvested, Ending Balance (in dollars per share) $ 61.53 $ 55.29