Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Fair value
Nuclear decommissioning trust fund	$ 608,747,000	$ 598,181,000
Long-term investments	565,662,000	518,065,000
Natural gas swaps
Fair value
Derivative liabilities	1,862,000	10,248,000
International equity trust
Fair value
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	$ 1,862,000	10,248,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	201,033,000	198,325,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	121,388,000	120,645,000
Long-term investments	31,582,000	31,378,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	89,338,000	98,129,000
Long-term investments	23,668,000	29,870,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	51,899,000	46,963,000
Long-term investments	13,468,000	7,437,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	42,280,000	45,039,000
Long-term investments	14,493,000	11,432,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	71,299,000	70,813,000
Long-term investments	231,055,000	224,536,000
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund	1,063,000	1,362,000
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	12,190,000	6,054,000
Long-term investments	449,000	537,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	3,263,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	245,746,000	209,165,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	14,994,000	10,851,000
Long-term investments	5,201,000	3,710,000
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	201,033,000	198,325,000
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	51,899,000	46,963,000
Long-term investments	13,468,000	7,437,000
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	71,299,000	70,813,000
Long-term investments	231,055,000	224,536,000
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	14,994,000	7,720,000
Long-term investments	5,201,000	3,710,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	1,862,000	10,248,000
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	121,388,000	120,645,000
Long-term investments	31,582,000	31,378,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	89,000,000	97,788,000
Long-term investments	23,461,000	29,661,000
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	42,280,000	45,039,000
Long-term investments	14,493,000	11,432,000
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	1,063,000	1,362,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	12,190,000	6,054,000
Long-term investments	449,000	537,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	3,263,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Treasury STRIPS
Fair value
Long-term investments	245,746,000	209,165,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Other
Fair value
Nuclear decommissioning trust fund	0	3,131,000
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	338,000	341,000
Long-term investments	207,000	209,000
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) | 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