Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Fair value
Nuclear decommissioning trust fund	$ 679,298,000	$ 641,239,000
Long-term investments	709,815,000	690,732,000
Short-term investments	135,341,000	143,931,000
International equity trust
Fair value
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	$ 10,381,000	13,445,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	259,404,000	234,979,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	141,293,000	134,911,000
Long-term investments	46,098,000	43,202,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	64,861,000	67,986,000
Long-term investments	14,949,000	14,151,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	42,814,000	43,917,000
Long-term investments	14,783,000	17,243,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	56,607,000	58,763,000
Long-term investments	17,068,000	15,024,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	92,626,000	85,481,000
Long-term investments	387,897,000	378,387,000
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund	303,000	303,000
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	8,231,000	7,256,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	226,643,000	220,765,000
Short-term investments	135,341,000	143,931,000
Recurring basis | International mutual funds
Fair value
Nuclear decommissioning trust fund	2,449,000	2,012,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	3,708,000	2,717,000
Long-term investments	1,775,000	1,568,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	7,002,000	2,914,000
Long-term investments	602,000	392,000
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	259,404,000	234,979,000
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	42,814,000	43,917,000
Long-term investments	14,783,000	17,243,000
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	92,626,000	85,481,000
Long-term investments	387,897,000	378,387,000
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	7,002,000	2,914,000
Long-term investments	602,000	392,000
Recurring basis | (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	10,381,000	13,445,000
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	141,293,000	134,911,000
Long-term investments	46,098,000	43,202,000
Recurring basis | (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	64,772,000	67,900,000
Long-term investments	14,949,000	14,151,000
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	56,607,000	58,763,000
Long-term investments	17,068,000	15,024,000
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	303,000	303,000
Recurring basis | (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	8,231,000	7,256,000
Recurring basis | (Level 2) | Treasury STRIPS
Fair value
Long-term investments	226,643,000	220,765,000
Short-term investments	135,341,000	143,931,000
Recurring basis | (Level 2) | International mutual funds
Fair value
Nuclear decommissioning trust fund	2,449,000	2,012,000
Recurring basis | (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	3,708,000	2,717,000
Long-term investments	1,775,000	1,568,000
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	89,000	86,000
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