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Provisions - Additional Information (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of other provisions [line items]      
Accretion charged to earnings as finance expense $ 6,524 $ 5,973  
Reclamation expenditures (7,849) (8,749)  
Provision accrued for labour claims 75,155 69,493 $ 59,943
Contingent liability for decommissioning, restoration and rehabilitation costs [member]      
Disclosure of other provisions [line items]      
Inflated and undiscounted estimate of future cash flows 159,100 142,200  
Accretion charged to earnings as finance expense 6,524 5,973  
Reclamation expenditures (7,849) (8,749)  
Provision accrued for labour claims 70,587 65,396 55,611
Litigation [member]      
Disclosure of other provisions [line items]      
Accretion charged to earnings as finance expense 0 0  
Reclamation expenditures 0 0  
Provision accrued for labour claims $ 4,568 $ 4,097 $ 4,332
Bottom of range [member] | Contingent liability for decommissioning, restoration and rehabilitation costs [member]      
Disclosure of other provisions [line items]      
Inflation rate (percent) 2.00% 2.00%  
Discount rate (percent) 2.00% 2.00%  
Top of range [member] | Contingent liability for decommissioning, restoration and rehabilitation costs [member]      
Disclosure of other provisions [line items]      
Inflation rate (percent) 17.00% 25.00%  
Discount rate (percent) 22.00% 24.00%