Derivative Instruments - Schedule of Location and Fair Value of Outstanding Commodity Derivative Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative instruments — current assets	$ 130,747	$ 302,159
Derivative instruments — non-current assets	4,699	13,348
Derivative instruments — non-current liabilities	(90)	0
Total derivative instruments	135,356
Crude oil [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments — current assets	130,747	302,159
Derivative instruments — non-current assets	4,699	13,348
Derivative instruments — non-current liabilities	(90)	0
Total derivative instruments	$ 135,356	$ 315,507