Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets
Commodity contracts, assets	$ 195,277	$ 4,749
Commodity contracts, liabilities	(2,606)	(4,271)
Nuclear decommissioning trust	1,225,485	1,138,435
Nuclear decommissioning trust, other	682,403	592,227
Other special use funds	374,843	254,509
Other special use funds, other	1,392	504
Total assets	1,795,605	1,397,693
Total assets, other	681,189	588,460
Liabilities
Gross derivative liability, other	1,048	2,986
Amount reported on balance sheet	(1,652)	(18,619)
Equity securities
Assets
Nuclear decommissioning trust	10,251	11,968
Nuclear decommissioning trust, other	(23,543)	(17,828)
Other special use funds	26,463	37,841
Other special use funds, other	1,392	504
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	705,946	610,055
U.S. Treasury debt
Assets
Nuclear decommissioning trust	183,084	164,514
Other special use funds	336,494	203,220
Corporate debt
Assets
Nuclear decommissioning trust	141,744	149,509
Mortgage-backed securities
Assets
Nuclear decommissioning trust	115,714	99,623
Municipal bonds
Assets
Nuclear decommissioning trust	60,188	89,705
Other special use funds	11,886	13,448
Other fixed income
Assets
Nuclear decommissioning trust	8,558	13,061
Level 1
Assets
Commodity contracts, assets	0	0
Nuclear decommissioning trust	216,878	194,310
Other special use funds	361,565	240,557
Total assets	578,443	434,867
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	33,794	29,796
Other special use funds	25,071	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	183,084	164,514
Other special use funds	336,494	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
Level 1 | Other fixed income
Assets
Nuclear decommissioning trust	0	0
Level 2
Assets
Commodity contracts, assets	197,874	9,016
Nuclear decommissioning trust	326,204	351,898
Other special use funds	11,886	13,448
Total assets	535,964	374,362
Liabilities
Gross derivative liability	(2,674)	(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	141,744	149,509
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	115,714	99,623
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	60,188	89,705
Other special use funds	11,886	13,448
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	8,558	13,061
Level 3
Assets
Commodity contracts, assets	9	4
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	9	4
Liabilities
Gross derivative liability	(26)	(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	$ 705,946	$ 610,055