Summary of Significant Accounting Policies - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Interest paid	$ 160	$ 147	$ 150
Income taxes paid	409	408	306
Treasury stock purchases settled after the balance sheet date	$ 5	$ 10	$ 15