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Share-Based Compensation Plans - Summary of Information Relative to Stock Option Plan Activity (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]      
Number of Options, Outstanding, January 1 (in shares) 45,091    
Number of Options, Granted (in shares) 4,232    
Number of Options, Exercised (in shares) (11,512)    
Number of Options, Forfeited (in shares) (1,537)    
Number of Options, Outstanding, December 31 (in shares) 36,274 45,091  
Number of Options, Exercisable (in shares) 26,778    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]      
Weighted Average Exercise Price, Options Outstanding, January 1 (in dollars per share) $ 44.47    
Weighted Average Exercise Price, Granted (in dollars per share) 63.88 $ 54.63 $ 59.73
Weighted Average Exercise Price, Exercised (in dollars per share) 43.38    
Weighted Average Exercise Price, Forfeited (in dollars per share) 51.78    
Weighted Average Exercise Price, Options Outstanding, December 31 (in dollars per share) 46.77 $ 44.47  
Weighted Average Exercise Price, Exercisable (in dollars per share) $ 42.54    
Weighted Average Remaining Contractual Term, Outstanding 4 years 10 months 21 days    
Weighted Average Remaining Contractual Term, Exercisable 3 years 7 months 21 days    
Aggregate Intrinsic Value, Outstanding $ 397    
Aggregate Intrinsic Value, Exercisable $ 384