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Acquisition of Asanko Gold - Summary of Key Assumptions Used to Determine Fair Value of Redeemable Preference Shares at Acquisition (Detail) - USD ($)
$ in Millions
 1 Months Ended 12 Months Ended
Jan. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about business combination [abstract]      
Par value of the preference shares     $ 165.0
Market-related interest rate 7.85% 8.50% 7.85%
Expected redemption period - 2020 to 2023 5 years 8 years 5 years