Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Investments	$ 7,509	$ 6,711
Energy Marketing Contracts Assets, Current	9,221	13,165
Liabilities:
Energy Marketing Contract Liabilities, Current	6,798	14,871
Equity Securities [Member]
Assets:
Investments	25	22
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Assets, Fair Value Disclosure	6,814	6,025
Quoted Prices in Active Markets (Level 1) [Member] \| Equity Securities [Member]
Assets:
Investments	25	22
Quoted Prices in Active Markets (Level 1) [Member] \| Investments - Mutual funds and other [Member]
Assets:
Investments	6,789	6,003
Quoted Prices in Active Markets (Level 1) [Member] \| Total Investments [Member]
Assets:
Investments	6,814	6,025
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Assets, Fair Value Disclosure	9,221	13,165
Liabilities:
Energy Marketing Contract Liabilities, Current	6,798
Significant Other Observable Inputs (Level 2) [Member] \| Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Derivative assets	9,221
Significant Unobservable Inputs (Level 3) [Member]
Assets:
Assets, Fair Value Disclosure	695	686
Significant Unobservable Inputs (Level 3) [Member] \| Investments in guaranteed income fund [Member]
Assets:
Investments	695	686
Significant Unobservable Inputs (Level 3) [Member] \| Total Investments [Member]
Assets:
Investments	695	686
Recurring [Member]
Assets:
Assets, Fair Value Disclosure	16,730	19,876
Liabilities:
Energy Marketing Contract Liabilities, Current	6,798	14,871
Recurring [Member] \| Equity Securities [Member]
Assets:
Investments	25	22
Recurring [Member] \| Investments in guaranteed income fund [Member]
Assets:
Investments	695	686
Recurring [Member] \| Investments - Mutual funds and other [Member]
Assets:
Investments	6,789	6,003
Recurring [Member] \| Total Investments [Member]
Assets:
Investments	7,509	6,711
Recurring [Member] \| Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Derivative assets	$ 9,221	$ 13,165

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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