Derivative Instruments (Details 3) (USD $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Realized and unrealized gains (losses) [Line Items]
Realized gain (loss)	$ (18)	$ 53	$ (54)	$ 40
Unrealized gain (loss)	50	(47)	77	(62)
Components of mark-to-market activities for realized/unrealized gains (losses) [Abstract]
Power contracts included in operating revenues	48	41	(9)	12
Natural gas contracts included in fuel and purchased energy expense	20	(13)	22	121
Interest rate swaps included in interest expense	1	(30)	2	(28)
Gain (Loss) on interest rate derivatives, net	(37)	8	(146)	(3)
Total mark-to-market activity	32	6	(131)	102
Interest Rate Swaps
Realized and unrealized gains (losses) [Line Items]
Realized gain (loss)	(60)	(6)	(106)	(12)
Unrealized gain (loss)	24	(16)	(38)	(19)
Commodity instruments
Realized and unrealized gains (losses) [Line Items]
Realized gain (loss)	42	59	52	52
Unrealized gain (loss)	$ 26	$ (31)	$ (39)	$ 81

Interest rate swaps included in interest expense

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Natural gas contracts included in fuel and purchased energy expense

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Power contracts included in operating revenues

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- Definition

Realized gain (loss) on derivatives

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- Definition

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5618551-113959

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Amount of gain (loss) included in earnings for the period from the increase (decrease) in fair value of interest rate derivatives not designated as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 44
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624171-113959

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5618551-113959

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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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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 95
-Paragraph 28
-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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
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-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

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