Fair Value Measurements (Narrative) (Details) (USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Fair Value Measurements [Abstract]
Gain (loss) on Level 3 energy derivative assets	$ 1		$ (1)	$ 47
Gains (loss) on Level 3 energy derivative liabilities		14
Loss on Level 3 energy derivative assets for purchased power costs	(13)		26
Gain (loss) on Level 3 energy derivative liabilities for purchased power costs		(33)		(73)
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in other income	(10)	(3)	(35)	(2)
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in purchased power costs	$ (5)	$ (22)	$ 31	$ (29)

Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) Included in Purchased Power Costs

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Fair Value, Assets Measured on Recurring Basis, Gain (Loss) Included in Purchased Power Costs

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Fair Value, Liabilities Measured on Recurring Basis, Gain (Loss) Included in purchased power costs

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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) in other income. Such unrealized (holding) gains or losses relate to those assets still held at the reporting date for which fair value 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 Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Topic 820

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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) in other income; the fair value of which assets 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 Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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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) in other income; the fair value of which 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 Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

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-Section 50

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