PROPERTY, PLANT, EQUIPMENT AND MINERAL PROPERTIES (Schedule of Property, Plant, Equipment and Mineral Properties) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Total depreciable assets	$ 602,863	$ 544,574
Accumulated depreciation	(224,354)	(198,365)
Total property, plant, equipment, and mineral properties, net	378,509	346,209
Land [Member]
Property, Plant and Equipment [Line Items]
Total depreciable assets	27,274	519
Ponds and land improvements [Member]
Property, Plant and Equipment [Line Items]
Total depreciable assets	65,992	58,961
Mineral properties and development costs [Member]
Property, Plant and Equipment [Line Items]
Total depreciable assets	143,988	139,418
Buildings and Plant [Member]
Property, Plant and Equipment [Line Items]
Total depreciable assets	81,468	81,429
Machinery and equipment [Member]
Property, Plant and Equipment [Line Items]
Total depreciable assets	253,536	241,977
Vehicles [Member]
Property, Plant and Equipment [Line Items]
Total depreciable assets	6,222	5,669
Office equipment and improvements [Member]
Property, Plant and Equipment [Line Items]
Total depreciable assets	9,136	13,779
Right Of Use Assets [Member]
Property, Plant and Equipment [Line Items]
Total depreciable assets	8,123	0
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Total depreciable assets	$ 7,124	$ 2,822