Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Assets
Commodity contracts, assets	$ 109,614	$ 4,749
Commodity contracts, liabilities	(1,368)	(4,271)
Nuclear decommissioning trust	1,223,088	1,138,435
Nuclear decommissioning trust, other	693,330	592,227
Other special use funds	358,436	254,509
Other special use funds, other	952	504
Total assets	1,691,138	1,397,693
Total assets, other	692,914	588,460
Liabilities
Gross derivative liability, other	83	2,986
Amount reported on balance sheet	(1,512)	(18,619)
Equity securities
Assets
Nuclear decommissioning trust	13,243	11,968
Nuclear decommissioning trust, other	(9,506)	(17,828)
Other special use funds	20,856	37,841
Other special use funds, other	952	504
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	702,836	610,055
U.S. Treasury debt
Assets
Nuclear decommissioning trust	167,584	164,514
Other special use funds	324,418	203,220
Corporate debt
Assets
Nuclear decommissioning trust	150,681	149,509
Mortgage-backed securities
Assets
Nuclear decommissioning trust	119,481	99,623
Municipal bonds
Assets
Nuclear decommissioning trust	59,876	89,705
Other special use funds	13,162	13,448
Other fixed income
Assets
Nuclear decommissioning trust	9,387	13,061
Level 1
Assets
Commodity contracts, assets	0	0
Nuclear decommissioning trust	190,333	194,310
Other special use funds	344,322	240,557
Total assets	534,655	434,867
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	22,749	29,796
Other special use funds	19,904	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	167,584	164,514
Other special use funds	324,418	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	0
Level 1 | Other fixed income
Assets
Nuclear decommissioning trust	0	0
Level 2
Assets
Commodity contracts, assets	84,594	9,016
Nuclear decommissioning trust	339,425	351,898
Other special use funds	13,162	13,448
Total assets	437,181	374,362
Liabilities
Gross derivative liability	(1,586)	(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	150,681	149,509
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	119,481	99,623
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	59,876	89,705
Other special use funds	13,162	13,448
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	9,387	13,061
Level 3
Assets
Commodity contracts, assets	26,388	4
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	26,388	4
Liabilities
Gross derivative liability	(9)	(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	$ 702,836	$ 610,055