Stock-Based Compensation - Stock Option Exercises (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Net cash proceeds from the exercise of stock options	$ 88	$ 86	$ 193
Total intrinsic value of stock option exercises	95	94	168
Income tax benefit realized from stock option exercises	$ 41	$ 49	$ 73