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Production Costs (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Production Costs [Line Items]    
Consumption of raw materials and consumables $ 184,484 $ 160,224
Employee compensation and benefits expense 167,879 169,109
Contractors and outside services 88,475 83,012
Utilities 26,320 24,764
Severance costs related to mine operations 0 3,509
Other expenses (1) 31,417 34,339
Changes in inventories (2) 17,061 25,713
Production Costs 515,636 500,670
NRV adjustments to inventory $ 24,300 12,300
Decomissioning Liability Adjustments [Member]    
Production Costs [Line Items]    
Other expenses (1)   $ (1,200)