Derivative Activities - Effects of Non-Hedge Derivatives on Consolidated Statement of Operations (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 Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	$ 142,057	$ (40,316)	$ (28,902)	$ 460
Gain (Loss) Recognized in Income (Ineffective Portion)	0	(39)	0	(2,930)
Derivative Fair Value Income (Loss)	142,057	(40,355)	(28,902)	(2,470)
Swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	105,767	(48,277)	23,173	(26,350)
Gain (Loss) Recognized in Income (Ineffective Portion)		(39)		(2,034)
Derivative Fair Value Income (Loss)	105,767	(48,316)	23,173	(28,384)
Re-purchased swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)		1,595		1,117
Derivative Fair Value Income (Loss)		1,595		1,117
Collars
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	30,119	6,366	(7,997)	25,783
Gain (Loss) Recognized in Income (Ineffective Portion)				(896)
Derivative Fair Value Income (Loss)	30,119	6,366	(7,997)	24,887
Basis Swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	6,171		(44,078)	(90)
Derivative Fair Value Income (Loss)	$ 6,171		$ (44,078)	$ (90)