Investment in Unconsolidated Entities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Equity method investments, combined income statements
Revenues	$ 1,679	$ 1,738	$ 5,008	$ 5,199
Operating expenses	1,254	1,267	3,662	3,840
Operating income	425	471	1,346	1,359
Other income (expense), net	(2)	(11)	(10)	(17)
Net income	$ 423	$ 460	$ 1,336	$ 1,342