Derivative Instruments - Fair Values of Derivative Contracts Recorded in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Energy Marketing Contracts Assets, Current	$ 13,165	$ 1,286
Energy Marketing Contract Liabilities, Current	14,871	6,247
Hedging Liability [Member] \| Natural Gas Futures [Member]
Derivatives, Fair Value [Line Items]
Energy Marketing Contract Liabilities, Current	8,705
Not Designated as Hedging Instrument [Member] \| Mark-to-market energy liabilities \| Natural Gas Futures [Member]
Derivatives, Fair Value [Line Items]
Energy Marketing Contract Liabilities, Current	4,562	5,776
Not Designated as Hedging Instrument [Member] \| Mark To Market Energy Assets \| Propane Swap Agreement
Derivatives, Fair Value [Line Items]
Energy Marketing Contracts Assets, Current		13
Not Designated as Hedging Instrument [Member] \| Mark To Market Energy Assets \| Natural Gas Futures [Member]
Derivatives, Fair Value [Line Items]
Energy Marketing Contracts Assets, Current	4,024
Designated as Hedging Instrument [Member] \| Mark-to-market energy liabilities \| Propane Swap Agreement
Derivatives, Fair Value [Line Items]
Energy Marketing Contract Liabilities, Current	0	469
Designated as Hedging Instrument [Member] \| Mark To Market Energy Assets \| Propane Swap Agreement
Derivatives, Fair Value [Line Items]
Energy Marketing Contracts Assets, Current	11	1,181
Designated as Hedging Instrument [Member] \| Mark To Market Energy Assets \| Put Option
Derivatives, Fair Value [Line Items]
Energy Marketing Contracts Assets, Current	71
Designated as Hedging Instrument [Member] \| Mark To Market Energy Assets \| Natural Gas Futures [Member]
Derivatives, Fair Value [Line Items]
Energy Marketing Contracts Assets, Current	9,059	92
2017 [Member] \| Designated as Hedging Instrument [Member] \| Mark-to-market energy liabilities \| Propane Swap Agreement
Derivatives, Fair Value [Line Items]
Energy Marketing Contract Liabilities, Current	$ 1,604	$ 2

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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