Risk Management and Hedging Activities - Balance Sheet Location (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ (150)		$ (248)
Cash collateral, net receivable (payable), offset against derivative positions	42		62
Derivative Assets (Liabilities), at Fair Value, Net	(108)		(186)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	12		16
Cash collateral, net receivable (payable), offset against derivative positions	(1)		0
Derivative Assets (Liabilities), at Fair Value, Net	11		16
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	37		32
Cash collateral, net receivable (payable), offset against derivative positions	1		0
Derivative Assets (Liabilities), at Fair Value, Net	38		32
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(108)		(178)
Cash collateral, net receivable (payable), offset against derivative positions	40		62
Derivative Assets (Liabilities), at Fair Value, Net	(68)		(116)
Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(91)		(118)
Cash collateral, net receivable (payable), offset against derivative positions	2		0
Derivative Assets (Liabilities), at Fair Value, Net	(89)		(118)
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(138)	[1]	(204)	[1]
Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	9	[1]	16	[1]
Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	36	[1]	32	[1]
Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(100)	[1]	(152)	[1]
Not Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(83)	[1]	(100)	[1]
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(12)		(44)
Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	3		0
Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	1		0
Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(8)		(26)
Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(8)		(18)
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	85	[1]	92	[1]
Derivative liabilities	(223)	[1]	(295)	[1]
Derivative, Fair Value, Net	(138)		(203)
Net regulatory asset	174		235		415	400
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	14	[1]	30	[1]
Derivative liabilities	(5)	[1]	(14)	[1]
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	37	[1]	34	[1]
Derivative liabilities	(1)	[1]	(2)	[1]
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	32	[1]	25	[1]
Derivative liabilities	(132)	[1]	(177)	[1]
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	2	[1]	3	[1]
Derivative liabilities	(85)	[1]	(102)	[1]
Commodity derivative [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	12		3
Derivative liabilities	(14)		(35)
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	7		1
Derivative liabilities	(4)		(1)
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	3		0
Derivative liabilities	(2)		0
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	1		1
Derivative liabilities	(4)		(22)
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	1		1
Derivative liabilities	(4)		(12)
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities			(1)	[1]
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities			0	[1]
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities			0	[1]
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities			0	[1]
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities			(1)	[1]
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	(10)		(12)
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	0		0
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	0		0
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	(5)		(5)
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ (5)		$ (7)

[1] The Company's commodity derivatives not designated as hedging contracts are generally included in regulated rates, and as of March 31, 2013 and December 31, 2012, a net regulatory asset of $174 million and $235 million, respectively, was recorded related to the net derivative liability of $138 million and $203 million, respectively.