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Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Assets: |  |
Investments | $ 6,711 | $ 6,756
Fair Value, Inputs, Level 1 [Member] |  |
Assets: |  |
Assets, Fair Value Disclosure | 6,025 | 6,108
Fair Value, Inputs, Level 1 [Member] | Mark-to-market energy liabilities |  |
Liabilities: |  |
Mark-to-market energy liabilities | 0 | 0
Significant Other Observable Inputs (Level 2) |  |
Assets: |  |
Assets, Fair Value Disclosure | 13,165 | 1,286
Significant Other Observable Inputs (Level 2) | Mark-to-market energy liabilities |  |
Liabilities: |  |
Mark-to-market energy liabilities | 14,871 | 6,247
Significant Unobservable Inputs (Level 3) |  |
Assets: |  |
Assets, Fair Value Disclosure | 686 | 648
Significant Unobservable Inputs (Level 3) | Mark-to-market energy liabilities |  |
Liabilities: |  |
Mark-to-market energy liabilities | 0 | 0
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Assets: |  |
Investments | 22 | 22
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) |  |
Assets: |  |
Investments | 0 | 0
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) |  |
Assets: |  |
Investments | 0 | 0
Guaranteed Income Fund [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Assets: |  |
Investments | 0 | 0
Guaranteed Income Fund [Member] | Significant Other Observable Inputs (Level 2) |  |
Assets: |  |
Investments | 0 | 0
Guaranteed Income Fund [Member] | Significant Unobservable Inputs (Level 3) |  |
Assets: |  |
Investments | 686 | 648
Investments - other | Fair Value, Inputs, Level 1 [Member] |  |
Assets: |  |
Investments | 6,003 | 6,086
Investments - other | Significant Other Observable Inputs (Level 2) |  |
Assets: |  |
Investments | 0 | 0
Investments - other | Significant Unobservable Inputs (Level 3) |  |
Assets: |  |
Investments | 0 | 0
Investments [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Assets: |  |
Investments | 6,025 | 6,108
Investments [Member] | Significant Other Observable Inputs (Level 2) |  |
Assets: |  |
Investments | 0 | 0
Investments [Member] | Significant Unobservable Inputs (Level 3) |  |
Assets: |  |
Investments | 686 | 648
Mark-to-market energy assets, including put option | Fair Value, Inputs, Level 1 [Member] |  |
Assets: |  |
Mark-to-market energy assets, including put option | 0 | 0
Mark-to-market energy assets, including put option | Significant Other Observable Inputs (Level 2) |  |
Assets: |  |
Mark-to-market energy assets, including put option | 13,165 | 1,286
Mark-to-market energy assets, including put option | Significant Unobservable Inputs (Level 3) |  |
Assets: |  |
Mark-to-market energy assets, including put option | 0 | 0
Fair Value |  |
Assets: |  |
Assets, Fair Value Disclosure | 19,876 | 8,042
Fair Value | Mark-to-market energy liabilities |  |
Liabilities: |  |
Mark-to-market energy liabilities | 14,871 | 6,247
Fair Value | Equity Securities [Member] |  |
Assets: |  |
Investments | 22 | 22
Fair Value | Guaranteed Income Fund [Member] |  |
Assets: |  |
Investments | 686 | 648
Fair Value | Investments - other |  |
Assets: |  |
Investments | 6,003 | 6,086
Fair Value | Investments [Member] |  |
Assets: |  |
Investments | 6,711 | 6,756
Fair Value | Mark-to-market energy assets, including put option |  |
Assets: |  |
Mark-to-market energy assets, including put option | $ 13,165 | $ 1,286

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