Derivative Activities - Effects of Non-Hedge Derivatives on Consolidated Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	$ 122,839	$ (146,850)
Derivative Fair Value Income (Loss)	122,839	(146,850)
Swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	125,777	(44,073)
Derivative Fair Value Income (Loss)	125,777	(44,073)
Collars
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	8,415	(39,148)
Derivative Fair Value Income (Loss)	8,415	(39,148)
Basis Swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	(11,353)	(63,629)
Derivative Fair Value Income (Loss)	$ (11,353)	$ (63,629)