Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Investments	$ 6,380	$ 4,902
Liabilities:
Energy Marketing Contract Liabilities, Current	1,732	773
Equity Securities [Member]
Assets:
Investments	22	21
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Assets, Fair Value Disclosure	5,738	4,341
Quoted Prices in Active Markets (Level 1) [Member] \| Equity Securities [Member]
Assets:
Investments	22	21
Quoted Prices in Active Markets (Level 1) [Member] \| Investments - other [Member]
Assets:
Investments	5,716	4,320
Quoted Prices in Active Markets (Level 1) [Member] \| Total Investments [Member]
Assets:
Investments	5,738	4,341
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Assets, Fair Value Disclosure	1,526	823
Liabilities:
Energy Marketing Contract Liabilities, Current	1,732	773
Significant Other Observable Inputs (Level 2) [Member] \| Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Derivative assets	1,526	823
Significant Unobservable Inputs (Level 3) [Member]
Assets:
Assets, Fair Value Disclosure	642	561
Significant Unobservable Inputs (Level 3) [Member] \| Investments in guaranteed income fund [Member]
Assets:
Investments	642	561
Significant Unobservable Inputs (Level 3) [Member] \| Total Investments [Member]
Assets:
Investments	642	561
Recurring [Member]
Assets:
Assets, Fair Value Disclosure	7,906	5,725
Liabilities:
Energy Marketing Contract Liabilities, Current	1,732	773
Recurring [Member] \| Equity Securities [Member]
Assets:
Investments	22	21
Recurring [Member] \| Investments in guaranteed income fund [Member]
Assets:
Investments	642	561
Recurring [Member] \| Investments - other [Member]
Assets:
Investments	5,716	4,320
Recurring [Member] \| Total Investments [Member]
Assets:
Investments	6,380	4,902
Recurring [Member] \| Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Derivative assets	$ 1,526	$ 823

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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-Name Accounting Standards Codification
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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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