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Changes in Significant Accounting Policies - Summary of Impact to IFRS 16 on the Measuring Lease Liabilities For Leases That Classified As Operating Leases (Detail)
Jan. 01, 2019
Australia [member] | Bottom of range [member]  
Disclosure Of Changes In Sigificant Accounting Policies [line items]  
Operating lease payments discounted rate 3.46%
Australia [member] | Top of range [member]  
Disclosure Of Changes In Sigificant Accounting Policies [line items]  
Operating lease payments discounted rate 6.39%
Ghana [member] | Bottom of range [member]  
Disclosure Of Changes In Sigificant Accounting Policies [line items]  
Operating lease payments discounted rate 6.83%
Ghana [member] | Top of range [member]  
Disclosure Of Changes In Sigificant Accounting Policies [line items]  
Operating lease payments discounted rate 7.68%
South Africa [member]  
Disclosure Of Changes In Sigificant Accounting Policies [line items]  
Operating lease payments discounted rate 9.84%
Peru [member] | Bottom of range [member]  
Disclosure Of Changes In Sigificant Accounting Policies [line items]  
Operating lease payments discounted rate 4.50%
Peru [member] | Top of range [member]  
Disclosure Of Changes In Sigificant Accounting Policies [line items]  
Operating lease payments discounted rate 4.76%
Corporate and other [Member] | Bottom of range [member]  
Disclosure Of Changes In Sigificant Accounting Policies [line items]  
Operating lease payments discounted rate 4.00%
Corporate and other [Member] | Top of range [member]  
Disclosure Of Changes In Sigificant Accounting Policies [line items]  
Operating lease payments discounted rate 10.25%