Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets
Nuclear decommissioning trust	$ 751,954	$ 735,196
Total assets	23,847	30,364
Recurring
Assets
Derivative instruments, other	(11,759)	(25,345)
Derivative assets	24,701	28,011
Nuclear decommissioning trust, other	318,073	314,622
Nuclear decommissioning trust	751,954	735,196
Total, other	306,314	289,277
Total assets	776,655	763,207
Liabilities
Total, other	25,022	39,698
Derivative liability	(178,886)	(167,689)
Recurring | US commingled equity funds
Assets
Nuclear decommissioning trust, other	319,971	314,957
Nuclear decommissioning trust	319,971	314,957
Recurring | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust, other	(1,898)	(335)
Nuclear decommissioning trust	9,637	11,925
Recurring | U.S. Treasury
Assets
Nuclear decommissioning trust	107,698	117,245
Recurring | Corporate debt
Assets
Nuclear decommissioning trust	101,687	96,243
Recurring | Mortgage-backed securities
Assets
Nuclear decommissioning trust	109,298	99,065
Recurring | Municipality bonds
Assets
Nuclear decommissioning trust	80,344	72,206
Recurring | Other
Assets
Nuclear decommissioning trust	23,319	23,555
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Decommissioning fund investments, gross fair value	119,233	129,505
Gross assets, fair value disclosure	119,233	129,505
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalent funds
Assets
Decommissioning fund investments, gross fair value	11,535	12,260
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	107,698	117,245
Recurring | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	12,613	22,992
Decommissioning fund investments, gross fair value	314,648	291,069
Gross assets, fair value disclosure	327,261	314,061
Liabilities
Gross derivative liability	(140,554)	(144,044)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Decommissioning fund investments, gross fair value	101,687	96,243
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	109,298	99,065
Recurring | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Decommissioning fund investments, gross fair value	80,344	72,206
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	23,319	23,555
Recurring | Significant Unobservable Inputs (a) (Level 3)
Assets
Gross derivative assets	23,847	30,364
Gross assets, fair value disclosure	23,847	30,364
Liabilities
Gross derivative liability	$ (63,354)	$ (63,343)