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Derivative Instruments - Realized and Unrealized Gains and Losses from Commodity Derivative Instruments (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]    
Net gain (loss) on derivative instruments $ 47,072us-gaap_DerivativeGainLossOnDerivativeNet $ (17,603)us-gaap_DerivativeGainLossOnDerivativeNet