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Acquisition Of Asanko Gold - Summary of Gain on Acquisition (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about business combination [abstract]      
Total fair value of assets acquired     $ 216.8
Redeemable preference shares equity financial asset acquired     129.9
Fair value of identifiable net assets acquired     86.9
Consideration transferred     (165.0)
Gain on acquisition $ 0.0 $ 0.0 $ 51.8