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Provisions - Schedule of Assumption in Provision Calculation (Detail)
Dec. 31, 2019
Dec. 31, 2018
South Africa [member]    
Disclosure of Provisions [line items]    
Inflation rate 5.40% 5.50%
Discount rate 10.30% 10.00%
Ghana [member]    
Disclosure of Provisions [line items]    
Inflation rate 2.50% 2.20%
Discount rate   10.30%
Australia [member]    
Disclosure of Provisions [line items]    
Inflation rate 2.50% 2.50%
Peru [member]    
Disclosure of Provisions [line items]    
Inflation rate 2.50% 2.20%
Discount rate 3.00% 4.20%
CHILE    
Disclosure of Provisions [line items]    
Inflation rate 2.50% 2.20%
Discount rate 2.60% 3.60%
Bottom of range [member] | Ghana [member]    
Disclosure of Provisions [line items]    
Discount rate 7.70%  
Bottom of range [member] | Australia [member]    
Disclosure of Provisions [line items]    
Discount rate 1.20% 2.30%
Top of range [member] | Ghana [member]    
Disclosure of Provisions [line items]    
Discount rate 7.90%  
Top of range [member] | Australia [member]    
Disclosure of Provisions [line items]    
Discount rate 1.60% 2.50%