Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Investments	$ 7,951	$ 6,756
Liabilities:
Energy Marketing Contract Liabilities, Current	9,774	6,247
Equity Securities [Member]
Assets:
Investments	22	22
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Assets, Fair Value Disclosure	7,272	6,108
Quoted Prices in Active Markets (Level 1) [Member] \| Equity Securities [Member]
Assets:
Investments	22	22
Quoted Prices in Active Markets (Level 1) [Member] \| Investments - Mutual funds and other [Member]
Assets:
Investments	7,250	6,086
Quoted Prices in Active Markets (Level 1) [Member] \| Total Investments [Member]
Assets:
Investments	7,272	6,108
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Assets, Fair Value Disclosure	10,568	1,286
Liabilities:
Energy Marketing Contract Liabilities, Current	9,774	6,247
Significant Other Observable Inputs (Level 2) [Member] \| Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Derivative assets	10,568	1,286
Significant Unobservable Inputs (Level 3) [Member]
Assets:
Assets, Fair Value Disclosure	679	648
Significant Unobservable Inputs (Level 3) [Member] \| Investments in guaranteed income fund [Member]
Assets:
Investments	679	648
Significant Unobservable Inputs (Level 3) [Member] \| Total Investments [Member]
Assets:
Investments	679	648
Recurring [Member]
Assets:
Assets, Fair Value Disclosure	18,519	8,042
Liabilities:
Energy Marketing Contract Liabilities, Current	9,774	6,247
Recurring [Member] \| Equity Securities [Member]
Assets:
Investments	22	22
Recurring [Member] \| Investments in guaranteed income fund [Member]
Assets:
Investments	679	648
Recurring [Member] \| Investments - Mutual funds and other [Member]
Assets:
Investments	7,250	6,086
Recurring [Member] \| Total Investments [Member]
Assets:
Investments	7,951	6,756
Recurring [Member] \| Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Derivative assets	$ 10,568	$ 1,286

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Current liabilities attributable to energy marketing activities (trading activities). Current energy marketing (trading activities) are when entities engage in both financial and physical trading to increase profits, manage their commodity price risk and enhance system reliability. They may trade electricity, coal, natural gas and oil. These entities typically use a variety of financial instruments, including forward contracts, options and swaps.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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