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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments Gain Loss [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	$ (40,316)	$ (37,009)	$ 460	$ 48,618
Gain (Loss) Recognized in Income (Ineffective Portion)	(39)	(3,719)	(2,930)	(1,610)
Derivative Fair Value Income (Loss)	(40,355)	(40,728)	(2,470)	47,008
Swaps
Derivative Instruments Gain Loss [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	(48,277)	(45,998)	(26,350)	30,330
Gain (Loss) Recognized in Income (Ineffective Portion)	(39)	(1,556)	(2,034)	(890)
Derivative Fair Value Income (Loss)	(48,316)	(47,554)	(28,384)	29,440
Re-purchased swaps
Derivative Instruments Gain Loss [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	1,595	12,822	1,117	4,078
Derivative Fair Value Income (Loss)	1,595	12,822	1,117	4,078
Collars
Derivative Instruments Gain Loss [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	6,366	(1,714)	25,783	3,381
Gain (Loss) Recognized in Income (Ineffective Portion)		(2,163)	(896)	(720)
Derivative Fair Value Income (Loss)	6,366	(3,877)	24,887	2,661
Call Option
Derivative Instruments Gain Loss [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)		(2,119)		10,829
Derivative Fair Value Income (Loss)		(2,119)		10,829
Basis Swap
Derivative Instruments Gain Loss [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)			(90)
Derivative Fair Value Income (Loss)			$ (90)