Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Fair value measurement
Long-term investments	$ 151,399,000	$ 140,622,000
Natural gas swaps
Fair value measurement
Derivative liabilities	11,382,000	6,328,000
International equity trust
Fair value measurement
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value measurement
Derivative liabilities	$ 11,382,000	6,328,000
Recurring basis | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	146,480,000	142,419,000
Recurring basis | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	86,840,000	88,820,000
Long-term investments	19,632,000	20,071,000
Recurring basis | Corporate bonds and debt
Fair value measurement
Nuclear decommissioning trust funds	51,481,000	66,317,000
Long-term investments	13,200,000	16,215,000
Recurring basis | US Treasury securities
Fair value measurement
Nuclear decommissioning trust funds	42,148,000	38,791,000
Long-term investments	7,643,000	6,670,000
Recurring basis | Mortgage backed securities
Fair value measurement
Nuclear decommissioning trust funds	56,752,000	49,379,000
Long-term investments	11,062,000	7,267,000
Recurring basis | Domestic mutual funds
Fair value measurement
Nuclear decommissioning trust funds	50,776,000	47,833,000
Long-term investments	91,714,000	87,011,000
Recurring basis | Municipal bonds
Fair value measurement
Nuclear decommissioning trust funds	288,000	92,000
Recurring basis | Federal agency securities
Fair value measurement
Nuclear decommissioning trust funds	5,164,000	3,725,000
Long-term investments	452,000	259,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value measurement
Nuclear decommissioning trust funds	1,466,000
Recurring basis | Treasury STRIPS
Fair value measurement
Long-term investments	4,641,000
Recurring basis | Other
Fair value measurement
Nuclear decommissioning trust funds	5,590,000	7,679,000
Long-term investments	3,055,000	3,129,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	146,480,000	142,419,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury securities
Fair value measurement
Nuclear decommissioning trust funds	42,148,000	38,791,000
Long-term investments	7,643,000	6,670,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic mutual funds
Fair value measurement
Nuclear decommissioning trust funds	50,776,000	47,833,000
Long-term investments	91,714,000	87,011,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value measurement
Nuclear decommissioning trust funds	5,590,000	7,679,000
Long-term investments	3,055,000	3,129,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value measurement
Derivative liabilities	11,382,000	6,328,000
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	86,840,000	88,820,000
Long-term investments	19,632,000	20,071,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds and debt
Fair value measurement
Nuclear decommissioning trust funds	48,172,000	66,317,000
Long-term investments	11,512,000	16,215,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage backed securities
Fair value measurement
Nuclear decommissioning trust funds	56,752,000	49,379,000
Long-term investments	11,062,000	7,267,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair value measurement
Nuclear decommissioning trust funds	288,000	92,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Federal agency securities
Fair value measurement
Nuclear decommissioning trust funds	5,164,000	3,725,000
Long-term investments	452,000	$ 259,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Non-US Gov't bonds & private placements
Fair value measurement
Nuclear decommissioning trust funds	1,466,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Treasury STRIPS
Fair value measurement
Long-term investments	4,641,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Corporate bonds and debt
Fair value measurement
Nuclear decommissioning trust funds	3,309,000
Long-term investments	$ 1,688,000