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Inventories (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Inventory Disclosure [Abstract]    
Supplies $ 36,639 $ 21,277
Raw materials and work in process 72,238 28,034
Finished goods 58,538 43,065
Total inventories $ 167,415 $ 92,376