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Summary of Significant Accounting Policies (Narrative) (Details) - Accounting Standards Update 2017-07 [Member] - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Nonoperating Income (Expense) [Member] |  |
Organization And Basis Of Presentation [Line Items] |  |
New accounting pronouncement, quantified affect | $ 156 | $ 139
Operating Expense [Member] |  |
Organization And Basis Of Presentation [Line Items] |  |
New accounting pronouncement, quantified affect | $ 156 | $ 139

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