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Fair Value Measurements and Credit Concentration (Net Asset and Liability Measured on Recurring Basis Level 3 Gain Loss Included in Earnings and Other Recurring Numeric Data) (Details) - Level 3 - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2016 | Mar. 31, 2015
Net gains and losses on assets and liabilities [Abstract] |  |
Total gains (losses) included in earnings | $ 60 | $ (17)
Recurring | Energy Commodities, Net | Wholesale energy |  |
Net gains and losses on assets and liabilities [Abstract] |  |
Total gains (losses) included in earnings | 41 | 21
Change in unrealized gains (losses) relating to positions still held at the reporting date | 13 | 25
Recurring | Energy Commodities, Net | Retail Energy |  |
Net gains and losses on assets and liabilities [Abstract] |  |
Total gains (losses) included in earnings | 18 | (40)
Change in unrealized gains (losses) relating to positions still held at the reporting date | 7 | (9)
Recurring | Energy Commodities, Net | Fuel and energy purchases |  |
Net gains and losses on assets and liabilities [Abstract] |  |
Total gains (losses) included in earnings | 1 | 2
Change in unrealized gains (losses) relating to positions still held at the reporting date | $ (1) | $ 1

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- Definition This item represents the amount of the total change in unrealized (holding) gains or losses for the period which are included in the statement of income (or changes in net assets and liabilities) in earnings. Such unrealized (holding) gains or losses relate to those assets and liabilities still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3). + References No definition available. + Details
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- Definition This item represents the amount of the total realized and unrealized gains or losses for the period which are included in the statement of income (or changes in net assets); the fair value of which assets and liabilities was or is measured on a recurring basis using significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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