Derivative Instruments - Additional Information (Detail)	1 Months Ended	12 Months Ended
Sep. 30, 2016 MMBTU	Dec. 31, 2017 USD ($) MMBTU
Derivative [Line Items]
Derivatives, net liability position \| $		$ 1,500,000
Collateral posted with counterparties related to derivatives \| $		$ 0
Natural Gas Swap \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative [Line Items]
Aggregate notional amount outstanding (in mmBTUs) \| MMBTU		3,540,000
Interest Rate Swap \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative [Line Items]
Derivative instrument term (not beyond)	4 years
Aggregate notional amount outstanding (in mmBTUs) \| MMBTU	100,000,000
Average fixed rate (as a percent)		1.323%
Interest Rate Swap \| Cash Flow Hedging \| Designated as Hedging Instrument \| London Interbank Offered Rate (LIBOR)
Derivative [Line Items]
Floor rate (as a percent)		0.75%
Maximum
Derivative [Line Items]
Derivative instrument term (not beyond)		1 year

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624258-113959

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Balance Type:	debit
Period Type:	instant

- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ References

No definition available.

+ Details

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- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

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- Definition

Floor rate on an interest rate derivative such as an interest rate floor or collar. If market rates falls below the floor rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

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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.

+ Details

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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.

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- Details

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- Details

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