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Provisions - Additional Information (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of other provisions [line items]      
Accretion charged to earnings as finance expense $ 8,260 $ 9,903  
Reclamation expenditures (2,462) (2,264)  
Provision accrued for labour claims 241,953 195,384 $ 75,155
Contingent liability for decommissioning, restoration and rehabilitation costs [member]      
Disclosure of other provisions [line items]      
Inflated and undiscounted estimate of future cash flows 330,600 290,400  
Accretion charged to earnings as finance expense 8,260 9,903  
Reclamation expenditures (2,462) (2,264)  
Provision accrued for labour claims 235,110 188,455 70,587
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 $ 6,843 $ 6,929 $ 4,568
Bottom of range [member] | Contingent liability for decommissioning, restoration and rehabilitation costs [member]      
Disclosure of other provisions [line items]      
Inflation rate (percent) 0.00% 0.00%  
Discount rate (percent) 0.00% 2.00%  
Top of range [member] | Contingent liability for decommissioning, restoration and rehabilitation costs [member]      
Disclosure of other provisions [line items]      
Inflation rate (percent) 4.00% 5.00%  
Discount rate (percent) 8.00% 9.00%