Fair Value Measurements (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Assets
Risk management activities-derivative instruments: Commodity Contracts	$ 61,590,000	$ 79,586,000
Nuclear decommissioning trust	570,625,000	513,733,000
Total assets	62,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(159,005,000)	(136,463,000)
Cash equivalents maximum maturity period	3 months
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	(51,000,000)	(38,000,000)
Total net gains (losses) realized/unrealized:
Included in earnings	2,000,000	2,000,000
Included in OCI	(3,000,000)	(5,000,000)
Deferred as a regulatory asset or liability	7,000,000	(10,000,000)
Settlements	(5,000,000)	11,000,000
Transfers into Level 3 from Level 2	(2,000,000)	(4,000,000)
Transfers from Level 3 into Level 2	4,000,000	(7,000,000)
Net derivative balance at end of period	(48,000,000)	(51,000,000)
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
Cash Equivalents	16,000,000
Nuclear decommissioning trust	110,000,000	69,000,000
Total assets	126,000,000	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	104,000,000	69,000,000
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	6,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Risk management activities-derivative instruments: Commodity Contracts	22,000,000	70,000,000
Nuclear decommissioning trust	465,000,000	446,000,000
Total assets	487,000,000	516,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(96,000,000)	(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	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	9,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Nuclear decommissioning trust	80,000,000	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	78,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Nuclear decommissioning trust	74,000,000	90,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Nuclear decommissioning trust	11,000,000	21,000,000
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Risk management activities-derivative instruments: Commodity Contracts	62,000,000	74,000,000
Total assets	62,000,000	74,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(110,000,000)	(125,000,000)
Fair value measurement on a recurring basis | Other
Assets
Risk management activities-derivative instruments: Commodity Contracts	(22,000,000)	(64,000,000)
Nuclear decommissioning trust	(4,000,000)	(1,000,000)
Total assets	(26,000,000)	(65,000,000)
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	47,000,000	229,000,000
Fair value measurement on a recurring basis | Other | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	(4,000,000)	(1,000,000)
Fair value measurement on a recurring basis | Fair Value
Assets
Cash Equivalents	16,000,000
Risk management activities-derivative instruments: Commodity Contracts	62,000,000	80,000,000
Nuclear decommissioning trust	571,000,000	514,000,000
Total assets	649,000,000	594,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(159,000,000)	(137,000,000)
Fair value measurement on a recurring basis | Fair Value | US commingled equity funds
Assets
Nuclear decommissioning trust	204,000,000	175,000,000
Fair value measurement on a recurring basis | Fair Value | U.S. Treasury
Assets
Nuclear decommissioning trust	104,000,000	69,000,000
Fair value measurement on a recurring basis | Fair Value | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	15,000,000	8,000,000
Fair value measurement on a recurring basis | Fair Value | Corporate debt
Assets
Nuclear decommissioning trust	80,000,000	73,000,000
Fair value measurement on a recurring basis | Fair Value | Mortgage-backed securities
Assets
Nuclear decommissioning trust	83,000,000	78,000,000
Fair value measurement on a recurring basis | Fair Value | Municipality bonds
Assets
Nuclear decommissioning trust	74,000,000	90,000,000
Fair value measurement on a recurring basis | Fair Value | Other
Assets
Nuclear decommissioning trust	$ 11,000,000	$ 21,000,000