Risk Management and Hedging Activities - Balance Sheet Location (Details) (Commodity Contract [Member], USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2011		Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ (406)			$ (445)
Cash Collateral, Net Receivable (Payable), Offset Against Derivative Positions	133			156
Derivative Assets (Liabilities), at Fair Value, Net	(273)			(289)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	33			40
Cash Collateral, Net Receivable (Payable), Offset Against Derivative Positions	0			(2)
Derivative Assets (Liabilities), at Fair Value, Net	33			38
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	6			9
Cash Collateral, Net Receivable (Payable), Offset Against Derivative Positions	0			0
Derivative Assets (Liabilities), at Fair Value, Net	6			9
Derivative Contracts - Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(246)			(274)
Cash Collateral, Net Receivable (Payable), Offset Against Derivative Positions	96			114
Derivative Assets (Liabilities), at Fair Value, Net	(150)			(160)
Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(199)			(220)
Cash Collateral, Net Receivable (Payable), Offset Against Derivative Positions	37			44
Derivative Assets (Liabilities), at Fair Value, Net	(162)			(176)
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	144	[1]		193	[1]
Derivative Liability, Fair Value, Gross Liability	(501)	[1]		(592)	[1]
Derivative, Fair Value, Net	(357)	[1]		(399)	[1]
Net regulatory asset	357		415	400		498	543	564
Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	64	[1]		93	[1]
Derivative Liability, Fair Value, Gross Liability	(35)	[1]		(47)	[1]
Derivative, Fair Value, Net	29	[1]		46	[1]
Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	6	[1]		14	[1]
Derivative Liability, Fair Value, Gross Liability	(2)	[1]		(5)	[1]
Derivative, Fair Value, Net	4	[1]		9	[1]
Not Designated as Hedging Instrument [Member] | Derivative Contracts - Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	61	[1]		73	[1]
Derivative Liability, Fair Value, Gross Liability	(275)	[1]		(324)	[1]
Derivative, Fair Value, Net	(214)	[1]		(251)	[1]
Not Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	13	[1]		13	[1]
Derivative Liability, Fair Value, Gross Liability	(189)	[1]		(216)	[1]
Derivative, Fair Value, Net	(176)	[1]		(203)	[1]
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	11			1
Derivative Liability, Fair Value, Gross Liability	(60)			(47)
Derivative, Fair Value, Net	(49)			(46)
Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	5			0
Derivative Liability, Fair Value, Gross Liability	(1)			(6)
Derivative, Fair Value, Net	4			(6)
Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2			0
Derivative Liability, Fair Value, Gross Liability	0			0
Derivative, Fair Value, Net	2			0
Designated as Hedging Instrument [Member] | Derivative Contracts - Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2			1
Derivative Liability, Fair Value, Gross Liability	(34)			(24)
Derivative, Fair Value, Net	(32)			(23)
Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2			0
Derivative Liability, Fair Value, Gross Liability	(25)			(17)
Derivative, Fair Value, Net	$ (23)			$ (17)

[1] The Company's commodity derivatives not designated as hedging contracts are generally included in regulated rates, and as of June 30, 2012 and December 31, 2011, a net regulatory asset of $357 million and $400 million, respectively, was recorded related to the net derivative liability of $357 million and $399 million, respectively.