Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Investments	$ 10,776	$ 9,229
Fair Value, Inputs, Level 1 [Member]
Assets:
Assets, Fair Value Disclosure	8,620	8,426
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	3,269	0
Significant Other Observable Inputs (Level 2) \| Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	127	1,844
Significant Unobservable Inputs (Level 3)
Assets:
Assets, Fair Value Disclosure	2,156	803
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	21	27
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	2,156	803
Other Investments [Member] \| Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	8,599	8,399
Other Investments [Member] \| Significant Other Observable Inputs (Level 2)
Assets:
Investments	0	0
Other Investments [Member] \| Significant Unobservable Inputs (Level 3)
Assets:
Investments	0	0
Investments [Member] \| Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	8,620	8,426
Investments [Member] \| Significant Other Observable Inputs (Level 2)
Assets:
Investments	0	0
Investments [Member] \| Significant Unobservable Inputs (Level 3)
Assets:
Investments	2,156	803
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	3,269	0
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	14,045	9,229
Fair Value \| Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	127	1,844
Fair Value \| Equity Securities [Member]
Assets:
Investments	21	27
Fair Value \| Guaranteed Income Fund [Member]
Assets:
Investments	2,156	803
Fair Value \| Other Investments [Member]
Assets:
Investments	8,599	8,399
Fair Value \| Investments [Member]
Assets:
Investments	10,776	9,229
Fair Value \| Mark-to-market energy assets, including put option
Assets:
Mark-to-market energy assets, including put option	$ 3,269	$ 0

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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