Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Assets
Nuclear decommissioning trust	$ 767,416	$ 735,196
Total assets	18,118	30,364
Recurring
Assets
Cash equivalents	9,857
Derivative instruments, other	(22,487)	(25,345)
Derivative assets	22,140	28,011
Nuclear decommissioning trust, other	314,898	314,622
Nuclear decommissioning trust	767,416	735,196
Total, other	292,411	289,277
Total assets	799,413	763,207
Liabilities
Total, other	18,864	39,698
Derivative liability	(107,550)	(167,689)
Recurring | US commingled equity funds
Assets
Nuclear decommissioning trust, other	328,037	314,957
Nuclear decommissioning trust	328,037	314,957
Recurring | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust, other	(13,139)	(335)
Nuclear decommissioning trust	4,753	11,925
Recurring | U.S. Treasury
Assets
Nuclear decommissioning trust	117,448	117,245
Recurring | Corporate debt
Assets
Nuclear decommissioning trust	106,399	96,243
Recurring | Mortgage-backed securities
Assets
Nuclear decommissioning trust	112,771	99,065
Recurring | Municipality bonds
Assets
Nuclear decommissioning trust	73,847	72,206
Recurring | Other
Assets
Nuclear decommissioning trust	24,161	23,555
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash equivalents	9,857
Decommissioning fund investments, gross fair value	135,340	129,505
Gross assets, fair value disclosure	145,197	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	17,892	12,260
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	117,448	117,245
Recurring | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	26,509	22,992
Decommissioning fund investments, gross fair value	317,178	291,069
Gross assets, fair value disclosure	343,687	314,061
Liabilities
Gross derivative liability	(75,916)	(144,044)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Decommissioning fund investments, gross fair value	106,399	96,243
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	112,771	99,065
Recurring | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Decommissioning fund investments, gross fair value	73,847	72,206
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	24,161	23,555
Recurring | Significant Unobservable Inputs (a) (Level 3)
Assets
Gross derivative assets	18,118	30,364
Gross assets, fair value disclosure	18,118	30,364
Liabilities
Gross derivative liability	$ (50,498)	$ (63,343)