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Provisions - Schedule of Assumption in Provision Calculation (Detail)
Dec. 31, 2017
Dec. 31, 2016
South Africa [member]    
Disclosure of Provisions [Line Items]    
Inflation rate 5.50% 5.50%
Discount rate 9.80% 9.70%
Ghana [member]    
Disclosure of Provisions [Line Items]    
Inflation rate 2.20% 2.20%
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 3.80% 3.70%
Bottom of range [member] | Ghana [member]    
Disclosure of Provisions [Line Items]    
Discount rate 9.20% 9.70%
Bottom of range [member] | Australia [member]    
Disclosure of Provisions [Line Items]    
Discount rate 2.60% 1.90%
Top of range [member] | Ghana [member]    
Disclosure of Provisions [Line Items]    
Discount rate 9.30% 9.80%
Top of range [member] | Australia [member]    
Disclosure of Provisions [Line Items]    
Discount rate 2.90% 3.00%