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

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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