Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Assets
Nuclear decommissioning trust	$ 723,582	$ 713,866
Total assets	30,000	33,000
Recurring
Assets
Derivative instruments, other	(13,000)	(23,000)
Derivative assets	33,000	31,000
Nuclear decommissioning trust, other	(4,000)	(7,000)
Nuclear decommissioning trust	724,000	714,000
Total, other	(17,000)	(30,000)
Total assets	757,000	745,000
Liabilities
Derivative instruments, other	28,000	59,000
Derivative liability	(148,000)	(110,000)
Recurring | US commingled equity funds
Assets
Nuclear decommissioning trust	314,000	310,000
Recurring | U.S. Treasury
Assets
Nuclear decommissioning trust	95,000	119,000
Recurring | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust, other	(4,000)	(7,000)
Nuclear decommissioning trust	7,000	4,000
Recurring | Corporate debt
Assets
Nuclear decommissioning trust	116,000	109,000
Recurring | Mortgage-backed securities
Assets
Nuclear decommissioning trust	87,000	89,000
Recurring | Municipality bonds
Assets
Nuclear decommissioning trust	86,000	69,000
Recurring | Other
Assets
Nuclear decommissioning trust	19,000	14,000
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Decommissioning fund investments, gross fair value	93,000	119,000
Gross assets, fair value disclosure	93,000	119,000
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	93,000	119,000
Recurring | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	16,000	21,000
Decommissioning fund investments, gross fair value	635,000	602,000
Gross assets, fair value disclosure	651,000	623,000
Liabilities
Gross derivative liability	(103,000)	(95,000)
Recurring | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Decommissioning fund investments, gross fair value	314,000	310,000
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	2,000
Recurring | Significant Other Observable Inputs (Level 2) | Cash and cash equivalent funds
Assets
Decommissioning fund investments, gross fair value	11,000	11,000
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Decommissioning fund investments, gross fair value	116,000	109,000
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	87,000	89,000
Recurring | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Decommissioning fund investments, gross fair value	86,000	69,000
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	19,000	14,000
Recurring | Significant Unobservable Inputs (Level 3)
Assets
Gross derivative assets	30,000	33,000
Gross assets, fair value disclosure	30,000	33,000
Liabilities
Gross derivative liability	$ (73,000)	$ (74,000)