Fair Value Measurements (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Assets
Nuclear decommissioning trust	$ 596,168,000		$ 570,625,000
Total assets	50,000,000		62,000,000
Changes in fair value for our risk management activities assets and liabilities that are measured at fair value on a recurring basis using Level 3 inputs
Net derivative balance at beginning of period	(48,000,000)	(51,000,000)
Total net gains (losses) realized/unrealized:
Included in earnings		1,000,000
Included in OCI		(5,000,000)
Deferred as a regulatory asset or liability	(1,000,000)	(5,000,000)
Settlements	(2,000,000)	1,000,000
Transfers into Level 3 from Level 2	(1,000,000)	2,000,000
Transfers from Level 3 into Level 2	(1,000,000)	(1,000,000)
Net derivative balance at end of period	(53,000,000)	(58,000,000)
Transfers in or out of Level 1 to or from any other hierarchy level	0
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Nuclear decommissioning trust	120,000,000		110,000,000
Cash Equivalents			16,000,000
Total assets	120,000,000		126,000,000
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Nuclear decommissioning trust	115,000,000		104,000,000
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	5,000,000		6,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Nuclear decommissioning trust	485,000,000		465,000,000
Risk management activities-derivative instruments: Commodity Contracts	28,000,000		22,000,000
Total assets	513,000,000		487,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(69,000,000)		(96,000,000)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Nuclear decommissioning trust	226,000,000		204,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	17,000,000		13,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Nuclear decommissioning trust	80,000,000		80,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Nuclear decommissioning trust	89,000,000		83,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Nuclear decommissioning trust	63,000,000		74,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Nuclear decommissioning trust	10,000,000		11,000,000
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Risk management activities-derivative instruments: Commodity Contracts	50,000,000		62,000,000
Total assets	50,000,000		62,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(103,000,000)		(110,000,000)
Fair value measurement on a recurring basis | Other
Assets
Nuclear decommissioning trust	(9,000,000)		(4,000,000)
Risk management activities-derivative instruments: Commodity Contracts	(24,000,000)		(22,000,000)
Total assets	(33,000,000)		(26,000,000)
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	24,000,000		47,000,000
Fair value measurement on a recurring basis | Other | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	(9,000,000)		(4,000,000)
Fair value measurement on a recurring basis | Fair Value
Assets
Nuclear decommissioning trust	596,000,000		571,000,000
Cash Equivalents			16,000,000
Risk management activities-derivative instruments: Commodity Contracts	54,000,000		62,000,000
Total assets	650,000,000		649,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(148,000,000)		(159,000,000)
Fair value measurement on a recurring basis | Fair Value | US commingled equity funds
Assets
Nuclear decommissioning trust	226,000,000		204,000,000
Fair value measurement on a recurring basis | Fair Value | U.S. Treasury
Assets
Nuclear decommissioning trust	115,000,000		104,000,000
Fair value measurement on a recurring basis | Fair Value | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	13,000,000		15,000,000
Fair value measurement on a recurring basis | Fair Value | Corporate debt
Assets
Nuclear decommissioning trust	80,000,000		80,000,000
Fair value measurement on a recurring basis | Fair Value | Mortgage-backed securities
Assets
Nuclear decommissioning trust	89,000,000		83,000,000
Fair value measurement on a recurring basis | Fair Value | Municipality bonds
Assets
Nuclear decommissioning trust	63,000,000		74,000,000
Fair value measurement on a recurring basis | Fair Value | Other
Assets
Nuclear decommissioning trust	$ 10,000,000		$ 11,000,000