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Provisions - Schedule of Provisions (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Disclosure of Provisions [line items]          
Environmental rehabilitation costs $ 281.5 $ 283.1 $ 283.1 $ 289.6 $ 281.5
Silicosis settlement costs       25.1 31.9
Other       4.8 7.9
Total provisions       $ 319.5 $ 321.3
Environmental rehabilitation costs Balance at beginning of the year 281.5 283.1      
Changes in estimates - continuing operations 23.2 (5.4)      
Changes in estimates - discontinued operations 0.0 0.0      
Interest expense - continuing operations 11.7 12.1 10.7    
Interest expense - discontinued operations 0.0 0.2      
Payments (9.6) (8.1)      
Disposal of subsidiary 0.0 (12.9)      
Translation adjustment (17.2) 12.5      
Environmental rehabilitation costs balance at end of the year 289.6 281.5 $ 283.1    
Total gross closure cost estimates 399.9 381.0      
South Africa [member]          
Disclosure of Provisions [line items]          
Total gross closure cost estimates 41.8 41.8      
Ghana [member]          
Disclosure of Provisions [line items]          
Total gross closure cost estimates 100.4 98.1      
Australia [member]          
Disclosure of Provisions [line items]          
Total gross closure cost estimates 178.2 179.2      
Peru [member]          
Disclosure of Provisions [line items]          
Total gross closure cost estimates 79.1 61.9      
CHILE          
Disclosure of Provisions [line items]          
Total gross closure cost estimates $ 0.4 $ 0.0