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Inventories (Details) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Inventory Disclosure [Abstract]    
Supplies $ 7,030 $ 9,598
Raw materials 887 746
Work in process 2,926 4,608
Finished goods 1,630 2,531
Total inventories $ 12,473 $ 17,483