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Stock-Based Compensation - Stock Option Exercises (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Net cash proceeds from the exercise of stock options $ 75 $ 88 $ 86
Total intrinsic value of stock option exercises 118 95 94
Income tax benefit realized from stock option exercises $ 64 $ 41 $ 49