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Fair Value Disclosures (Estimated Fair Values and Carrying Amounts of Assets and Liabilities) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Cash and cash equivalents, carrying value $ 277 $ 314 $ 568
Cash and cash equivalents, fair value 277 314  
Restricted cash, carrying value 136 126  
Restricted cash, fair value 136 126  
Debt and other obligations, carrying amount 16,682 16,159  
Debt and other obligations, fair value $ 16,562 $ 16,644