Investments in Unconsolidated Entities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Equity method investments, combined income statements
Revenues	$ 1,610	$ 1,682
Operating expenses	1,212	1,230
Operating income	398	452
Other expense, net		(4)
Net income	$ 398	$ 448