PROPERTY, PLANT, EQUIPMENT, AND MINERAL PROPERTIES (Schedule of Property, Plant, Equipment, and Mineral Properties) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property, plant, equipment, and mineral properties
Depreciable assets	$ 379,850	$ 385,173
Accumulated depreciation	(135,491)	(116,194)
Depreciable assets, net	244,359	268,979
Mineral properties and development costs	138,882	138,578
Accumulated depletion	(25,410)	(21,974)
Total depletable assets, net	113,472	116,604
Land	519	719
Construction in progress	4,937	2,188
Total property, plant, equipment and mineral properties, net	363,287	388,490
Buildings Plant
Property, plant, equipment, and mineral properties
Depreciable assets	79,658	82,457
Machinery and Equipment
Property, plant, equipment, and mineral properties
Depreciable assets	227,654	227,987
Vehicles
Property, plant, equipment, and mineral properties
Depreciable assets	4,752	4,750
Office Equipment and Improvements
Property, plant, equipment, and mineral properties
Depreciable assets	12,690	12,505
Ponds and Land Improvements
Property, plant, equipment, and mineral properties
Depreciable assets	$ 55,096	$ 57,474

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Mineral properties and development costs which include acquisition costs, the cost of drilling wells, and the cost of other development work, net of depletion.

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Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount of structure or a modification to a structure under construction. Includes recently completed structures or modifications to structures that have not been placed into service.

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Amount before accumulated depletion of real estate held for productive use, excluding land held for sale.

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The accumulated decrease in the carrying value of the mineral properties as a result of mine or mineral production.

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Carrying amount at the balance sheet date of mineral properties, gross of adjustments.

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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