Crude Oil and Natural Gas Property Information - Schedule of Capitalized Exploratory Drilling Costs Pending Evaluation (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Well	Dec. 31, 2011 Well	Dec. 31, 2010 Well
Oil and Gas Properties [Line Items]
Balance at January 1	$ 128,123	$ 92,806	$ 22,856
Additions to capitalized exploratory well costs pending determination of proved reserves	485,530	500,046	185,713
Reclassification to proved crude oil and natural gas properties based on the determination of proved reserves	(520,187)	(456,780)	(112,739)
Capitalized exploratory well costs charged to expense	(767)	(7,949)	(3,024)
Balance at December 31	$ 92,699	$ 128,123	$ 92,806
Number of wells	46	56	87