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Fair Value Disclosures (Estimated Fair Values and Carrying Amounts of Assets and Liabilities) (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash and cash equivalents, at carrying value $ 323 $ 314
Cash and cash equivalents, fair value 323 314
Restricted cash, current and non-current, carrying value 130 126
Restricted cash, current and non-current, fair value 130 126
Long-term debt and other obligations, carrying amount 16,424 16,159
Long-term debt and other obligations, fair value $ 16,362 $ 16,644