Supplemental Oil and Natural Gas Information - Changes in Standardized Measure of Discounted Net Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized Measure, beginning of the year	$ 205,981	$ 212,865	$ 330,657
Net change in prices and production costs	653,347	(33,507)	(372,664)
Net change in future development costs	(385,042)	1,552	79,244
Sales, less production costs	(226,324)	(99,768)	(121,646)
Extensions	135,734	79,941	107,749
Acquisitions	2,365	1,045
Divestitures		(5,231)
Revisions of previous quantity estimates	322,917	15,754	(97,210)
Previously estimated development costs incurred	34,102	4,886	62,906
Accretion of discount	20,598	21,287	50,939
Net change in taxes			178,732
Changes in timing and other	(33,992)	7,157	(5,842)
Standardized Measure, end of year	$ 729,686	$ 205,981	$ 212,865