Derivative Activities - Effects of Non-Hedge Derivatives on Consolidated Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Financial Instruments [Line Items]
Derivative Fair Value Income (Loss)	$ 202,004	$ 142,057	$ 290,052	$ (28,902)
Swaps
Derivative Financial Instruments [Line Items]
Derivative Fair Value Income (Loss)	198,245	105,767	281,921	23,173
Re-purchased Swaps
Derivative Financial Instruments [Line Items]
Derivative Fair Value Income (Loss)	1,683	0	1,683	0
Collars
Derivative Financial Instruments [Line Items]
Derivative Fair Value Income (Loss)	5,626	30,119	12,391	(7,997)
Basis Swaps
Derivative Financial Instruments [Line Items]
Derivative Fair Value Income (Loss)	$ (3,550)	$ 6,171	$ (5,943)	$ (44,078)