Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets
Cash equivalents	$ 26,130
Commodity contracts, assets	2,108	$ 515
Commodity contracts, liabilities	(719)	(69)
Nuclear decommissioning trust	920,426	1,010,775
Nuclear decommissioning trust, other	418,553	521,245
Other special use funds	252,723	245,095
Other special use funds, other	1,331	474
Total assets	1,201,387	1,256,385
Total assets, other	419,165	521,650
Liabilities
Total, other	(346)	(711)
Amount reported on balance sheet	(87,361)	(72,132)
Equity securities
Assets
Nuclear decommissioning trust	13,542	13,273
Nuclear decommissioning trust, other	2,090	2,401
Other special use funds	8,083	7,616
Other special use funds, other	1,331	474
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	416,463	518,844
U.S. Treasury debt
Assets
Nuclear decommissioning trust	152,951	160,607
Other special use funds	238,637	232,848
Corporate debt
Assets
Nuclear decommissioning trust	112,667	115,869
Mortgage-backed securities
Assets
Nuclear decommissioning trust	115,790	118,795
Municipal bonds
Assets
Nuclear decommissioning trust	98,605	73,040
Other special use funds	6,003	4,631
Other fixed income
Assets
Nuclear decommissioning trust	10,408	10,347
Level 1
Assets
Cash equivalents	26,130
Commodity contracts, assets	0	0
Nuclear decommissioning trust	164,403	171,479
Other special use funds	245,389	239,990
Total assets	435,922	411,469
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	11,452	10,872
Other special use funds	6,752	7,142
Level 1 | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Level 1 | U.S. Treasury debt
Assets
Nuclear decommissioning trust	152,951	160,607
Other special use funds	238,637	232,848
Level 1 | Corporate debt
Assets
Nuclear decommissioning trust	0	0
Level 1 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	0	0
Level 1 | Municipal bonds
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 1 | Other fixed income
Assets
Nuclear decommissioning trust	0	0
Level 2
Assets
Commodity contracts, assets	2,325	551
Nuclear decommissioning trust	337,470	318,051
Other special use funds	6,003	4,631
Total assets	345,798	323,233
Liabilities
Gross derivative liability	(79,583)	(67,992)
Level 2 | Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 2 | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Level 2 | U.S. Treasury debt
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 2 | Corporate debt
Assets
Nuclear decommissioning trust	112,667	115,869
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	115,790	118,795
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	98,605	73,040
Other special use funds	6,003	4,631
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	10,408	10,347
Level 3
Assets
Commodity contracts, assets	502	33
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	502	33
Liabilities
Gross derivative liability	(8,124)	(3,429)
Level 3 | Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 3 | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Level 3 | U.S. Treasury debt
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 3 | Corporate debt
Assets
Nuclear decommissioning trust	0	0
Level 3 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	0	0
Level 3 | Municipal bonds
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 3 | Other fixed income
Assets
Nuclear decommissioning trust	0	0
Fair Value Measured at Net Asset Value Per Share | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	$ 416,463	$ 518,844