Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Assets
Commodity contracts, assets	$ 251,658	$ 110,389
Commodity contracts, liabilities	(11,095)	(4,690)
Nuclear decommissioning trust	1,025,913	1,294,757
Nuclear decommissioning trust, other	443,371	567,266
Other special use funds	359,394	358,410
Other special use funds, other	1,123	936
Total assets	1,636,965	1,763,556
Total assets, other	433,399	563,512
Liabilities
Gross derivative liability	(4,298)
Gross derivative liability, other		3,105
Amount reported on balance sheet	(7,861)	(4,373)
Equity securities
Assets
Nuclear decommissioning trust	8,098	17,482
Nuclear decommissioning trust, other	5,249	(27,782)
Other special use funds	39,692	48,506
Other special use funds, other	1,123	936
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	438,122	595,048
U.S. Treasury debt
Assets
Nuclear decommissioning trust	199,314	240,745
Other special use funds	315,643	298,170
Corporate debt
Assets
Nuclear decommissioning trust	164,700	203,454
Mortgage-backed securities
Assets
Nuclear decommissioning trust	142,214	155,574
Municipal bonds
Assets
Nuclear decommissioning trust	64,457	72,189
Other special use funds	4,059	11,734
Other fixed income
Assets
Nuclear decommissioning trust	9,008	10,265
Level 1
Assets
Commodity contracts, assets	0	0
Nuclear decommissioning trust	202,163	286,009
Other special use funds	354,212	345,740
Total assets	556,375	631,749
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	2,849	45,264
Other special use funds	38,569	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	199,314	240,745
Other special use funds	315,643	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	251,715	115,079
Nuclear decommissioning trust	380,379	441,482
Other special use funds	4,059	11,734
Total assets	636,153	568,295
Liabilities
Gross derivative liability	(2,933)	(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	164,700	203,454
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	142,214	155,574
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	64,457	72,189
Other special use funds	4,059	11,734
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	9,008	10,265
Level 3
Assets
Commodity contracts, assets	11,038	0
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	11,038	0
Liabilities
Gross derivative liability	(630)	(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	$ 438,122	$ 595,048