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Price Risk Management Net realized and unrealized gains and losses on derivative transactions (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Derivative [Line Items]
Net gain or (loss) recognized in the statement of income offset by regulatory accounting	$ (72)	$ (146)	$ (107)	$ (306)
Commodity contracts:
Electricity	44	76	75	135
Natural Gas	$ 30	$ 72	$ 41	$ 181

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Commodity contracts: [Abstract]

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Net gain or loss recognized in the statement of income for derivative instruments related to price risk management activities offset by regulatory accounting.

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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.

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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.

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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.

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