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Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	$ 295	$ 1,569
Derivative liabilities, Estimated Fair Value	(261)	(1,306)
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	295	1,569
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities, Estimated Fair Value	$ (261)	(1,306)
Oil [Member]
Derivatives, Fair Value [Line Items]
Notional Units	Barrels
Derivative assets, Notional Amount	$ 33	83
Derivative assets, Estimated Fair Value	279	576
Derivative liabilities, Notional Amount	(33)	(83)
Derivative liabilities, Estimated Fair Value	$ (246)	(496)
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Notional Units	MMBTUs
Derivative assets, Notional Amount	$ 20	1,900
Derivative assets, Estimated Fair Value	16	993
Derivative liabilities, Notional Amount	(20)	(1,900)
Derivative liabilities, Estimated Fair Value	$ (15)	$ (810)

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Notional units of derivative assets and liabilities.

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Nominal or face amount used to calculate payments on the derivative asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
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Fair value, after offset of derivative liability, of 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, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Nominal or face amount used to calculate payments on the derivative liability.

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