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Investment in Unconsolidated Entities (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Equity method investments, combined income statements    
Revenues $ 1,615 $ 1,688
Operating expenses 1,217 1,236
Operating income 398 452
Other expense, net   (4)
Net income $ 398 $ 448