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Royalties - Summary of Royalties (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (105.0) $ (73.7) $ (62.5)
Group excluding Asanko [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (105.0) $ (73.7) $ (62.5)
South Africa [member]      
Disclosure of information about royalty arrangements [line items]      
Royalty percentage 0.50% 0.50% 0.50%
South Africa [member] | South deep [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (2.0) $ (1.6) $ (1.0)
Foreign [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties (103.0) (72.1) (61.5)
Australia [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (44.3) $ (30.5) $ (27.9)
Royalty percentage 2.50% 2.50% 2.50%
Ghana [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (62.5) $ (43.8) $ (31.3)
Royalty percentage 4.10% 3.50% 3.00%
Peru [member]      
Disclosure of information about royalty arrangements [line items]      
Royalty percentage 3.90% 3.60% 4.00%