Offsetting of Assets and Liabilities Related to Derivative Contracts (Detail) (Commodity derivatives, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Commodity derivatives
Derivative Financial Instruments [Line Items]
Gross amounts of recognized liabilities	$ 4,667	$ 2,271
Gross amounts offset in the consolidated balance sheet	(115)	(2,169)
Net amounts of liabilities presented in the consolidated balance sheet	4,552	102
Gross Amounts of Recognized Assets	6,203	3,111
Gross amounts offset in the consolidated balance sheet	(115)	(2,169)
Net Amounts of Assets Presented in the Consolidated Balance Sheet	$ 6,088	$ 942

Fair value, after the effects of master netting arrangements, of a 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. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value 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, elected not to be offset against an obligation to return collateral under a master netting arrangement.

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- Definition

Fair value 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 offset against a right to receive collateral under a master netting arrangement.

+ References+ Details

- Definition

Fair value, after the effects of master netting arrangements, of a 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. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References+ Details