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Debt (Estimated Fair Value of Significant Financial Instruments) (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term debt $ 13 $ 312
Long-term debt 5,475 4,495
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term debt 13 313
Long-term debt $ 5,720 $ 4,625