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Provisions - Silicosis Settlement Costs - Additional Information (Detail)
R in Millions, $ in Millions
Dec. 31, 2020
USD ($)
Dec. 31, 2020
ZAR (R)
Dec. 31, 2019
USD ($)
Dec. 31, 2019
ZAR (R)
Dec. 31, 2018
USD ($)
Disclosure of Provisions [line items]          
Provisions $ 18.3   $ 21.2   $ 25.1
Silicosis [Member]          
Disclosure of Provisions [line items]          
Provisions 18.3 R 268.6 $ 21.2 R 297.1  
Nominal amount of provision $ 23.1 R 339.3      
Silicosis [Member] | Government bonds [member]          
Disclosure of Provisions [line items]          
Discount rate on government bonds 6.67% 6.67% 10.08% 10.08%