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Derivative Instruments - Summary of Gross and Net Information about Commodity Derivative Instruments (Detail) - USD ($)
$ in Thousands
Jun. 30, 2015
Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]    
Gross Amounts of Recognized Assets $ 127,254 $ 331,121
Derivative Assets, Gross Amounts Offset in the Balance Sheet (17,435) (15,614)
Net Amounts of Assets Presented in the Balance Sheet 109,819 315,507
Gross Amounts of Recognized Liabilities 17,435 15,614
Derivative Liabilities, Gross Amounts Offset in the Balance Sheet (17,435) (15,614)
Net Amounts of Liabilities Presented in the Balance Sheet $ 0 $ 0