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Production Costs (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Production Costs [Line Items]    
Consumption of raw materials and consumables $ 160,224 $ 163,675
Employee compensation and benefits expense (Note 22) 169,109 148,256
Contractors and outside services 83,012 81,241
Utilities 24,764 20,335
Severance costs related to mine operations 3,509 0
Other expenses (1) 34,339 43,400
Changes in inventories (2) 25,713 (28,574)
Production Costs 500,670 428,333
NRV adjustments to inventory 12,300 (42,800)
Decomissioning Liability Adjustments [Member]    
Production Costs [Line Items]    
Other expenses (1) $ (1,200) $ (1,700)