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Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	$ 2,903	$ 6,124
Derivative liabilities, Estimated Fair Value	(1,602)	(4,756)
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	2,903	6,124
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities, Estimated Fair Value	$ (1,602)	(4,756)
Oil [Member]
Derivatives, Fair Value [Line Items]
Notional Units	Barrels
Derivative assets, Notional Amount	$ 216	312
Derivative assets, Estimated Fair Value	1,768	4,629
Derivative liabilities, Notional Amount	(216)	(312)
Derivative liabilities, Estimated Fair Value	$ (1,426)	(4,271)
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Notional Units	MMBTUs
Derivative assets, Notional Amount	$ 5,525	2,010
Derivative assets, Estimated Fair Value	1,135	1,495
Derivative liabilities, Notional Amount	(5,525)	(2,010)
Derivative liabilities, Estimated Fair Value	$ (176)	$ (485)

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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.

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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.

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

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