Crude Oil and Natural Gas Property Information - Schedule of Capitalized Exploratory Drilling Costs Pending Evaluation (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Well	Dec. 31, 2016 USD ($) Well	Dec. 31, 2015 USD ($) Well
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]
Balance at January 1	$ 34,852	$ 59,397	$ 93,421
Additions to capitalized exploratory well costs pending determination of proved reserves	79,451	123,980	132,806
Reclassification to proved crude oil and natural gas properties based on the determination of proved reserves	(81,035)	(141,941)	(160,779)
Capitalized exploratory well costs charged to expense	(1,912)	(6,584)	(6,051)
Balance at December 31	$ 31,356	$ 34,852	$ 59,397
Number of wells | Well	37	54	73