Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets
Commodity contracts, assets	$ 25,811	$ 4,749
Commodity contracts, liabilities	(3,882)	(4,271)
Nuclear decommissioning trust	1,159,699	1,138,435
Nuclear decommissioning trust, other	632,661	592,227
Other special use funds	357,506	254,509
Other special use funds, other	1,401	504
Total assets	1,543,016	1,397,693
Total assets, other	630,180	588,460
Liabilities
Gross derivative liability, other	2,597	2,986
Amount reported on balance sheet	(9,995)	(18,619)
Equity securities
Assets
Nuclear decommissioning trust	6,622	11,968
Nuclear decommissioning trust, other	(15,538)	(17,828)
Other special use funds	20,612	37,841
Other special use funds, other	1,401	504
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	648,199	610,055
U.S. Treasury debt
Assets
Nuclear decommissioning trust	175,707	164,514
Other special use funds	323,589	203,220
Corporate debt
Assets
Nuclear decommissioning trust	143,876	149,509
Mortgage-backed securities
Assets
Nuclear decommissioning trust	110,073	99,623
Municipal bonds
Assets
Nuclear decommissioning trust	64,479	89,705
Other special use funds	13,305	13,448
Other fixed income
Assets
Nuclear decommissioning trust	10,743	13,061
Level 1
Assets
Commodity contracts, assets	0	0
Nuclear decommissioning trust	197,867	194,310
Other special use funds	342,800	240,557
Total assets	540,667	434,867
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	22,160	29,796
Other special use funds	19,211	37,337
Level 1 | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Level 1 | U.S. Treasury debt
Assets
Nuclear decommissioning trust	175,707	164,514
Other special use funds	323,589	203,220
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	14,353	9,016
Nuclear decommissioning trust	329,171	351,898
Other special use funds	13,305	13,448
Total assets	356,829	374,362
Liabilities
Gross derivative liability	(11,668)	(20,498)
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	143,876	149,509
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	110,073	99,623
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	64,479	89,705
Other special use funds	13,305	13,448
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	10,743	13,061
Level 3
Assets
Commodity contracts, assets	15,340	4
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	15,340	4
Liabilities
Gross derivative liability	(924)	(1,107)
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	$ 648,199	$ 610,055