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Property, Plant and Mine Development, net - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Property, Plant and Mine Development, net        
Depletion, depreciation and amortization $ 3,700 $ 4,000 $ 7,386 $ 6,727