DERIVATIVES - Summary of commodity derivatives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Unrealized gain (loss) on unsettled derivatives	$ (5,575)	$ 1,977
Net settlements paid on derivative contracts	(3,214)	(2,742)
Net settlements receivable (payable) on derivative contracts	(196)	820
Net gain (loss) on commodity derivatives	$ (8,985)	$ 55