Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Fair value
Nuclear decommissioning trust fund	$ 615,247,000	$ 659,910,000
Long-term investments	702,821,000	711,379,000
International equity trust
Fair value
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	$ 173,946,000	63,994,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	237,403,000	249,999,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	126,621,000	140,718,000
Long-term investments	36,959,000	35,873,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	47,987,000	72,936,000
Long-term investments	4,129,000	14,022,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	102,406,000	53,321,000
Long-term investments	40,382,000	15,259,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	22,192,000	40,460,000
Long-term investments	0	12,021,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	68,348,000	75,384,000
Long-term investments	271,155,000	277,937,000
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund	809,000	1,133,000
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	2,268,000	9,608,000
Long-term investments	0	257,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	350,275,000	350,532,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	7,213,000	16,351,000
Long-term investments		5,478,000
Other liabilities	(79,000)
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	237,403,000	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	102,406,000	53,321,000
Long-term investments	40,382,000	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	68,348,000	75,384,000
Long-term investments	271,155,000	277,937,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds
Fair value
Nuclear decommissioning trust fund	0	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	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Treasury STRIPS
Fair value
Long-term investments	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value
Nuclear decommissioning trust fund	7,213,000	13,623,000
Long-term investments		5,478,000
Other liabilities	(79,000)
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	173,946,000	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	126,621,000	140,718,000
Long-term investments	36,959,000	35,873,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	46,718,000	72,369,000
Long-term investments	2,938,000	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	22,192,000	40,460,000
Long-term investments	0	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	809,000	1,133,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	2,268,000	9,608,000
Long-term investments	0	257,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Treasury STRIPS
Fair value
Long-term investments	350,275,000	350,532,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Other
Fair value
Nuclear decommissioning trust fund	0	2,728,000
Long-term investments		0
Other liabilities	0
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	1,269,000	567,000
Long-term investments	1,191,000	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	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Treasury STRIPS
Fair value
Long-term investments	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Other
Fair value
Nuclear decommissioning trust fund	0	0
Long-term investments		$ 0
Other liabilities	$ 0