Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets:
Investments	$ 5,952	$ 4,902
Energy Marketing Contracts Assets, Current	229	823
Liabilities:
Energy Marketing Contract Liabilities, Current	188	773
Equity Securities [Member]
Assets:
Investments	22	21
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Assets, Fair Value Disclosure	5,322	4,341
Quoted Prices in Active Markets (Level 1) [Member] \| Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities including natural gas futures	0	0
Quoted Prices in Active Markets (Level 1) [Member] \| Equity Securities [Member]
Assets:
Investments	22	21
Quoted Prices in Active Markets (Level 1) [Member] \| Investments in guaranteed income fund [Member]
Assets:
Investments	0	0
Quoted Prices in Active Markets (Level 1) [Member] \| Investments - other [Member]
Assets:
Investments	5,300	4,320
Quoted Prices in Active Markets (Level 1) [Member] \| Total Investments [Member]
Assets:
Investments	5,322	4,341
Quoted Prices in Active Markets (Level 1) [Member] \| Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Mark-to-market energy assets	0	0
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Assets, Fair Value Disclosure	229	823
Significant Other Observable Inputs (Level 2) [Member] \| Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities including natural gas futures		773
Energy Marketing Contract Liabilities, Current	188
Significant Other Observable Inputs (Level 2) [Member] \| Equity Securities [Member]
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) [Member] \| Investments in guaranteed income fund [Member]
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) [Member] \| Investments - other [Member]
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) [Member] \| Total Investments [Member]
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) [Member] \| Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Mark-to-market energy assets		823
Significant Unobservable Inputs (Level 3) [Member]
Assets:
Assets, Fair Value Disclosure	630	561
Significant Unobservable Inputs (Level 3) [Member] \| Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities including natural gas futures	0	0
Significant Unobservable Inputs (Level 3) [Member] \| Equity Securities [Member]
Assets:
Investments	0	0
Significant Unobservable Inputs (Level 3) [Member] \| Investments in guaranteed income fund [Member]
Assets:
Investments	630	561
Significant Unobservable Inputs (Level 3) [Member] \| Investments - other [Member]
Assets:
Investments	0	0
Significant Unobservable Inputs (Level 3) [Member] \| Total Investments [Member]
Assets:
Investments	630	561
Significant Unobservable Inputs (Level 3) [Member] \| Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Mark-to-market energy assets	0	0
Recurring [Member]
Assets:
Assets, Fair Value Disclosure	6,181	5,725
Recurring [Member] \| Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities including natural gas futures	188	773
Recurring [Member] \| Equity Securities [Member]
Assets:
Investments	22	21
Recurring [Member] \| Investments in guaranteed income fund [Member]
Assets:
Investments	630	561
Recurring [Member] \| Investments - other [Member]
Assets:
Investments	5,300	4,320
Recurring [Member] \| Total Investments [Member]
Assets:
Investments	5,952	4,902
Recurring [Member] \| Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Mark-to-market energy assets	$ 229	$ 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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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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Carrying amount as of the balance sheet date of the asset arising from energy trading activities that involves the purchase and sale of energy under forward contracts at fixed and variable prices and the buying and selling of financial energy contracts that include exchange futures and options and over the counter options and swaps, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

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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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