Quarterly report pursuant to Section 13 or 15(d)

Inventories (Details)

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Inventories (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Inventory Disclosure [Abstract]    
Supplies $ 7,264 $ 9,437
Raw materials 321 219
Work in process 1,326 2,370
Finished goods 3,806 5,788
Total inventory $ 12,717 $ 17,814