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Fair Value Disclosures (Estimated Fair Values and Carrying Amounts of Assets and Liabilities) (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash and cash equivalents, at carrying value $ 245 $ 277
Cash and cash equivalents, fair value 245 277
Restricted cash, current and non-current, carrying value 163 136
Restricted cash, current and non-current, fair value 163 136
Long-term debt and other obligations, carrying amount 17,216 16,682
Long-term debt and other obligations, fair value $ 17,602 $ 16,562