Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	9 Months Ended
	Sep. 30, 2022	Dec. 31, 2021
Fair value
Nuclear decommissioning trust fund	$ 503,452,000	$ 659,910,000
Long-term investments	630,411,000	711,379,000
Short-term investments	52,061,000	0
International equity trust
Fair value
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	$ 192,026,000	63,994,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	189,706,000	249,999,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	97,294,000	140,718,000
Long-term investments	29,375,000	35,873,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	63,283,000	72,936,000
Long-term investments	13,444,000	14,022,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	44,191,000	53,321,000
Long-term investments	11,141,000	15,259,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	38,779,000	40,460,000
Long-term investments	12,725,000	12,021,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	52,740,000	75,384,000
Long-term investments	264,413,000	277,937,000
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund		1,133,000
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	2,958,000	9,608,000
Long-term investments		257,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	297,346,000	350,532,000
Short-term investments	52,061,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	11,659,000	16,351,000
Long-term investments	81,000	5,478,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	2,842,000
Long-term investments	1,886,000
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	189,706,000	249,999,000
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	44,191,000	53,321,000
Long-term investments	11,141,000	15,259,000
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	52,740,000	75,384,000
Long-term investments	264,413,000	277,937,000
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
Long-term investments		0
Recurring basis | (Level 1) | Treasury STRIPS
Fair value
Long-term investments	0	0
Short-term investments	0
Recurring basis | (Level 1) | Other
Fair value
Nuclear decommissioning trust fund	11,659,000	13,623,000
Long-term investments	81,000	5,478,000
Recurring basis | (Level 1) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	0
Long-term investments	0
Recurring basis | (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	192,026,000	63,994,000
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	97,294,000	140,718,000
Long-term investments	29,375,000	35,873,000
Recurring basis | (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	63,283,000	72,369,000
Long-term investments	13,346,000	12,656,000
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	38,779,000	40,460,000
Long-term investments	12,725,000	12,021,000
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		1,133,000
Recurring basis | (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	2,958,000	9,608,000
Long-term investments		257,000
Recurring basis | (Level 2) | Treasury STRIPS
Fair value
Long-term investments	297,346,000	350,532,000
Short-term investments	52,061,000
Recurring basis | (Level 2) | Other
Fair value
Nuclear decommissioning trust fund	0	2,728,000
Long-term investments	0	0
Recurring basis | (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	2,842,000
Long-term investments	1,886,000
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	0	567,000
Long-term investments	98,000	1,366,000
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
Long-term investments		0
Recurring basis | (Level 3) | Treasury STRIPS
Fair value
Long-term investments	0	0
Short-term investments	0
Recurring basis | (Level 3) | Other
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	$ 0
Recurring basis | (Level 3) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	0
Long-term investments	$ 0