Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	6 Months Ended
	Jun. 30, 2022	Dec. 31, 2021
Fair value
Nuclear decommissioning trust fund	$ 535,714,000	$ 659,910,000
Long-term investments	648,297,000	711,379,000
Short-term investments	42,691,000	0
International equity trust
Fair value
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	$ 184,859,000	63,994,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	199,520,000	249,999,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	109,140,000	140,718,000
Long-term investments	32,940,000	35,873,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	86,470,000	72,936,000
Long-term investments	19,749,000	14,022,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	41,590,000	53,321,000
Long-term investments	157,000	15,259,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	24,902,000	40,460,000
Long-term investments	10,976,000	12,021,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	54,219,000	75,384,000
Long-term investments	264,867,000	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	3,138,000	9,608,000
Long-term investments		257,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	308,949,000	350,532,000
Short-term investments	42,691,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	13,747,000	16,351,000
Long-term investments	9,218,000	5,478,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	2,988,000
Long-term investments	1,441,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Natural gas swaps
Fair value
Derivative liabilities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value
Nuclear decommissioning trust fund	199,520,000	249,999,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | International equity trust
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury securities
Fair value
Nuclear decommissioning trust fund	41,590,000	53,321,000
Long-term investments	157,000	15,259,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	54,219,000	75,384,000
Long-term investments	264,867,000	277,937,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds
Fair value
Nuclear decommissioning trust fund		0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments		0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Treasury STRIPS
Fair value
Long-term investments	0	0
Short-term investments	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value
Nuclear decommissioning trust fund	13,747,000	13,623,000
Long-term investments	9,218,000	5,478,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	0
Long-term investments	0
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	184,859,000	63,994,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Domestic equity
Fair value
Nuclear decommissioning trust fund	0	0
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value
Nuclear decommissioning trust fund	109,140,000	140,718,000
Long-term investments	32,940,000	35,873,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	86,470,000	72,369,000
Long-term investments	19,749,000	12,656,000
Recurring basis | Significant Other Observable Inputs (Level 2) | US Treasury securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	24,902,000	40,460,000
Long-term investments	10,976,000	12,021,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair value
Nuclear decommissioning trust fund		1,133,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	3,138,000	9,608,000
Long-term investments		257,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Treasury STRIPS
Fair value
Long-term investments	308,949,000	350,532,000
Short-term investments	42,691,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Other
Fair value
Nuclear decommissioning trust fund	0	2,728,000
Long-term investments	0	0
Recurring basis | Significant Other Observable Inputs (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	2,988,000
Long-term investments	1,441,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Natural gas swaps
Fair value
Derivative liabilities	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Domestic equity
Fair value
Nuclear decommissioning trust fund	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | International equity trust
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	0	567,000
Long-term investments	0	1,366,000
Recurring basis | Significant Unobservable Inputs (Level 3) | US Treasury securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Municipal bonds
Fair value
Nuclear decommissioning trust fund		0
Recurring basis | Significant Unobservable Inputs (Level 3) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments		0
Recurring basis | Significant Unobservable Inputs (Level 3) | Treasury STRIPS
Fair value
Long-term investments	0	0
Short-term investments	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Other
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	$ 0
Recurring basis | Significant Unobservable Inputs (Level 3) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	0
Long-term investments	$ 0