Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
ASSETS
Cash equivalents	$ 15	$ 10
Nuclear decommissioning trust:	1,239,024	1,201,246
Nuclear decommissioning trust, other	386,816	408,849
Other special use funds	364,225	362,781
Other special use funds, other	1,423	2,196
Total assets	1,603,342	1,570,845
Total assets, other	388,244	409,356
Commodity contracts
ASSETS
Commodity contracts, assets	78	6,808
Commodity contracts assets, other	5	(1,689)
LIABILITIES
Gross derivative liability, other	6,329	4,823
Interest rate swaps	(148,402)	(123,888)
Equity securities
ASSETS
Nuclear decommissioning trust:	11,719	10,297
Nuclear decommissioning trust, other	2,129	(767)
Other special use funds	48,088	43,187
Other special use funds, other	1,423	2,196
U.S. commingled equity funds
ASSETS
Nuclear decommissioning trust:	384,687	409,616
U.S. Treasury debt
ASSETS
Nuclear decommissioning trust:	343,985	319,734
Other special use funds	316,137	319,594
Corporate debt
ASSETS
Nuclear decommissioning trust:	218,595	188,317
Mortgage-backed securities
ASSETS
Nuclear decommissioning trust:	214,514	208,306
Municipal bonds
ASSETS
Nuclear decommissioning trust:	52,053	59,323
Other fixed income
ASSETS
Nuclear decommissioning trust:	13,471	5,653
Level 1
ASSETS
Cash equivalents	15	10
Nuclear decommissioning trust:	353,575	330,798
Other special use funds	362,802	360,585
Total assets	716,392	691,393
Level 1 | Commodity contracts
ASSETS
Commodity contracts, assets	0	0
LIABILITIES
Gross derivative liability	0	0
Level 1 | Equity securities
ASSETS
Nuclear decommissioning trust:	9,590	11,064
Other special use funds	46,665	40,991
Level 1 | U.S. commingled equity funds
ASSETS
Nuclear decommissioning trust:	0	0
Level 1 | U.S. Treasury debt
ASSETS
Nuclear decommissioning trust:	343,985	319,734
Other special use funds	316,137	319,594
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
Level 1 | Other fixed income
ASSETS
Nuclear decommissioning trust:	0	0
Level 2
ASSETS
Cash equivalents	0	0
Nuclear decommissioning trust:	498,633	461,599
Other special use funds	0	0
Total assets	498,633	463,480
Level 2 | Commodity contracts
ASSETS
Commodity contracts, assets	0	1,881
LIABILITIES
Gross derivative liability	(138,687)	(127,016)
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:	218,595	188,317
Level 2 | Mortgage-backed securities
ASSETS
Nuclear decommissioning trust:	214,514	208,306
Level 2 | Municipal bonds
ASSETS
Nuclear decommissioning trust:	52,053	59,323
Level 2 | Other fixed income
ASSETS
Nuclear decommissioning trust:	13,471	5,653
Level 3
ASSETS
Cash equivalents	0	0
Nuclear decommissioning trust:	0	0
Other special use funds	0	0
Total assets	73	6,616
Level 3 | Commodity contracts
ASSETS
Commodity contracts, assets	73	6,616
LIABILITIES
Gross derivative liability	(16,044)	(1,695)
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
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:	$ 384,687	$ 409,616