Fair Value Measurements (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Assets
Nuclear decommissioning trust	$ 566,960,000		$ 566,960,000		$ 513,733,000
Risk management activities: commodity contracts	64,706,000		64,706,000		79,586,000
Total assets	60,000,000		60,000,000
Liabilities
Risk management activities: commodity contracts	(137,507,000)		(137,507,000)		(136,463,000)
Changes in fair value for assets and liabilities that are measured at fair value on a recurring basis using Level 3 inputs
Net derivative balance at beginning of period	(45,000,000)	(47,000,000)	(51,000,000)	(38,000,000)
Total net gains (losses) realized/unrealized:
Included in earnings		1,000,000	2,000,000	2,000,000
Included in OCI		2,000,000	(2,000,000)	1,000,000
Deferred as a regulatory asset or liability	(3,000,000)	2,000,000	4,000,000	(4,000,000)
Settlements	(1,000,000)	6,000,000	(1,000,000)	10,000,000
Transfers into Level 3 from Level 2	(4,000,000)		(2,000,000)	(4,000,000)
Transfers from Level 3 into Level 2	3,000,000	(2,000,000)		(5,000,000)
Net derivative balance at end of period	(50,000,000)	(38,000,000)	(50,000,000)	(38,000,000)
Net unrealized gains included in earnings related to instruments still held at end of period				1,000,000
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Nuclear decommissioning trust	93,000,000		93,000,000		69,000,000
Cash Equivalents	46,000,000		46,000,000
Total assets	139,000,000		139,000,000		69,000,000
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Nuclear decommissioning trust	93,000,000		93,000,000		69,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Nuclear decommissioning trust	477,000,000		477,000,000		446,000,000
Risk management activities: commodity contracts	45,000,000		45,000,000		70,000,000
Total assets	522,000,000		522,000,000		516,000,000
Liabilities
Risk management activities: commodity contracts	(116,000,000)		(116,000,000)		(241,000,000)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Nuclear decommissioning trust	204,000,000		204,000,000		175,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	13,000,000		13,000,000		9,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Nuclear decommissioning trust	79,000,000		79,000,000		73,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Nuclear decommissioning trust	83,000,000		83,000,000		78,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Nuclear decommissioning trust	86,000,000		86,000,000		90,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Nuclear decommissioning trust	12,000,000		12,000,000		21,000,000
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Risk management activities: commodity contracts	60,000,000		60,000,000		74,000,000
Total assets	60,000,000		60,000,000		74,000,000
Liabilities
Risk management activities: commodity contracts	(110,000,000)		(110,000,000)		(125,000,000)
Fair value measurement on a recurring basis | Other
Assets
Nuclear decommissioning trust	(3,000,000)		(3,000,000)		(1,000,000)
Risk management activities: commodity contracts	(40,000,000)		(40,000,000)		(64,000,000)
Total assets	(43,000,000)		(43,000,000)		(65,000,000)
Liabilities
Risk management activities: commodity contracts	88,000,000		88,000,000		229,000,000
Fair value measurement on a recurring basis | Other | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	(3,000,000)		(3,000,000)		(1,000,000)
Fair value measurement on a recurring basis | Fair Value
Assets
Nuclear decommissioning trust	567,000,000		567,000,000		514,000,000
Cash Equivalents	46,000,000		46,000,000
Risk management activities: commodity contracts	65,000,000		65,000,000		80,000,000
Total assets	678,000,000		678,000,000		594,000,000
Liabilities
Risk management activities: commodity contracts	(138,000,000)		(138,000,000)		(137,000,000)
Fair value measurement on a recurring basis | Fair Value | US commingled equity funds
Assets
Nuclear decommissioning trust	204,000,000		204,000,000		175,000,000
Fair value measurement on a recurring basis | Fair Value | U.S. Treasury
Assets
Nuclear decommissioning trust	93,000,000		93,000,000		69,000,000
Fair value measurement on a recurring basis | Fair Value | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	10,000,000		10,000,000		8,000,000
Fair value measurement on a recurring basis | Fair Value | Corporate debt
Assets
Nuclear decommissioning trust	79,000,000		79,000,000		73,000,000
Fair value measurement on a recurring basis | Fair Value | Mortgage-backed securities
Assets
Nuclear decommissioning trust	83,000,000		83,000,000		78,000,000
Fair value measurement on a recurring basis | Fair Value | Municipality bonds
Assets
Nuclear decommissioning trust	86,000,000		86,000,000		90,000,000
Fair value measurement on a recurring basis | Fair Value | Other
Assets
Nuclear decommissioning trust	$ 12,000,000		$ 12,000,000		$ 21,000,000