DERIVATIVES - Summary of commodity derivatives (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Unrealized gain (loss) on unsettled derivatives	$ 19,993	$ (8,976)
Net settlements paid on derivative contracts	(198)	(842)
Net settlements receivable (payable) on derivative contracts	1,403	(759)
Net gain (loss) on commodity derivatives	$ 21,198	$ (10,577)