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Acquisition of Asanko Gold - Summary of Key Assumptions Used to Determine Fair Value of the Net Identifiable Assets Acquired (Detail) - $ / oz
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about business combination [line items]    
Discount rate   10.27%
Life-of-mine - 2019 to 2030   12 years
Within one year [member] | US Dollars [member]    
Disclosure of detailed information about business combination [line items]    
US$ gold price 1,200 1,200
Later than one year [member] | US Dollars [member]    
Disclosure of detailed information about business combination [line items]    
US$ gold price 1,300 1,300
Life-of-mine - 2019 to 2030 75 years