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Derivative Instruments and Hedging Activities (Intro) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2016 | Mar. 31, 2015
Cash Flow Hedges |  |
Net unrealized after-tax gains (losses) expected to be reclassified into earnings within 12 months related to commodity price risk cash flow hedge contracts | $ 15 |
Commodity Price Risk Nontrading Economic Activity Numeric [Abstract] |  |
Year of expiration of the maximum maturity date of economic activity derivative contracts | 2020 |
Wholesale energy |  |
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract] |  |
Pre-tax gain (loss) | $ 60 | $ 19
Retail Energy |  |
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract] |  |
Pre-tax gain (loss) | 1 | (13)
Fuel and energy purchases |  |
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract] |  |
Pre-tax gain (loss) | $ 17 | $ 34

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- Definition Maximum length of time over which the entity is economically hedging its exposure to the variability in future cash flows for forecasted transactions. + References No definition available. + Details
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- Definition Amount of pre-tax gains (losses) from economic activity related to price and volume risk derivative instruments not designated as hedging instruments, the ineffective portion of qualifying cash flow hedges, and the entire change in fair value of certain cash flow hedges that fail retrospective effectiveness testing. + References No definition available. + Details
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- Definition The estimated net amount of unrealized gains or losses on price risk cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994 + Details
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