Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Assets
Commodity contracts, assets	$ 248,875	$ 110,389
Commodity contracts, liabilities	(40,737)	(4,690)
Nuclear decommissioning trust	1,076,088	1,294,757
Nuclear decommissioning trust, other	462,020	567,266
Other special use funds	346,406	358,410
Other special use funds, other	938	936
Total assets	1,671,369	1,763,556
Total assets, other	422,221	563,512
Liabilities
Gross derivative liability	(1,543)
Gross derivative liability, other		3,105
Amount reported on balance sheet	(5,596)	(4,373)
Equity securities
Assets
Nuclear decommissioning trust	14,174	17,482
Nuclear decommissioning trust, other	1,401	(27,782)
Other special use funds	30,031	48,506
Other special use funds, other	938	936
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	460,619	595,048
U.S. Treasury debt
Assets
Nuclear decommissioning trust	211,284	240,745
Other special use funds	310,667	298,170
Corporate debt
Assets
Nuclear decommissioning trust	177,297	203,454
Mortgage-backed securities
Assets
Nuclear decommissioning trust	135,858	155,574
Municipal bonds
Assets
Nuclear decommissioning trust	67,943	72,189
Other special use funds	5,708	11,734
Other fixed income
Assets
Nuclear decommissioning trust	8,913	10,265
Level 1
Assets
Commodity contracts, assets	0	0
Nuclear decommissioning trust	224,057	286,009
Other special use funds	339,760	345,740
Total assets	563,817	631,749
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	12,773	45,264
Other special use funds	29,093	47,570
Level 1 | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Level 1 | U.S. Treasury debt
Assets
Nuclear decommissioning trust	211,284	240,745
Other special use funds	310,667	298,170
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	281,107	115,079
Nuclear decommissioning trust	390,011	441,482
Other special use funds	5,708	11,734
Total assets	676,826	568,295
Liabilities
Gross derivative liability	(94)	(4,740)
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	177,297	203,454
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	135,858	155,574
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	67,943	72,189
Other special use funds	5,708	11,734
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	8,913	10,265
Level 3
Assets
Commodity contracts, assets	8,505	0
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	8,505	0
Liabilities
Gross derivative liability	(3,959)	(2,738)
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	$ 460,619	$ 595,048