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Derivative Instruments - Summary of Gross and Net Information about Commodity Derivative Instruments (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]    
Gross Amounts of Recognized Assets $ 56,752 $ 22,743
Derivative Assets, Gross Amounts Offset in the Balance Sheet (16,456) (19,146)
Net Amounts of Assets Presented in the Balance Sheet 40,296 3,597
Gross Amounts of Recognized Liabilities 16,456 27,473
Derivative Liabilities, Gross Amounts Offset in the Balance Sheet (16,456) (19,146)
Net Amounts of Liabilities Presented in the Balance Sheet $ 0 $ 8,327