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Related Party Transactions (Summary of Significant Related Party Transactions) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Significant transactions with related parties      
Operating revenues and other income $ 2,174 $ 2,452 $ 6,514
Purchases 8,109 8,142 15,647
Operating expenses and selling, general and administrative expenses $ 125 $ 129 $ 133