Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Fair value
Nuclear decommissioning trust fund	$ 462,043,000	$ 420,818,000
Long-term investments	178,186,000	164,125,000
Natural gas swaps
Fair value
Derivative liabilities	8,867,000	13,154,000
International equity trust
Fair value
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	$ 8,867,000	13,154,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	154,952,000	136,196,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	86,813,000	76,852,000
Long-term investments	20,885,000	17,382,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	57,108,000	51,356,000
Long-term investments	11,658,000	12,571,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	51,850,000	47,712,000
Long-term investments	14,277,000	12,062,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	51,222,000	56,004,000
Long-term investments	11,791,000	11,517,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	50,292,000	43,359,000
Long-term investments	94,326,000	94,494,000
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund	425,000	278,000
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	3,942,000	6,066,000
Long-term investments	934,000	941,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	1,425,000	964,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	23,677,000	14,113,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	4,014,000	2,031,000
Long-term investments	638,000	1,045,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value
Nuclear decommissioning trust fund	154,952,000	136,196,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury securities
Fair value
Nuclear decommissioning trust fund	51,850,000	47,712,000
Long-term investments	14,277,000	12,062,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	50,292,000	43,359,000
Long-term investments	94,326,000	94,494,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value
Nuclear decommissioning trust fund	4,014,000	2,031,000
Long-term investments	638,000	1,045,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	8,867,000	13,154,000
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value
Nuclear decommissioning trust fund	86,813,000	76,852,000
Long-term investments	20,885,000	17,382,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	56,297,000	48,853,000
Long-term investments	11,152,000	11,366,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	51,222,000	56,004,000
Long-term investments	11,791,000	11,517,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair value
Nuclear decommissioning trust fund	425,000	278,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	3,942,000	6,066,000
Long-term investments	934,000	941,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	1,425,000	964,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Treasury STRIPS
Fair value
Long-term investments	23,677,000	14,113,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	811,000	2,503,000
Long-term investments	$ 506,000	$ 1,205,000