Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Fair value measurement
Long-term investments	$ 118,659,000	$ 99,874,000
Natural gas swaps
Fair value measurement
Derivative assets	858,000	15,090,000
Derivative liabilities	(858,000)	(15,090,000)
Recurring basis \| Natural gas swaps
Fair value measurement
Derivative assets	858,000
Derivative liabilities		(15,090,000)
Recurring basis \| Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	132,230,000	170,408,000
Recurring basis \| International equity trust
Fair value measurement
Nuclear decommissioning trust funds	77,338,000	66,861,000
Long-term investments	19,707,000	15,946,000
Recurring basis \| Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	66,221,000	60,019,000
Long-term investments	14,025,000	11,853,000
Recurring basis \| US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	74,414,000	65,725,000
Long-term investments	12,134,000	12,187,000
Recurring basis \| Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	14,743,000	17,410,000
Long-term investments	1,344,000	1,651,000
Recurring basis \| Mutual funds
Fair value measurement
Nuclear decommissioning trust funds	45,353,000
Long-term investments	71,098,000	57,932,000
Recurring basis \| Municipal bonds
Fair value measurement
Nuclear decommissioning trust funds	471,000	943,000
Recurring basis \| Other
Fair value measurement
Nuclear decommissioning trust funds	3,360,000	4,663,000
Long-term investments	351,000	305,000
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	132,230,000	170,408,000
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	74,414,000	65,725,000
Long-term investments	12,134,000	12,187,000
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Mutual funds
Fair value measurement
Nuclear decommissioning trust funds	45,353,000
Long-term investments	71,098,000	57,932,000
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Other
Fair value measurement
Nuclear decommissioning trust funds	3,360,000	4,663,000
Long-term investments	351,000	305,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Natural gas swaps
Fair value measurement
Derivative assets	858,000
Derivative liabilities		(15,090,000)
Recurring basis \| Significant Other Observable Inputs (Level 2) \| International equity trust
Fair value measurement
Nuclear decommissioning trust funds	77,338,000	66,861,000
Long-term investments	19,707,000	15,946,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	66,221,000	60,019,000
Long-term investments	14,025,000	11,853,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	14,743,000	17,410,000
Long-term investments	1,344,000	1,651,000
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Municipal bonds
Fair value measurement
Nuclear decommissioning trust funds	$ 471,000	$ 943,000

The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning facilities, whether such amount is presented as a separate caption or as a parenthetical disclosure on the balance sheet. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
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-Section 50
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