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Provisions - Silicosis Settlement Costs - Additional Information (Detail)
R in Millions, $ in Millions
Dec. 31, 2018
USD ($)
Dec. 31, 2018
ZAR (R)
Dec. 31, 2017
USD ($)
Dec. 31, 2017
ZAR (R)
Disclosure of Provisions [line items]        
Provisions $ 25.1   $ 31.9  
Silicosis [Member]        
Disclosure of Provisions [line items]        
Provisions 25.1 R 367.8 $ 31.9 R 401.6
Nominal amount of provision $ 34.7 R 507.0    
Silicosis [Member] | Government bonds [member]        
Disclosure of Provisions [line items]        
Discount rate on government bonds 8.74% 8.74% 8.24% 8.24%