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Price Risk Management Net realized and unrealized gains and losses on derivative transactions (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Derivative [Line Items] |  |  |
Net gain or loss recognized in the statement of income offset by regulatory accounting | $ 42 | $ 192 | $ 258
Commodity contracts: [Abstract] |  |  |
Commodity Contracts, Electricity | 56 | 117 | 127
Contracts, Natural Gas | $ 19 | $ 98 | $ 192

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- Definition Net gain or loss recognized in the statement of income for derivative instruments related to price risk management activities offset by regulatory accounting. + References No definition available. + Details
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- Definition Realized and unrealized gains and losses, net on commodity contracts related to electricity not designated as hedging instruments included in purchased power and fuel expense in the statement of income. + References No definition available. + Details
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- Definition Realized and unrealized gains and losses, net on commodity contracts related to natural gas not designated as hedging instruments included in purchased power and fuel expense in the statement of income. + References No definition available. + Details
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