Summary of Significant Accounting Policies - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Interest paid	$ 147	$ 150	$ 144
Income taxes paid	408	306	336
Treasury stock purchases settled after the balance sheet date	$ 10	$ 15	$ 19