Derivative Instruments - Realized and Unrealized Gains and Losses from Commodity Derivative Instruments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Non-cash change in fair value of derivative instruments	$ 114,555	$ (31,750)	$ 45,026	$ (36,703)
Derivative settlements	(11,129)	(8,067)	(24,773)	(5,135)
Total net gain (loss) on derivative instruments	$ 103,426	$ (39,817)	$ 20,253	$ (41,838)