Fair Value Measurements (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Assets
Nuclear decommissioning trust	$ 595,231,000		$ 595,231,000		$ 570,625,000
Total assets	53,000,000		53,000,000		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	(53,000,000)	(58,000,000)	(48,000,000)	(51,000,000)
Total net gains (losses) realized/unrealized:
Included in earnings		1,000,000		2,000,000
Included in OCI		3,000,000		(2,000,000)
Deferred as a regulatory asset or liability	(4,000,000)	12,000,000	(5,000,000)	7,000,000
Settlements	4,000,000	(1,000,000)	2,000,000
Transfers into Level 3 from Level 2			(1,000,000)	2,000,000
Transfers from Level 3 into Level 2		(2,000,000)	(1,000,000)	(3,000,000)
Net derivative balance at end of period	(53,000,000)	(45,000,000)	(53,000,000)	(45,000,000)
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
Cash Equivalents	3,000,000		3,000,000		16,000,000
Nuclear decommissioning trust	111,000,000		111,000,000		110,000,000
Total assets	114,000,000		114,000,000		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	110,000,000		110,000,000		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	1,000,000		1,000,000		6,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Risk management activities-derivative instruments: Commodity Contracts	13,000,000		13,000,000		22,000,000
Nuclear decommissioning trust	490,000,000		490,000,000		465,000,000
Total assets	503,000,000		503,000,000		487,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(70,000,000)		(70,000,000)		(96,000,000)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Nuclear decommissioning trust	233,000,000		233,000,000		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	10,000,000		10,000,000		13,000,000
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		80,000,000
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		83,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Nuclear decommissioning trust	71,000,000		71,000,000		74,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Nuclear decommissioning trust	14,000,000		14,000,000		11,000,000
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Risk management activities-derivative instruments: Commodity Contracts	53,000,000		53,000,000		62,000,000
Total assets	53,000,000		53,000,000		62,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(106,000,000)		(106,000,000)		(110,000,000)
Fair value measurement on a recurring basis | Other
Assets
Risk management activities-derivative instruments: Commodity Contracts	(11,000,000)		(11,000,000)		(22,000,000)
Nuclear decommissioning trust	(6,000,000)		(6,000,000)		(4,000,000)
Total assets	(17,000,000)		(17,000,000)		(26,000,000)
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	23,000,000		23,000,000		47,000,000
Fair value measurement on a recurring basis | Other | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	(6,000,000)		(6,000,000)		(4,000,000)
Fair value measurement on a recurring basis | Fair Value
Assets
Cash Equivalents	3,000,000		3,000,000		16,000,000
Risk management activities-derivative instruments: Commodity Contracts	55,000,000		55,000,000		62,000,000
Nuclear decommissioning trust	595,000,000		595,000,000		571,000,000
Total assets	653,000,000		653,000,000		649,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(153,000,000)		(153,000,000)		(159,000,000)
Fair value measurement on a recurring basis | Fair Value | US commingled equity funds
Assets
Nuclear decommissioning trust	233,000,000		233,000,000		204,000,000
Fair value measurement on a recurring basis | Fair Value | U.S. Treasury
Assets
Nuclear decommissioning trust	110,000,000		110,000,000		104,000,000
Fair value measurement on a recurring basis | Fair Value | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	5,000,000		5,000,000		15,000,000
Fair value measurement on a recurring basis | Fair Value | Corporate debt
Assets
Nuclear decommissioning trust	79,000,000		79,000,000		80,000,000
Fair value measurement on a recurring basis | Fair Value | Mortgage-backed securities
Assets
Nuclear decommissioning trust	83,000,000		83,000,000		83,000,000
Fair value measurement on a recurring basis | Fair Value | Municipality bonds
Assets
Nuclear decommissioning trust	71,000,000		71,000,000		74,000,000
Fair value measurement on a recurring basis | Fair Value | Other
Assets
Nuclear decommissioning trust	$ 14,000,000		$ 14,000,000		$ 11,000,000