Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Fair value
Nuclear decommissioning trust fund	$ 444,820,000	$ 511,339,000
Long-term investments	307,412,000	254,864,000
Natural gas swaps
Fair value
Derivative liabilities	36,067,000	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	$ 36,067,000	32,256,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	134,999,000	179,346,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	78,208,000	96,204,000
Long-term investments	18,257,000	23,161,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	65,999,000	63,849,000
Long-term investments	19,744,000	20,395,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	51,244,000	45,522,000
Long-term investments	8,932,000	9,257,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	64,729,000	62,400,000
Long-term investments	15,305,000	12,867,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	38,564,000	55,522,000
Long-term investments	130,940,000	126,380,000
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund	1,302,000	1,189,000
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	3,499,000	2,586,000
Long-term investments	867,000	1,082,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	242,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	111,297,000	59,816,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	6,034,000	4,721,000
Long-term investments	2,070,000	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	134,999,000	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	51,244,000	45,522,000
Long-term investments	8,932,000	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	38,564,000	55,522,000
Long-term investments	130,940,000	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	6,034,000	4,450,000
Long-term investments	2,070,000	1,906,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	36,067,000	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	78,208,000	96,204,000
Long-term investments	18,257,000	23,161,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	65,999,000	63,849,000
Long-term investments	19,744,000	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	64,729,000	62,400,000
Long-term investments	15,305,000	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	1,302,000	1,189,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	3,499,000	2,586,000
Long-term investments	867,000	1,082,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	242,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Treasury STRIPS
Fair value
Long-term investments	111,297,000	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	0	0
Long-term investments	0	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