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Summary of Significant Accounting Policies Operations and Maintenance (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Operating Expense [Member] |  |  |
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items] |  |  |
Results of Operations, Expense from Oil and Gas Producing Activities | $ 342 | $ 349 | $ 366

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- Definition The amount of expenses related to oil and gas producing activities. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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