Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Assets
Nuclear decommissioning trust	$ 690,226,000	$ 642,007,000
Total assets	30,000,000	41,000,000
Recurring
Assets
Risk management activities-derivative instruments: Commodity contracts	29,000,000	41,000,000
Nuclear decommissioning trust	690,000,000	642,000,000
Total assets	719,000,000	683,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(52,000,000)	(102,000,000)
Recurring | US commingled equity funds
Assets
Nuclear decommissioning trust	295,000,000	272,000,000
Recurring | U.S. Treasury
Assets
Nuclear decommissioning trust	126,000,000	107,000,000
Recurring | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	6,000,000	8,000,000
Recurring | Corporate debt
Assets
Nuclear decommissioning trust	109,000,000	88,000,000
Recurring | Mortgage-backed securities
Assets
Nuclear decommissioning trust	83,000,000	85,000,000
Recurring | Municipality bonds
Assets
Nuclear decommissioning trust	56,000,000	71,000,000
Recurring | Other
Assets
Nuclear decommissioning trust	15,000,000	11,000,000
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Nuclear decommissioning trust	126,000,000	107,000,000
Total assets	126,000,000	107,000,000
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Nuclear decommissioning trust	126,000,000	107,000,000
Recurring | Significant Other Observable Inputs (Level 2)
Assets
Risk management activities-derivative instruments: Commodity contracts	7,000,000	9,000,000
Nuclear decommissioning trust	566,000,000	538,000,000
Total assets	573,000,000	547,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(24,000,000)	(33,000,000)
Recurring | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Nuclear decommissioning trust	295,000,000	272,000,000
Recurring | Significant Other Observable Inputs (Level 2) | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	8,000,000	11,000,000
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Nuclear decommissioning trust	109,000,000	88,000,000
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Nuclear decommissioning trust	83,000,000	85,000,000
Recurring | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Nuclear decommissioning trust	56,000,000	71,000,000
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets
Nuclear decommissioning trust	15,000,000	11,000,000
Recurring | Significant Unobservable Inputs (Level 3)
Assets
Risk management activities-derivative instruments: Commodity contracts	30,000,000	41,000,000
Total assets	30,000,000	41,000,000
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	(68,000,000)	(90,000,000)
Recurring | Other
Assets
Risk management activities-derivative instruments: Commodity contracts	(8,000,000)	(9,000,000)
Nuclear decommissioning trust	(2,000,000)	(3,000,000)
Total assets	(10,000,000)	(12,000,000)
Liabilities
Risk management activities-derivative instruments: Commodity Contracts	40,000,000	21,000,000
Recurring | Other | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	$ (2,000,000)	$ (3,000,000)