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Provisions - Schedule of Provisions (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of Provisions [line items]      
Silicosis settlement costs $ 18.3 $ 21.2  
Other 3.1 4.2  
Total provisions 402.9 395.7  
Environmental rehabilitation costs balance at beginning of the year 370.3 289.6  
Changes in estimates (5.1) 79.5  
Interest expense 10.7 11.7 $ 11.7
Payments (12.9) (10.7)  
Translation adjustment 18.5 0.2  
Environmental rehabilitation costs balance at end of the year 381.5 370.3 $ 289.6
Total gross closure cost estimates 466.6 436.3  
Current portion of provisions (23.6) (4.6)  
Non-current portion of provisions 379.3 391.1  
Current portion of environmental rehabilitation costs (19.6) 0.0  
Non-current portion of environmental rehabilitation costs 361.9 370.3  
South Africa [member]      
Disclosure of Provisions [line items]      
Total gross closure cost estimates 43.9 45.9  
Ghana [member]      
Disclosure of Provisions [line items]      
Total gross closure cost estimates 104.4 105.6  
Australia [member]      
Disclosure of Provisions [line items]      
Total gross closure cost estimates 218.8 198.2  
Peru [member]      
Disclosure of Provisions [line items]      
Total gross closure cost estimates 97.4 86.2  
CHILE      
Disclosure of Provisions [line items]      
Total gross closure cost estimates $ 2.1 $ 0.4