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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 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]        
Non-cash change in fair value of derivative instruments $ (54,165) $ 11,345 $ (69,529) $ (4,953)
Derivative settlements (11,405) 1,246 (13,644) 2,932
Total net gain (loss) on derivative instruments $ (65,570) $ 12,591 $ (83,173) $ (2,021)