Derivative Financial Instruments By Balance Sheet Location (Details) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010		Sep. 30, 2010
Derivatives Fair Value [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	$ 0		$ 0		$ 4
Derivative Assets (Liabilities), at Fair Value, Net	48		75		86
Footnotes to Derivatives Balance Sheet Location [Abstract]
Collateral Already Posted Aggregate Fair Value	79		105		91
Cash flow hedges [Member]
Derivatives Fair Value [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	13		(2)		(21)
Cash flow hedges [Member] | Natural gas contracts
Derivatives Fair Value [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	8	[1],[2]	3	[1],[2]	13	[1],[2]
Derivative Liability Designated as Hedging Instrument, Fair Value	(8)	[1],[2]	(5)	[1],[2]	(15)	[1],[2]
Cash flow hedges [Member] | Interest rate swap contracts [Member]
Derivatives Fair Value [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	13	[1],[2]	0	[1],[2]	0	[1],[2]
Derivative Liability Designated as Hedging Instrument, Fair Value	0	[1],[2]	0	[1],[2]	(23)	[1],[2]
Derivatives not designated as hedging instruments [Member]
Derivatives Fair Value [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	35		77		107
Derivatives not designated as hedging instruments [Member] | Natural gas contracts
Derivatives Fair Value [Line Items]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	78	[1],[2]	105	[1],[2]	139	[1],[2]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	(82)	[1],[2]	(80)	[1],[2]	(117)	[1],[2]
Derivatives not designated as hedging instruments [Member] | Natural gas contracts | Asset Financial Instruments Current [Member]
Derivatives Fair Value [Line Items]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	486	[1],[2]	541	[1],[2]	727	[1],[2]
Derivatives not designated as hedging instruments [Member] | Natural gas contracts | Liability Financial Instruments Current [Member]
Derivatives Fair Value [Line Items]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	$ (447)	[1],[2]	$ (489)	[1],[2]	$ (642)	[1],[2]

[1] As required by the authoritative guidance related to derivatives and hedging, the fair value amounts above are presented on a gross basis. As a result, the amounts above do not include cash collateral held on deposit in broker margin accounts of $79 million as of September 30, 2011, $91 million as of September 30, 2010 and $105 million as of December 31, 2010. Accordingly, the amounts above will differ from the amounts presented on our Condensed Consolidated Statements of Financial Position and the fair value information presented for our derivative financial instruments in the recurring values table of this note.

[2] These amounts are netted within our Condensed Consolidated Statements of Financial Position. Some of our derivative financial instruments have asset positions which are presented as a liability in our Condensed Consolidated Statements of Financial Position, and we have derivative instruments that have liability positions which are presented as an asset in our consolidated statements of financial position.