DERIVATIVE COMMODITY INSTRUMENTS - Gain (Loss) on Derivative Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Gain (loss) on derivative instruments, net	$ (26,393)	$ (88,477)	$ 115,293
Commodity contracts
Derivative [Line Items]
Open non-cash mark-to-market losses on derivative instruments	(10,759)	(71,190)	(281,752)
Closed gains (losses) on derivative instruments	(15,634)	(17,287)	397,045
Gain (loss) on derivative instruments, net	$ (26,393)	$ (88,477)	$ 115,293