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Provisions - Schedule of Provisions (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2019
Dec. 31, 2018
Jan. 01, 2018
Disclosure of Provisions [line items]            
Environmental rehabilitation costs $ 289.6 $ 281.5 $ 281.5 $ 370.3 $ 289.6  
Silicosis settlement costs       21.2 25.1  
Other       4.2 4.8  
Total provisions       $ 391.1 $ 319.5 [1] $ 321.3
Environmental rehabilitation costs Balance at beginning of the year 289.6 281.5        
Changes in estimates 79.5 23.2        
Interest expense 11.7 11.7 12.1      
Payments (10.7) (9.6)        
Translation adjustment 0.2 (17.2)        
Environmental rehabilitation costs balance at end of the year 370.3 289.6 $ 281.5      
Total gross closure cost estimates 436.3 399.9        
South Africa [member]            
Disclosure of Provisions [line items]            
Total gross closure cost estimates 45.9 41.8        
Ghana [member]            
Disclosure of Provisions [line items]            
Total gross closure cost estimates 105.6 100.4        
Australia [member]            
Disclosure of Provisions [line items]            
Total gross closure cost estimates 198.2 178.2        
Peru [member]            
Disclosure of Provisions [line items]            
Total gross closure cost estimates 86.2 79.1        
CHILE            
Disclosure of Provisions [line items]            
Total gross closure cost estimates $ 0.4 $ 0.4        
[1] Refer note 42.