INCOME AND MINING TAXES - Tax Effects of Temporary Differences (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforward	$ 57,667	$ 55,515
Mineral properties	60,299	62,345
Other temporary differences	14,356	15,198
Total gross deferred tax assets	132,322	133,058
Less: valuation allowance	(121,212)	(124,153)	$ (123,648)	$ (111,621)
Net deferred tax assets	11,110	8,905
Deferred tax liabilities:
Acquired mineral property interests	(16,024)	(15,331)
Total deferred tax liabilities	(16,024)	(15,331)
Total net deferred tax liability	(4,914)	$ (6,426)
Valuation allowances	$ 2,900