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Derivative Instruments - Realized and Unrealized Gains and Losses from Commodity Derivative Instruments (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]        
Net gain (loss) on derivative instruments $ (39,424) $ (65,570) $ 7,648 $ (83,173)