Employee Stock and Savings Plans - Schedule of Additional Information Related to Stock Option and Restricted Stock Unit Activity (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]
Total intrinsic value of stock options exercised	$ 116	$ 113	$ 123
Fair value of restricted stock units vested	61	58	41
Income tax benefit from stock options exercised and restricted stock units vested	66	62	61
Cash received from stock options exercised	$ 36	$ 39	$ 35