Derivative Instruments - Additional Information (Detail) gal in Thousands, $ in Thousands, Mcf in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2017 USD ($) $ / gal gal Mcf	Dec. 31, 2016 USD ($) $ / gal gal
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| Mcf	1.4
Energy Marketing Contracts Assets, Current	$ 1,526	$ 823
Energy Marketing Contract Liabilities, Current	(1,732)	(773)
Hedging Liability [Member] \| Natural Gas Futures [Member]
Derivative [Line Items]
Energy Marketing Contract Liabilities, Current	(1,307)
Hedging Liability [Member] \| Natural Gas Swaps [Member]
Derivative [Line Items]
Energy Marketing Contract Liabilities, Current	(412)
Designated as Hedging Instrument [Member] \| Mark To Market Energy Assets [Member] \| Propane Swap Agreement [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	1,510
Not Designated as Hedging Instrument [Member] \| Mark To Market Energy Assets [Member] \| Natural Gas Swaps [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	1
Not Designated as Hedging Instrument [Member] \| Mark To Market Energy Assets [Member] \| Propane Swap Agreement [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	15	8
Not Designated as Hedging Instrument [Member] \| Mark To Market Energy Assets [Member] \| Put Option [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current		9
Not Designated as Hedging Instrument [Member] \| Mark-to-market energy liabilities [Member] \| Natural Gas Futures [Member]
Derivative [Line Items]
Energy Marketing Contract Liabilities, Current	$ (13)	$ (773)
PESCO [Member] \| Natural Gas Futures [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| Mcf	4.0
Derivative, Term of Contract	2 years
Energy Marketing Contract Liabilities, Current	$ (1,300)
Sharp Energy Inc [Member] \| Put Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal		630
Payments for Derivative Instrument, Financing Activities		$ 33
Derivative, Price Risk Option Strike Price \| $ / gal		0.5650
2017 [Member] \| Natural Gas Swaps [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| Mcf	12.0
2017 [Member] \| Propane Swap Agreement [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	11,100
2017 [Member] \| Strike Price 4 [Member] \| Propane Swap Agreement [Member]
Derivative [Line Items]
Strike Price Per Gallon For The Propane Swap Agreements \| $ / gal	0.59
2017 [Member] \| Strike Price 2 [Member] \| Propane Swap Agreement [Member]
Derivative [Line Items]
Strike Price Per Gallon For The Propane Swap Agreements \| $ / gal	0.675
2017 [Member] \| Designated as Hedging Instrument [Member] \| Mark-to-market energy liabilities [Member] \| Propane Swap Agreement [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	$ 1,500
2016 [Member] \| Propane Swap Agreement [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	4,800
Gain (Loss) on Hedging Activity	$ 193
2016 [Member] \| Strike Price 4 [Member] \| Propane Swap Agreement [Member]
Derivative [Line Items]
Strike Price Per Gallon For The Propane Swap Agreements \| $ / gal	0.5225
2016 [Member] \| Strike Price 2 [Member] \| Propane Swap Agreement [Member]
Derivative [Line Items]
Strike Price Per Gallon For The Propane Swap Agreements \| $ / gal	0.5650

Strike Price Per Gallon For The Propane Swap Agreements

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- Definition

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.

+ References

No definition available.

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- Definition

Nominal volume used to calculate payments on a derivative instrument.

+ References

No definition available.

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- Definition

The strike price on the price risk option contract such as a put option or a call option.

+ References

No definition available.

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- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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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- Definition

Gain (loss) incurred in hedging activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959

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- Definition

The cash outflow for derivative instruments during the period, which are classified as financing activities, excluding those designated as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3505-108585

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