Risk Management and Hedging Activities - Balance Sheet Location (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Mar. 31, 2013	Dec. 31, 2012		Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ (170)			$ (248)
Cash collateral, net receivable, offset against derivative positions	35			62
Derivative Assets (Liabilities), at Fair Value, Net	(135)			(186)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	9			16
Cash collateral, net receivable, offset against derivative positions	0			0
Derivative Assets (Liabilities), at Fair Value, Net	9			16
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	34			32
Cash collateral, net receivable, offset against derivative positions	0			0
Derivative Assets (Liabilities), at Fair Value, Net	34			32
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(122)			(178)
Cash collateral, net receivable, offset against derivative positions	35			62
Derivative Assets (Liabilities), at Fair Value, Net	(87)			(116)
Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(91)			(118)
Cash collateral, net receivable, offset against derivative positions	0			0
Derivative Assets (Liabilities), at Fair Value, Net	(91)			(118)
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(143)			(204)
Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	9			16
Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	31			32
Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(101)			(152)
Not Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(82)			(100)
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(27)			(44)
Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0			0
Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	3			0
Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(21)			(26)
Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(9)			(18)
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	81	[1]		92	[1]
Derivative liabilities	(226)	[1]		(295)	[1]
Derivative, Fair Value, Net	(145)			(203)
Net regulatory asset	172		174	235		357	415	400
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	26	[1]		30	[1]
Derivative liabilities	(17)	[1]		(14)	[1]
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	30	[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	24	[1]		25	[1]
Derivative liabilities	(125)	[1]		(177)	[1]
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	1	[1]		3	[1]
Derivative liabilities	(83)	[1]		(102)	[1]
Commodity derivative [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	2			3
Derivative liabilities	(28)			(35)
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	0			1
Derivative liabilities	0			(1)
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	(1)			0
Derivative liabilities	0			0
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	2			1
Derivative liabilities	(18)			(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	(10)			(12)
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	2
Derivative liabilities				(1)
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	0
Derivative liabilities				0
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	2
Derivative liabilities				0
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	0
Derivative liabilities				0
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	0
Derivative liabilities				(1)
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	4
Derivative liabilities	(5)			(12)
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	0
Derivative liabilities	0			0
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	4
Derivative liabilities	0			0
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	0
Derivative liabilities	(5)			(5)
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	0
Derivative liabilities	$ 0			$ (7)

[1] The Company's commodity derivatives not designated as hedging contracts are generally included in regulated rates, and as of June 30, 2013 and December 31, 2012, a net regulatory asset of $172 million and $235 million, respectively, was recorded related to the net derivative liability of $145 million and $203 million, respectively.