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Royalties - Summary of Royalties (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of information about royalty arrangements [line items]      
Total royalties [1] $ (73.7) $ (62.5) $ (62.0)
South Africa [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (1.6) $ (1.0) $ (1.8)
Royalty percentage 0.50% 0.50% 0.50%
Foreign [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (72.1) $ (61.5) $ (60.2)
Australia [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (30.5) $ (27.9) $ (27.8)
Royalty percentage 2.50% 2.50% 2.50%
Ghana [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (43.8) $ (31.3) $ (27.1)
Royalty percentage 3.50% 3.00% 3.00%
Peru [member]      
Disclosure of information about royalty arrangements [line items]      
Royalty percentage 3.60% 4.00% 4.60%
[1] Refer note 42.