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Equity (Restricted Stock, Performance Share Awards and Options) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted Stock Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Activity [Roll Forward]      
Nonvested balance at beginning of year (in shares) 556,648    
Granted (in shares) 237,662    
Vested (in shares) (261,940)    
Forfeited (in shares) (21,057)    
Nonvested balance at end of year (in shares) 511,313 556,648  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]      
Nonvested balance at beginning of year, weighted-average grant date fair value (in dollars per share) $ 103.26    
Granted, weighted-average grant date fair value (in dollars per share) 130.16 $ 112.86 $ 103.58
Vested, weighted-average grant date fair value (in dollars per share) 101.31    
Forfeited, weighted-average grant date fair value (in dollars per share) 112.91    
Nonvested balance at end of year, weighted-average grant date fair value (in dollars per share) $ 116.36 $ 103.26  
Performance Share Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Activity [Roll Forward]      
Nonvested balance at beginning of year (in shares) 834,433    
Granted (in shares) 483,958    
Vested (in shares) (463,511)    
Forfeited (in shares) (46,472)    
Nonvested balance at end of year (in shares) 808,408 834,433  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]      
Nonvested balance at beginning of year, weighted-average grant date fair value (in dollars per share) $ 95.76    
Granted, weighted-average grant date fair value (in dollars per share) 107.39 $ 89.23 $ 77.12
Vested, weighted-average grant date fair value (in dollars per share) 87.24    
Forfeited, weighted-average grant date fair value (in dollars per share) 100.38    
Nonvested balance at end of year, weighted-average grant date fair value (in dollars per share) $ 110.98 $ 95.76  
Restricted Stock and Performance Share Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]      
Total fair value of awards vested $ 96 $ 99 $ 108