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Provisions - Additional Information (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of other provisions [line items]      
Accretion charged to earnings as finance expense $ 5,973 $ 4,363  
Reclamation expenditures (8,749) (6,080)  
Provision accrued for labour claims 69,493 59,943 $ 54,871
Contingent liability for decommissioning, restoration and rehabilitation costs [member]      
Disclosure of other provisions [line items]      
Inflated and undiscounted estimate of future cash flows 142,200 122,100  
Accretion charged to earnings as finance expense 5,973 4,363  
Reclamation expenditures (8,749) (6,080)  
Provision accrued for labour claims 65,396 55,611 50,453
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 $ 4,097 $ 4,332 $ 4,418
Bottom of range [member] | Contingent liability for decommissioning, restoration and rehabilitation costs [member]      
Disclosure of other provisions [line items]      
Inflation rate (percent) 2.00% 1.00%  
Discount rate (percent) 2.00% 1.00%  
Top of range [member] | Contingent liability for decommissioning, restoration and rehabilitation costs [member]      
Disclosure of other provisions [line items]      
Inflation rate (percent) 25.00% 23.00%  
Discount rate (percent) 24.00% 30.00%