Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Fair value
Nuclear decommissioning trust fund	$ 686,619	$ 641,239
Long-term investments	630,385	690,732
Short-term investments	147,156	143,931
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	22,090	13,445
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	244,363	234,979
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	148,275	134,911
Long-term investments	45,497	43,202
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	72,196	67,986
Long-term investments	14,544	14,151
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	46,970	43,917
Long-term investments	16,127	17,243
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	57,536	58,763
Long-term investments	16,481	15,024
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	88,247	85,481
Long-term investments	353,127	378,387
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund	298	303
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	8,201	7,256
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	181,052	220,765
Short-term investments	147,156	143,931
Recurring basis | International mutual funds
Fair value
Nuclear decommissioning trust fund	2,912	2,012
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	3,963	2,717
Long-term investments	2,494	1,568
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	13,658	2,914
Long-term investments	1,063	392
Recurring basis | (Level 1) | Natural gas swaps
Fair value
Derivative liabilities	0	0
Recurring basis | (Level 1) | Domestic equity
Fair value
Nuclear decommissioning trust fund	244,363	234,979
Recurring basis | (Level 1) | International equity trust
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 1) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 1) | US Treasury securities
Fair value
Nuclear decommissioning trust fund	46,970	43,917
Long-term investments	16,127	17,243
Recurring basis | (Level 1) | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 1) | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	88,247	85,481
Long-term investments	353,127	378,387
Recurring basis | (Level 1) | Municipal bonds
Fair value
Nuclear decommissioning trust fund	0	0
Recurring basis | (Level 1) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	0	0
Recurring basis | (Level 1) | Treasury STRIPS
Fair value
Long-term investments	0	0
Short-term investments	0	0
Recurring basis | (Level 1) | International mutual funds
Fair value
Nuclear decommissioning trust fund	0	0
Recurring basis | (Level 1) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 1) | Other
Fair value
Nuclear decommissioning trust fund	13,658	2,914
Long-term investments	1,063	392
Recurring basis | (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	22,090	13,445
Recurring basis | (Level 2) | Domestic equity
Fair value
Nuclear decommissioning trust fund	0	0
Recurring basis | (Level 2) | International equity trust
Fair value
Nuclear decommissioning trust fund	148,275	134,911
Long-term investments	45,497	43,202
Recurring basis | (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	72,110	67,900
Long-term investments	14,544	14,151
Recurring basis | (Level 2) | US Treasury securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 2) | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	57,536	58,763
Long-term investments	16,481	15,024
Recurring basis | (Level 2) | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 2) | Municipal bonds
Fair value
Nuclear decommissioning trust fund	298	303
Recurring basis | (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	8,201	7,256
Recurring basis | (Level 2) | Treasury STRIPS
Fair value
Long-term investments	181,052	220,765
Short-term investments	147,156	143,931
Recurring basis | (Level 2) | International mutual funds
Fair value
Nuclear decommissioning trust fund	2,912	2,012
Recurring basis | (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	3,963	2,717
Long-term investments	2,494	1,568
Recurring basis | (Level 2) | Other
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 3) | Natural gas swaps
Fair value
Derivative liabilities	0	0
Recurring basis | (Level 3) | Domestic equity
Fair value
Nuclear decommissioning trust fund	0	0
Recurring basis | (Level 3) | International equity trust
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 3) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	86	86
Long-term investments	0	0
Recurring basis | (Level 3) | US Treasury securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 3) | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 3) | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 3) | Municipal bonds
Fair value
Nuclear decommissioning trust fund	0	0
Recurring basis | (Level 3) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	0	0
Recurring basis | (Level 3) | Treasury STRIPS
Fair value
Long-term investments	0	0
Short-term investments	0	0
Recurring basis | (Level 3) | International mutual funds
Fair value
Nuclear decommissioning trust fund	0	0
Recurring basis | (Level 3) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | (Level 3) | Other
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	$ 0	$ 0