v3.19.1

INVENTORY AND LONG-TERM PARTS INVENTORY (Summary of Inventory) (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Inventory Disclosure [Abstract] |  |
Finished goods product inventory | $ 48,370 | $ 54,577
In-process mineral inventory | 24,325 | 19,822
Total product inventory | 72,695 | 74,399
Current parts inventory, net | 9,351 | 8,727
Total current inventory, net | 82,046 | 83,126
Long-term parts inventory, net | 30,031 | 30,611
Total inventory, net | $ 112,077 | $ 113,737

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