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Lease Obligations (Schedule of Lease Obligations) (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Disclosure of quantitative information about right-of-use assets [line items]    
Operating leases $ 31,703 $ 37,792
Less: future finance charges (12,206) (13,913)
Present value of minimum lease payments 19,497 23,879
Less current portion (6,978) (8,831)
Non-current portion 12,519 15,048
Lease Obligation Mining Equipment    
Disclosure of quantitative information about right-of-use assets [line items]    
Operating leases 16,829  
Lease Obligation Other Leases    
Disclosure of quantitative information about right-of-use assets [line items]    
Operating leases 2,668  
Less than one year    
Disclosure of quantitative information about right-of-use assets [line items]    
Operating leases 7,367 9,313
Between one and five years    
Disclosure of quantitative information about right-of-use assets [line items]    
Operating leases 10,209 13,521
More than five years    
Disclosure of quantitative information about right-of-use assets [line items]    
Operating leases $ 14,127 $ 14,958