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