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, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Financial Instruments [Line Items]
Gain (Loss)Recognized in Income (Non-hedge Derivatives	$ (37,009)	$ 67,696	$ 48,618	$ 70,877
Gain (Loss) Recognized in Income (Ineffective Portion)	(3,719)	(1,935)	(1,610)	7,089
Derivative Fair Value Income (Loss)	(40,728)	65,761	47,008	77,966
Swaps [Member]
Derivative Financial Instruments [Line Items]
Gain (Loss)Recognized in Income (Non-hedge Derivatives	(45,998)	26,219	30,330	39,968
Gain (Loss) Recognized in Income (Ineffective Portion)	(1,556)		(890)
Derivative Fair Value Income (Loss)	(47,554)	26,219	29,440	39,968
Purchased swaps [Member]
Derivative Financial Instruments [Line Items]
Gain (Loss)Recognized in Income (Non-hedge Derivatives	12,822		4,078
Derivative Fair Value Income (Loss)	12,822		4,078
Collars [Member]
Derivative Financial Instruments [Line Items]
Gain (Loss)Recognized in Income (Non-hedge Derivatives	(1,714)	15,828	3,381	14,693
Gain (Loss) Recognized in Income (Ineffective Portion)	(2,163)	(1,935)	(720)	7,089
Derivative Fair Value Income (Loss)	(3,877)	13,893	2,661	21,782
Call options [Member]
Derivative Financial Instruments [Line Items]
Gain (Loss)Recognized in Income (Non-hedge Derivatives	(2,119)	25,649	10,829	16,259
Derivative Fair Value Income (Loss)	(2,119)	25,649	10,829	16,259
Basis swaps [Member]
Derivative Financial Instruments [Line Items]
Gain (Loss)Recognized in Income (Non-hedge Derivatives				(43)
Derivative Fair Value Income (Loss)				$ (43)