UNAUDITED CRUDE OIL AND NATURAL GAS RESERVES INFORMATION - Changes in Standardized Measure of Estimated Discounted Future Net Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
UNAUDITED CRUDE OIL AND NATURAL GAS RESERVES INFORMATION
Standardized measure, beginning of year	$ 40,164		$ 5
Sales of oil and gas, net of production cost	(9,392)	$ (126)	(3)
Net change in sales prices, net of production cost	10,263	489
Discoveries, extensions and improved recoveries	11,979	76,445
Change in future development costs	(4,050)	(37,667)
Development costs incurred during the period that reduced future development cost	1,144
Revisions of quantity estimates	(559)		$ (2)
Accretion of discount	(4,275)	(130)
Net change in income tax	(6,810)	(2,587)
Purchase of reserves		6,021
Changes in timing of rates of production	2,672	(2,541)
Standardized measure, end of year	$ 49,686	$ 40,164