Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Investments	$ 9,229	$ 6,711
Fair Value, Inputs, Level 1 [Member]
Assets:
Assets, Fair Value Disclosure	8,426	6,025
Fair Value, Inputs, Level 1 [Member] | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	0
Significant Other Observable Inputs (Level 2)
Assets:
Assets, Fair Value Disclosure	0	82
Significant Other Observable Inputs (Level 2) | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	1,844	1,604	[1]
Significant Unobservable Inputs (Level 3)
Assets:
Assets, Fair Value Disclosure	803	686
Significant Unobservable Inputs (Level 3) | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	0
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	27	22
Equity Securities [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0
Equity Securities [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments	0
Guaranteed Income Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	0
Guaranteed Income Fund [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0
Guaranteed Income Fund [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments	803	686
Other Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	8,399	6,003
Other Investments [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0
Other Investments [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments	0
Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	8,426	6,025
Investments [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0	0
Investments [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments	803	686
Mark-to-market energy assets, including put option | Fair Value, Inputs, Level 1 [Member]
Assets:
Mark-to-market energy assets, including put option	0
Mark-to-market energy assets, including put option | Significant Other Observable Inputs (Level 2)
Assets:
Mark-to-market energy assets, including put option	0	82	[1]
Mark-to-market energy assets, including put option | Significant Unobservable Inputs (Level 3)
Assets:
Mark-to-market energy assets, including put option	0
Fair Value
Assets:
Assets, Fair Value Disclosure	9,229	6,793
Fair Value | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	1,844	1,604	[1]
Fair Value | Equity Securities [Member]
Assets:
Investments	27	22
Fair Value | Guaranteed Income Fund [Member]
Assets:
Investments	803	686
Fair Value | Other Investments [Member]
Assets:
Investments	8,399	6,003
Fair Value | Investments [Member]
Assets:
Investments	9,229	6,711
Fair Value | Mark-to-market energy assets, including put option
Assets:
Mark-to-market energy assets, including put option	$ 0	$ 82	[1]

[1] ⁽¹⁾As discussed in Note 4, Acquisitions and Divestitures, during the fourth quarter of 2019, we sold PESCO's assets and contracts. PESCO's derivative assets and liabilities are reflected as assets held-for-sale in the consolidated balance sheet as of December 31, 2018