Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	9 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Fair value
Nuclear decommissioning trust fund	$ 576,499,000	$ 540,716,000
Long-term investments	666,714,000	669,479,000
Short-term investments	124,919,000	61,702,000
International equity trust
Fair value
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	$ 47,054,000	131,804,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	210,135,000	204,129,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	121,884,000	111,266,000
Long-term investments	37,721,000	33,606,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	68,052,000	60,806,000
Long-term investments	12,523,000	10,473,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	43,616,000	49,775,000
Long-term investments	15,056,000	15,488,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	42,443,000	41,210,000
Long-term investments	14,288,000	12,113,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	71,496,000	57,348,000
Long-term investments	338,327,000	302,302,000
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund	282,000
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	8,199,000	2,037,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	246,911,000	293,281,000
Short-term investments	124,919,000	61,702,000
Recurring basis | International mutual funds
Fair value
Nuclear decommissioning trust fund	861,000	653,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	2,483,000	2,890,000
Long-term investments	1,377,000	1,976,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	7,048,000	10,602,000
Long-term investments	511,000	240,000
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	210,135,000	204,129,000
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	43,616,000	49,775,000
Long-term investments	15,056,000	15,488,000
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	71,496,000	57,348,000
Long-term investments	338,327,000	302,302,000
Recurring basis | (Level 1) | Municipal bonds
Fair value
Nuclear decommissioning trust fund	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	7,048,000	10,602,000
Long-term investments	511,000	240,000
Recurring basis | (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	47,054,000	131,804,000
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	121,884,000	111,266,000
Long-term investments	37,721,000	33,606,000
Recurring basis | (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	68,013,000	60,788,000
Long-term investments	12,523,000	10,473,000
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	42,443,000	41,210,000
Long-term investments	14,288,000	12,113,000
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	282,000
Recurring basis | (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	8,199,000	2,037,000
Recurring basis | (Level 2) | Treasury STRIPS
Fair value
Long-term investments	246,911,000	293,281,000
Short-term investments	124,919,000	61,702,000
Recurring basis | (Level 2) | International mutual funds
Fair value
Nuclear decommissioning trust fund	861,000	653,000
Recurring basis | (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	2,483,000	2,890,000
Long-term investments	1,377,000	1,976,000
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	39,000	18,000
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
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