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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, 2014
Dec. 31, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]    
Gross Amounts of Recognized Assets $ 22,988 $ 22,743
Derivative Assets, Gross Amounts Offset in the Balance Sheet (22,988) (19,146)
Net Amounts of Assets Presented in the Balance Sheet 0 3,597
Gross Amounts of Recognized Liabilities 97,247 27,473
Derivative Liabilities, Gross Amounts Offset in the Balance Sheet (22,988) (19,146)
Net Amounts of Liabilities Presented in the Balance Sheet $ 74,259 $ 8,327