Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) (USD $) In Thousands, unless otherwise specified			9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Oil [Member]	Sep. 30, 2014 Natural Gas [Member]	Sep. 30, 2014 Other Liabilities [Member]	Sep. 30, 2014 Other Assets [Member]
Derivatives, Fair Value [Line Items]
Notional Units			Barrels	MMBTUs
Derivative assets, Notional Amount			$ 288	$ 1,718
Derivative assets, Estimated Fair Value	2,648	3,049	935	1,713		2,648
Derivative liabilities, Notional Amount			(288)	(1,718)
Derivative liabilities, Estimated Fair Value	$ (1,250)	$ (1,886)	$ (558)	$ (692)	$ 1,250

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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Fair value, after offset of derivative asset, of financial liability or 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, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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