Fair Value Measurements (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Assets
Nuclear decommissioning trust	$ 682,359,000	$ 642,007,000
Total assets	33,000,000	41,000,000
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Nuclear decommissioning trust	122,000,000	107,000,000
Total assets	122,000,000	107,000,000
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Nuclear decommissioning trust	122,000,000	107,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Risk management activities-derivative instruments: Commodity contracts	17,000,000	9,000,000
Nuclear decommissioning trust	565,000,000	538,000,000
Total assets	582,000,000	547,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(19,000,000)	(33,000,000)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Nuclear decommissioning trust	291,000,000	272,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	9,000,000	11,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Nuclear decommissioning trust	106,000,000	88,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Nuclear decommissioning trust	82,000,000	85,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Nuclear decommissioning trust	63,000,000	71,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Nuclear decommissioning trust	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	33,000,000	41,000,000
Total assets	33,000,000	41,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(75,000,000)	(90,000,000)
Fair value measurement on a recurring basis | Other
Assets
Risk management activities-derivative instruments: Commodity contracts	(13,000,000)	(9,000,000)
Nuclear decommissioning trust	(5,000,000)	(3,000,000)
Total assets	(18,000,000)	(12,000,000)
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	49,000,000	21,000,000
Fair value measurement on a recurring basis | Other | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	(5,000,000)	(3,000,000)
Fair value measurement on a recurring basis | Fair Value
Assets
Risk management activities-derivative instruments: Commodity contracts	37,000,000	41,000,000
Nuclear decommissioning trust	682,000,000	642,000,000
Total assets	719,000,000	683,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(45,000,000)	(102,000,000)
Fair value measurement on a recurring basis | Fair Value | US commingled equity funds
Assets
Nuclear decommissioning trust	291,000,000	272,000,000
Fair value measurement on a recurring basis | Fair Value | U.S. Treasury
Assets
Nuclear decommissioning trust	122,000,000	107,000,000
Fair value measurement on a recurring basis | Fair Value | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	4,000,000	8,000,000
Fair value measurement on a recurring basis | Fair Value | Corporate debt
Assets
Nuclear decommissioning trust	106,000,000	88,000,000
Fair value measurement on a recurring basis | Fair Value | Mortgage-backed securities
Assets
Nuclear decommissioning trust	82,000,000	85,000,000
Fair value measurement on a recurring basis | Fair Value | Municipality bonds
Assets
Nuclear decommissioning trust	63,000,000	71,000,000
Fair value measurement on a recurring basis | Fair Value | Other
Assets
Nuclear decommissioning trust	$ 14,000,000	$ 11,000,000