Derivative Activities - Effects of Non-Hedge Derivatives on Consolidated Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	$ (146,850)	$ (96,984)
Gain (Loss) Recognized in Income (Ineffective Portion)	0	(2,891)
Derivative fair value loss	(146,850)	(99,875)
Swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	(44,073)	(43,076)
Gain (Loss) Recognized in Income (Ineffective Portion)		(1,995)
Derivative fair value loss	(44,073)	(45,071)
Re-purchased swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)		1,185
Derivative fair value loss		1,185
Collars
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	(39,148)	(55,003)
Gain (Loss) Recognized in Income (Ineffective Portion)		(896)
Derivative fair value loss	(39,148)	(55,899)
Call options
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	(63,629)	(90)
Derivative fair value loss	$ (63,629)	$ (90)