Fair Value Measurements (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Assets
Nuclear decommissioning trust	$ 612,640,000	$ 570,625,000
Total assets	48,000,000	62,000,000
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	109,000,000	110,000,000
Total assets	109,000,000	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	109,000,000	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		6,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Risk management activities-derivative instruments: Commodity Contracts	12,000,000	22,000,000
Nuclear decommissioning trust	507,000,000	465,000,000
Total assets	519,000,000	487,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(49,000,000)	(96,000,000)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Nuclear decommissioning trust	245,000,000	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	13,000,000	13,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Nuclear decommissioning trust	83,000,000	80,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Nuclear decommissioning trust	82,000,000	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	74,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Nuclear decommissioning trust	13,000,000	11,000,000
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Risk management activities-derivative instruments: Commodity Contracts	48,000,000	62,000,000
Total assets	48,000,000	62,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(92,000,000)	(110,000,000)
Fair value measurement on a recurring basis | Other
Assets
Risk management activities-derivative instruments: Commodity Contracts	(11,000,000)	(22,000,000)
Nuclear decommissioning trust	(3,000,000)	(4,000,000)
Total assets	(14,000,000)	(26,000,000)
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	20,000,000	47,000,000
Fair value measurement on a recurring basis | Other | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	(3,000,000)	(4,000,000)
Fair value measurement on a recurring basis | Fair Value
Assets
Cash Equivalents		16,000,000
Risk management activities-derivative instruments: Commodity Contracts	49,000,000	62,000,000
Nuclear decommissioning trust	613,000,000	571,000,000
Total assets	662,000,000	649,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(121,000,000)	(159,000,000)
Fair value measurement on a recurring basis | Fair Value | US commingled equity funds
Assets
Nuclear decommissioning trust	245,000,000	204,000,000
Fair value measurement on a recurring basis | Fair Value | U.S. Treasury
Assets
Nuclear decommissioning trust	109,000,000	104,000,000
Fair value measurement on a recurring basis | Fair Value | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	10,000,000	15,000,000
Fair value measurement on a recurring basis | Fair Value | Corporate debt
Assets
Nuclear decommissioning trust	83,000,000	80,000,000
Fair value measurement on a recurring basis | Fair Value | Mortgage-backed securities
Assets
Nuclear decommissioning trust	82,000,000	83,000,000
Fair value measurement on a recurring basis | Fair Value | Municipality bonds
Assets
Nuclear decommissioning trust	71,000,000	74,000,000
Fair value measurement on a recurring basis | Fair Value | Other
Assets
Nuclear decommissioning trust	$ 13,000,000	$ 11,000,000