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Provisions - Schedule of Assumption in Provision Calculation (Detail)
Dec. 31, 2018
Dec. 31, 2017
South Africa [member]    
Disclosure of Provisions [line items]    
Inflation rate 5.50% 5.50%
Discount rate 10.00% 9.80%
Ghana [member]    
Disclosure of Provisions [line items]    
Inflation rate 2.20% 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 2.20% 2.20%
Discount rate 4.20% 3.80%
CHILE    
Disclosure of Provisions [line items]    
Inflation rate 2.20%  
Discount rate 3.60%  
Bottom of range [member] | Ghana [member]    
Disclosure of Provisions [line items]    
Discount rate   9.20%
Bottom of range [member] | Australia [member]    
Disclosure of Provisions [line items]    
Discount rate 2.30% 2.60%
Top of range [member] | Ghana [member]    
Disclosure of Provisions [line items]    
Discount rate   9.30%
Top of range [member] | Australia [member]    
Disclosure of Provisions [line items]    
Discount rate 2.50% 2.90%