Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Assets
Commodity contracts, assets	$ 297,623	$ 110,389
Commodity contracts, liabilities	(8,621)	(4,690)
Nuclear decommissioning trust	1,227,465	1,294,757
Nuclear decommissioning trust, other	568,428	567,266
Other special use funds	349,042	358,410
Other special use funds, other	1,112	936
Total assets	1,874,130	1,763,556
Total assets, other	560,919	563,512
Liabilities
Gross derivative liability	(1,554)
Gross derivative liability, other		3,105
Amount reported on balance sheet	(1,706)	(4,373)
Equity securities
Assets
Nuclear decommissioning trust	17,265	17,482
Nuclear decommissioning trust, other	478	(27,782)
Other special use funds	28,180	48,506
Other special use funds, other	1,112	936
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	567,950	595,048
U.S. Treasury debt
Assets
Nuclear decommissioning trust	225,902	240,745
Other special use funds	312,613	298,170
Corporate debt
Assets
Nuclear decommissioning trust	196,300	203,454
Mortgage-backed securities
Assets
Nuclear decommissioning trust	145,845	155,574
Municipal bonds
Assets
Nuclear decommissioning trust	65,494	72,189
Other special use funds	8,249	11,734
Other fixed income
Assets
Nuclear decommissioning trust	8,709	10,265
Level 1
Assets
Commodity contracts, assets	0	0
Nuclear decommissioning trust	242,689	286,009
Other special use funds	339,681	345,740
Total assets	582,370	631,749
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	16,787	45,264
Other special use funds	27,068	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	225,902	240,745
Other special use funds	312,613	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
Level 1 | Other fixed income
Assets
Nuclear decommissioning trust	0	0
Level 2
Assets
Commodity contracts, assets	296,442	115,079
Nuclear decommissioning trust	416,348	441,482
Other special use funds	8,249	11,734
Total assets	721,039	568,295
Liabilities
Gross derivative liability	0	(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	196,300	203,454
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	145,845	155,574
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	65,494	72,189
Other special use funds	8,249	11,734
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	8,709	10,265
Level 3
Assets
Commodity contracts, assets	9,802	0
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	9,802	0
Liabilities
Gross derivative liability	(152)	(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	$ 567,950	$ 595,048