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Provisions - Schedule of Assumption in Provision Calculation (Detail)
Dec. 31, 2020
Dec. 31, 2019
South Africa [member]    
Disclosure of Provisions [line items]    
Inflation rate 4.80% 5.40%
Discount rate 10.80% 10.30%
Ghana [member]    
Disclosure of Provisions [line items]    
Inflation rate 2.20% 2.50%
Australia [member]    
Disclosure of Provisions [line items]    
Inflation rate 2.50% 2.50%
Discount rate 2.50%  
Peru [member]    
Disclosure of Provisions [line items]    
Inflation rate 2.20% 2.50%
Discount rate 2.20% 3.00%
CHILE    
Disclosure of Provisions [line items]    
Inflation rate 2.20% 2.50%
Discount rate 2.20% 2.60%
Bottom of range [member] | Ghana [member]    
Disclosure of Provisions [line items]    
Discount rate 6.70% 7.70%
Bottom of range [member] | Australia [member]    
Disclosure of Provisions [line items]    
Discount rate   1.20%
Top of range [member] | Ghana [member]    
Disclosure of Provisions [line items]    
Discount rate 7.20% 7.90%
Top of range [member] | Australia [member]    
Disclosure of Provisions [line items]    
Discount rate   1.60%