STANDARDIZED MEASURE OF DISCOUNTED FUTURE NET CASH FLOWS RELATING TO PROVED OIL, NATURAL GAS AND NATURAL GAS LIQUIDS RESERVES (UNAUDITED) (Principal Sources of Changes in Standardized Measure of Future Net Cash Flows) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Standardized Measure of Discontinued Future Net Cash Flows Relating to Proved Oil, Natural Gas and Natural Gas Liquids Reserves [Abstract]
Standardized measure at beginning of period	$ 579,405	$ 371,117	$ 536,450
Sales and transfers of oil, natural gas and natural gas liquids produced, net of production costs	(126,181)	(111,118)	(71,939)
Net changes in prices and production costs	48,322	202,810	(83,146)
Extensions, discoveries and improved recovery, less related costs	3,148	5,451	1,712
Development costs incurred during the period	16	2,261	2,065
Revisions and other	88,587	13,516	(23,328)
Accretion of 10% timing discount	58,222	37,356	53,998
Changes in income taxes	(70,390)	(371)	1,093
Changes in estimated future development costs	3,174	20,177	3,976
Changes in timing and other	21,551	8,302	(26,920)
Purchase of minerals in place		32,949
Sales of minerals in place	(169,421)	(3,045)	(22,844)
Standardized measure of discounted future net cash flows	$ 436,433	$ 579,405	$ 371,117
Accretion timing discount rate	10.00%	10.00%	10.00%