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Debt (Estimated Fair Value Of Significant Financial Instruments) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Estimate of Fair Value Measurement |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Short-term debt | $ 264 | $ 313
Long-term debt | 5,444 | 4,625
Reported Value Measurement |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Short-term debt | 262 | 312
Long-term debt | $ 5,225 | $ 4,495

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