Oil and Gas Reserves Information (Unaudited) - Changes in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Reserves (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Extractive Industries [Abstract]
Standardized Measure, Beginning of Year	$ 429,275	$ 372,139	$ 1,090,660
Net change in sales price, net of production costs	326,662	(45,379)	(751,774)
Development costs incurred during the year which were previously estimated	119,864	45,648	157,390
Revisions of quantity estimates	57,042	113,583	(111,454)
Accretion of discount	43,130	37,251	114,427
Changes in future development and abandonment costs	(62,509)	5,315	14,901
Changes in timing and other	(15,565)	(38,071)	(44,439)
Extensions and discoveries	167,135	70,149	56,216
Sales of minerals in place	(6,027)	(22,449)	(43,694)
Sales, net of production costs	(194,562)	(107,253)	(163,336)
Net changes in income taxes	17,099	(1,658)	53,242
Standardized Measure, End of Year	$ 881,544	$ 429,275	$ 372,139