PROPERTY, PLANT, EQUIPMENT AND MINERAL PROPERTIES (Schedule of Depreciation, Depletion, and Accretion) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Depreciation	$ 27,889	$ 27,858	$ 28,323
Depletion	4,173	4,357	4,886
Amortization of ROU assets	2,059	0	0
Depreciation, depletion, and amortization	$ 34,121	$ 32,215	$ 33,209