Fair Value Measurements (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Assets
Nuclear decommissioning trust	$ 544,933,000		$ 544,933,000		$ 513,733,000
Risk management activities: commodity contracts	73,517,000		73,517,000		79,586,000
Total assets	69,000,000		69,000,000
Liabilities
Risk management activities: commodity contracts	(172,996,000)		(172,996,000)		(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	(58,000,000)	(48,000,000)	(51,000,000)	(38,000,000)
Total net gains (losses) realized/unrealized:
Included in earnings	1,000,000		2,000,000	1,000,000
Included in OCI	3,000,000	(3,000,000)	(2,000,000)	(1,000,000)
Deferred as a regulatory asset or liability	12,000,000	1,000,000	7,000,000	(6,000,000)
Settlements	(1,000,000)	4,000,000		4,000,000
Transfers into Level 3 from Level 2		1,000,000	2,000,000	(4,000,000)
Transfers from Level 3 into Level 2	(2,000,000)	(2,000,000)	(3,000,000)	(3,000,000)
Net derivative balance at end of period	(45,000,000)	(47,000,000)	(45,000,000)	(47,000,000)
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	84,000,000		84,000,000		69,000,000
Cash Equivalents	2,000,000		2,000,000
Total assets	86,000,000		86,000,000		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	84,000,000		84,000,000		69,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Nuclear decommissioning trust	467,000,000		467,000,000		446,000,000
Risk management activities: commodity contracts	50,000,000		50,000,000		70,000,000
Total assets	517,000,000		517,000,000		516,000,000
Liabilities
Risk management activities: commodity contracts	(199,000,000)		(199,000,000)		(241,000,000)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Nuclear decommissioning trust	192,000,000		192,000,000		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		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	69,000,000		69,000,000		73,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Nuclear decommissioning trust	84,000,000		84,000,000		78,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Nuclear decommissioning trust	94,000,000		94,000,000		90,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Nuclear decommissioning trust	13,000,000		13,000,000		21,000,000
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Risk management activities: commodity contracts	69,000,000		69,000,000		74,000,000
Total assets	69,000,000		69,000,000		74,000,000
Liabilities
Risk management activities: commodity contracts	(114,000,000)		(114,000,000)		(125,000,000)
Fair value measurement on a recurring basis | Other
Assets
Nuclear decommissioning trust	(6,000,000)		(6,000,000)		(1,000,000)
Risk management activities: commodity contracts	(45,000,000)		(45,000,000)		(64,000,000)
Total assets	(51,000,000)		(51,000,000)		(65,000,000)
Liabilities
Risk management activities: commodity contracts	140,000,000		140,000,000		229,000,000
Fair value measurement on a recurring basis | Other | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	(6,000,000)		(6,000,000)		(1,000,000)
Fair value measurement on a recurring basis | Fair Value
Assets
Nuclear decommissioning trust	545,000,000		545,000,000		514,000,000
Cash Equivalents	2,000,000		2,000,000
Risk management activities: commodity contracts	74,000,000		74,000,000		80,000,000
Total assets	621,000,000		621,000,000		594,000,000
Liabilities
Risk management activities: commodity contracts	(173,000,000)		(173,000,000)		(137,000,000)
Fair value measurement on a recurring basis | Fair Value | US commingled equity funds
Assets
Nuclear decommissioning trust	192,000,000		192,000,000		175,000,000
Fair value measurement on a recurring basis | Fair Value | U.S. Treasury
Assets
Nuclear decommissioning trust	84,000,000		84,000,000		69,000,000
Fair value measurement on a recurring basis | Fair Value | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	9,000,000		9,000,000		8,000,000
Fair value measurement on a recurring basis | Fair Value | Corporate debt
Assets
Nuclear decommissioning trust	69,000,000		69,000,000		73,000,000
Fair value measurement on a recurring basis | Fair Value | Mortgage-backed securities
Assets
Nuclear decommissioning trust	84,000,000		84,000,000		78,000,000
Fair value measurement on a recurring basis | Fair Value | Municipality bonds
Assets
Nuclear decommissioning trust	94,000,000		94,000,000		90,000,000
Fair value measurement on a recurring basis | Fair Value | Other
Assets
Nuclear decommissioning trust	$ 13,000,000		$ 13,000,000		$ 21,000,000