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Derivative Instruments - Summary of Gross and Net Information about Commodity Derivative Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Gross Amounts of Recognized Assets	$ 44,565	$ 68,970
Derivative Assets, Gross Amounts Offset in the Balance Sheet	(26,658)	(44,973)
Net Amounts of Assets Presented in the Balance Sheet	17,907	23,997
Gross Amounts of Recognized Liabilities	26,949	46,401
Derivative Liabilities, Gross Amounts Offset in the Balance Sheet	(26,658)	(44,973)
Net Amounts of Liabilities Presented in the Balance Sheet	$ 291	$ 1,428

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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- 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.

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