Derivative Instruments - Summary of Gross and Net Information about Commodity Derivative Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Gross Amounts of Recognized Assets	$ 153,394	$ 331,121
Derivative Assets, Gross Amounts Offset in the Balance Sheet	(17,948)	(15,614)
Net Amounts of Assets Presented in the Balance Sheet	135,446	315,507
Gross Amounts of Recognized Liabilities	18,038	15,614
Derivative Liabilities, Gross Amounts Offset in the Balance Sheet	(17,948)	(15,614)
Net Amounts of Liabilities Presented in the Balance Sheet	$ 90	$ 0

Fair value of liability associated with 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.

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