Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets
Nuclear decommissioning trust	$ 861,439	$ 871,000
Total assets	1,548	1,036
Recurring
Assets
Cash equivalents		10,630
Derivative instruments, other	(4,809)	(4,737)
Derivative assets	1,994	1,982
Nuclear decommissioning trust, other	413,623	417,499
Nuclear decommissioning trust	861,439	871,000
Other special use funds, net purchases and sales	1,482	525
Other special use funds	217,992	32,542
Total Assets, Other	410,296	413,287
Total assets	1,081,425	916,154
Liabilities
Total, other	3,075	1,516
Amount Reported on Balance Sheet	(115,369)	(96,422)
Recurring | Equity Securities
Assets
Nuclear decommissioning trust, other	(67)
Nuclear decommissioning trust	6,508
Other special use funds, net purchases and sales	1,482
Other special use funds	16,445
Recurring | US commingled equity funds
Assets
Nuclear decommissioning trust, other	413,690	417,390
Nuclear decommissioning trust	413,690	417,390
Recurring | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust, other		109
Nuclear decommissioning trust		7,333
Recurring | U.S. Treasury
Assets
Nuclear decommissioning trust	128,396	127,662
Other special use funds, net purchases and sales	0
Other special use funds	174,111
Recurring | Corporate debt
Assets
Nuclear decommissioning trust	111,735	114,007
Recurring | Mortgage-backed securities
Assets
Nuclear decommissioning trust	108,951	111,874
Recurring | Municipal bonds
Assets
Nuclear decommissioning trust	80,604	79,049
Other special use funds	27,436
Recurring | Other
Assets
Nuclear decommissioning trust	11,555	13,685
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash equivalents		10,630
Decommissioning fund investments, gross fair value	134,971	134,886
Other special use funds, gross	189,074	455
Gross assets, fair value disclosure	324,045	145,971
Liabilities
Gross derivative liability	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity Securities
Assets
Decommissioning fund investments, gross fair value	6,575
Other special use funds, gross	14,963
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust		7,224
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	128,396	127,662
Other special use funds, gross	174,111
Recurring | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	5,255	5,683
Decommissioning fund investments, gross fair value	312,845	318,615
Other special use funds, gross	27,436	31,562
Gross assets, fair value disclosure	345,536	355,860
Liabilities
Gross derivative liability	(97,142)	(78,646)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Decommissioning fund investments, gross fair value	111,735	114,007
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	108,951	111,874
Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds
Assets
Decommissioning fund investments, gross fair value	80,604	79,049
Other special use funds, gross	27,436
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	11,555	13,685
Recurring | Significant Unobservable Inputs (Level 3)
Assets
Gross derivative assets	1,548	1,036
Gross assets, fair value disclosure	1,548	1,036
Liabilities
Gross derivative liability	$ (21,302)	$ (19,292)