Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Assets
Commodity contracts, assets	$ 110,389	$ 4,749
Commodity contracts, liabilities	(4,690)	(4,271)
Nuclear decommissioning trust	1,294,757	1,138,435
Nuclear decommissioning trust, other	567,266	592,227
Other special use fund	358,410	254,509
Other special use funds, other	936	504
Total assets	1,763,556	1,397,693
Total assets, other	563,512	588,460
Liabilities
Derivative instruments, other	3,105	2,986
Derivative instruments, total	(4,373)	(18,619)
Equity securities
Assets
Nuclear decommissioning trust	17,482	11,968
Nuclear decommissioning trust, other	(27,782)	(17,828)
Other special use fund	48,506	37,841
Other special use funds, other	936	504
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	595,048	610,055
U.S. Treasury debt
Assets
Nuclear decommissioning trust	240,745	164,514
Other special use fund	298,170	203,220
Corporate debt
Assets
Nuclear decommissioning trust	203,454	149,509
Mortgage-backed securities
Assets
Nuclear decommissioning trust	155,574	99,623
Municipal bonds
Assets
Nuclear decommissioning trust	72,189	89,705
Other special use fund	11,734	13,448
Other fixed income
Assets
Nuclear decommissioning trust	10,265	13,061
Level 1
Assets
Commodity contracts, assets	0	0
Nuclear decommissioning trust	286,009	194,310
Other special use fund	345,740	240,557
Total assets	631,749	434,867
Liabilities
Derivative instruments	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	45,264	29,796
Other special use fund	47,570	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	240,745	164,514
Other special use fund	298,170	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 fund	0	0
Level 1 | Other fixed income
Assets
Nuclear decommissioning trust	0	0
Level 2
Assets
Commodity contracts, assets	115,079	9,016
Nuclear decommissioning trust	441,482	351,898
Other special use fund	11,734	13,448
Total assets	568,295	374,362
Liabilities
Derivative instruments	(4,740)	(20,498)
Level 2 | Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use fund	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 fund	0	0
Level 2 | Corporate debt
Assets
Nuclear decommissioning trust	203,454	149,509
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	155,574	99,623
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	72,189	89,705
Other special use fund	11,734	13,448
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	10,265	13,061
Level 3
Assets
Commodity contracts, assets	0	4
Nuclear decommissioning trust	0	0
Other special use fund	0	0
Total assets	0	4
Liabilities
Derivative instruments	(2,738)	(1,107)
Level 3 | Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use fund	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 fund	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 fund	0	0
Level 3 | Other fixed income
Assets
Nuclear decommissioning trust	0	0
NAV | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	$ 595,048	$ 610,055