Stock-Based Compensation - Stock Option Exercises (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Net cash proceeds from the exercise of stock options	$ 34	$ 75	$ 88
Total intrinsic value of stock option exercises	77	118	95
Income tax benefit realized from stock option exercises	$ 27	$ 64	$ 41