Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair value
Nuclear decommissioning trust fund	$ 540,726,000	$ 511,339,000
Long-term investments	416,418,000	254,864,000
Natural gas swaps
Fair value
Derivative liabilities	4,808,000	32,256,000
International equity trust
Fair value
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	$ 4,808,000	32,256,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	177,002,000	179,346,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	102,877,000	96,204,000
Long-term investments	26,751,000	23,161,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	95,579,000	63,849,000
Long-term investments	30,260,000	20,395,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	40,416,000	45,522,000
Long-term investments	4,034,000	9,257,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	49,749,000	62,400,000
Long-term investments	12,757,000	12,867,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	55,568,000	55,522,000
Long-term investments	162,346,000	126,380,000
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund	1,407,000	1,189,000
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	8,699,000	2,586,000
Long-term investments	625,000	1,082,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	2,055,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	176,672,000	59,816,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	7,374,000	4,721,000
Long-term investments	2,973,000	1,906,000
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	177,002,000	179,346,000
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	40,416,000	45,522,000
Long-term investments	4,034,000	9,257,000
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	55,568,000	55,522,000
Long-term investments	162,346,000	126,380,000
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) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	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	7,374,000	4,450,000
Long-term investments	2,973,000	1,906,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	4,808,000	32,256,000
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	102,877,000	96,204,000
Long-term investments	26,751,000	23,161,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	95,237,000	63,849,000
Long-term investments	30,050,000	20,395,000
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	49,749,000	62,400,000
Long-term investments	12,757,000	12,867,000
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	1,407,000	1,189,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	8,699,000	2,586,000
Long-term investments	625,000	1,082,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	2,055,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Treasury STRIPS
Fair value
Long-term investments	176,672,000	59,816,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Other
Fair value
Nuclear decommissioning trust fund	0	271,000
Long-term investments	0	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	342,000	0
Long-term investments	210,000	0
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) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	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	$ 0