Fair Value Measurements (Details) (USD $)	3 Months Ended
	Mar. 31, 2012 M	Mar. 31, 2011	Dec. 31, 2011
Assets
Nuclear decommissioning trust	$ 541,989,000		$ 513,733,000
Risk management activities: commodity contracts	87,741,000		79,586,000
Total assets	84,000,000		74,000,000
Liabilities
Risk management activities: commodity contracts	(153,375,000)		(136,463,000)
Cash equivalents maximum maturity period (in months)	3
Changes in fair value for assets and liabilities that are measured at fair value on a recurring basis using Level 3 inputs
Net risk management activities at beginning of period	(51,000,000)	(38,000,000)
Total net gains (losses) realized/unrealized:
Included in earnings	1,000,000	1,000,000
Included in OCI	(5,000,000)	2,000,000
Deferred as a regulatory asset or liability	(5,000,000)	(7,000,000)
Settlements	1,000,000
Transfers into Level 3 from Level 2	2,000,000	(5,000,000)
Transfers from Level 3 into Level 2	(1,000,000)	(1,000,000)
Net risk management activities at end of period	(58,000,000)	(48,000,000)
Net unrealized gains (losses) 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	76,000,000		69,000,000
Cash equivalents	12,000,000
Total assets	88,000,000		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	76,000,000		69,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Nuclear decommissioning trust	467,000,000		446,000,000
Risk management activities: commodity contracts	63,000,000		70,000,000
Total assets	530,000,000		516,000,000
Liabilities
Risk management activities: commodity contracts	(271,000,000)		(241,000,000)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Nuclear decommissioning trust	196,000,000		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	15,000,000		9,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Nuclear decommissioning trust	67,000,000		73,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Nuclear decommissioning trust	82,000,000		78,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Nuclear decommissioning trust	88,000,000		90,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Nuclear decommissioning trust	19,000,000		21,000,000
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Risk management activities: commodity contracts	84,000,000		74,000,000
Total assets	84,000,000		74,000,000
Liabilities
Risk management activities: commodity contracts	(142,000,000)		(125,000,000)
Fair value measurement on a recurring basis | Other
Assets
Nuclear decommissioning trust	(1,000,000)		(1,000,000)
Risk management activities: commodity contracts	(59,000,000)		(64,000,000)
Total assets	(60,000,000)		(65,000,000)
Liabilities
Risk management activities: commodity contracts	260,000,000		229,000,000
Fair value measurement on a recurring basis | Other | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	(1,000,000)		(1,000,000)
Fair value measurement on a recurring basis | Fair Value
Assets
Nuclear decommissioning trust	542,000,000		514,000,000
Cash equivalents	12,000,000
Risk management activities: commodity contracts	88,000,000		80,000,000
Total assets	642,000,000		594,000,000
Liabilities
Risk management activities: commodity contracts	(153,000,000)		(137,000,000)
Fair value measurement on a recurring basis | Fair Value | US commingled equity funds
Assets
Nuclear decommissioning trust	196,000,000		175,000,000
Fair value measurement on a recurring basis | Fair Value | U.S. Treasury
Assets
Nuclear decommissioning trust	76,000,000		69,000,000
Fair value measurement on a recurring basis | Fair Value | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	14,000,000		8,000,000
Fair value measurement on a recurring basis | Fair Value | Corporate debt
Assets
Nuclear decommissioning trust	67,000,000		73,000,000
Fair value measurement on a recurring basis | Fair Value | Mortgage-backed securities
Assets
Nuclear decommissioning trust	82,000,000		78,000,000
Fair value measurement on a recurring basis | Fair Value | Municipality bonds
Assets
Nuclear decommissioning trust	88,000,000		90,000,000
Fair value measurement on a recurring basis | Fair Value | Other
Assets
Nuclear decommissioning trust	$ 19,000,000		$ 21,000,000