Quarterly report pursuant to Section 13 or 15(d)

Inventories - Summary (Details)

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Inventories - Summary (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Inventory Disclosure [Abstract]    
Consumables and materials $ 7,442 $ 7,566
Spare parts 17,194 15,898
Inventories $ 24,636 $ 23,464