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Inventory (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Inventory.    
Raw materials and supplies $ 42,851 $ 26,298
Work-in-process 3,492 1,865
Finished goods 2,433 1,777
Total inventory 48,776 29,940
Inventory held at service locations $ 5,500 $ 3,300