ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Asset Retirement Obligations [Line Items]
Inflation rate	2.00%
Undiscounted decommissioning liability	$ 41,950	$ 41,750
Revision of estimate	7,845	$ 2,063
Change in estimated undiscounted cash flow	200
Change in estimated timing of future reclamation activities	$ 7,650
Minimum
Schedule of Asset Retirement Obligations [Line Items]
Risk Free Rate	9.50%
Maximum
Schedule of Asset Retirement Obligations [Line Items]
Risk Free Rate	11.50%