Segment Information - Foreign and U.S. Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 9,967	$ 9,523	$ 29,962	$ 28,766