Fair Value Measurements - Fair Value of Assets and Liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Assets
Nuclear decommissioning trust	$ 713,866,000	$ 642,007,000
Total assets	33,000,000	41,000,000
Fair value measurement on a recurring basis
Assets
Other	(23,000,000)	(9,000,000)
Derivative assets	31,000,000	41,000,000
Other	(7,000,000)	(3,000,000)
Nuclear decommissioning trust	714,000,000	642,000,000
Other	(30,000,000)	(12,000,000)
Total assets	745,000,000	683,000,000
Liabilities
Other	59,000,000	21,000,000
Derivative Liability	(110,000,000)	(102,000,000)
Fair value measurement on a recurring basis | US commingled equity funds
Assets
Nuclear decommissioning trust	310,000,000	272,000,000
Fair value measurement on a recurring basis | U.S. Treasury
Assets
Nuclear decommissioning trust	119,000,000	107,000,000
Fair value measurement on a recurring basis | Cash and cash equivalent funds
Assets
Other	(7,000,000)	(3,000,000)
Nuclear decommissioning trust	4,000,000	8,000,000
Fair value measurement on a recurring basis | Corporate debt
Assets
Nuclear decommissioning trust	109,000,000	88,000,000
Fair value measurement on a recurring basis | Mortgage-backed securities
Assets
Nuclear decommissioning trust	89,000,000	85,000,000
Fair value measurement on a recurring basis | Municipality bonds
Assets
Nuclear decommissioning trust	69,000,000	71,000,000
Fair value measurement on a recurring basis | Other
Assets
Nuclear decommissioning trust	14,000,000	11,000,000
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Decommissioning fund investments, gross fair value	119,000,000	107,000,000
Gross assets, fair value disclosure	119,000,000	107,000,000
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	119,000,000	107,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	21,000,000	9,000,000
Decommissioning fund investments, gross fair value	602,000,000	538,000,000
Gross assets, fair value disclosure	623,000,000	547,000,000
Liabilities
Gross derivative liability	(95,000,000)	(33,000,000)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Decommissioning fund investments, gross fair value	310,000,000	272,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Cash and cash equivalent funds
Assets
Decommissioning fund investments, gross fair value	11,000,000	11,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Decommissioning fund investments, gross fair value	109,000,000	88,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	89,000,000	85,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Decommissioning fund investments, gross fair value	69,000,000	71,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	14,000,000	11,000,000
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Gross derivative assets	33,000,000	41,000,000
Gross assets, fair value disclosure	33,000,000	41,000,000
Liabilities
Gross derivative liability	$ (74,000,000)	$ (90,000,000)