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Share-Based Compensation - Additional Information (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 42 $ 48  
Deferred compensation costs related to unvested awards $ 318   $ 168
RSUs and PSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock awards, weighted average grant-date fair value (in USD per share) $ 133    
RSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock awards, granted (in shares) 1.3    
Stock awards, vesting period 4 years    
Stock awards, period of service after grant date 1 year    
PSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock awards, granted (in shares) 0.1    
Stock awards, vesting period 3 years