Natural Gas Derivative Instruments - Additional Information (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) MMBTU $ / MMBTU	Dec. 31, 2016 USD ($) $ / MMBTU
Natural gas derivative contract
Derivative [Line Items]
Estimated cash flows, discount rate	8.00%
Last derivative contract expiration month and year	2018-12
Contracts volume, derivative instruments \| MMBTU	1,680,000
Average price \| $ / MMBTU	3.11	2.46
Natural gas derivative contract \| 2015 through December 31, 2018
Derivative [Line Items]
Contracts volume, derivative instruments \| MMBTU	1,800,000
Average price \| $ / MMBTU	4.40
Cost of Sales
Derivative [Line Items]
Gain (loss) on derivative instruments \| $	$ (917)	$ 1,886

Derivative maturity month year.

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- Definition

The average forward price on the group of price risk forward or futures contracts, such as gas futures contracts.

+ References

No definition available.

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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 energy measure used to calculate payments on a derivative instrument.

+ References

No definition available.

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ 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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