Derivative Activities - Effects of Non-Hedge Derivatives on Consolidated Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	$ 137,760	$ 146,697	$ 40,776	$ 85,627
Gain (Loss) Recognized in Income (Ineffective Portion)		1,872	(2,891)	2,109
Derivative Fair Value Income (Loss)	137,760	148,569	37,885	87,736
Swap
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	65,003	129,313	21,927	76,328
Gain (Loss) Recognized in Income (Ineffective Portion)		562	(1,995)	666
Derivative Fair Value Income (Loss)	65,003	129,875	19,932	76,994
Re-purchased swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	(1,663)	(8,744)	(478)	(8,744)
Gain (Loss) Recognized in Income (Ineffective Portion)
Derivative Fair Value Income (Loss)	(1,663)	(8,744)	(478)	(8,744)
Collars
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	74,420	7,597	19,417	5,095
Gain (Loss) Recognized in Income (Ineffective Portion)		1,310	(896)	1,443
Derivative Fair Value Income (Loss)	74,420	8,907	18,521	6,538
Call Option
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)		18,531	(90)	12,948
Gain (Loss) Recognized in Income (Ineffective Portion)
Derivative Fair Value Income (Loss)		$ 18,531	$ (90)	$ 12,948