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Provisions - Summary of Silicosis Settlement Costs (Detail)
R in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2020
ZAR (R)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Statement [line items]        
Balance at the beginning of the year $ 21.2   $ 25.1  
Changes in estimates 0.3   (1.6) $ (4.5)
Unwinding of provision recognised as finance expense 1.5   1.3  
Payment (3.5)   (4.6)  
Translation (1.2)   1.0  
Balance at end of the year 18.3   21.2 $ 25.1
Silicosis [Member]        
Statement [line items]        
Balance at the beginning of the year 21.2 R 297.1    
Balance at end of the year 18.3 R 268.6 21.2  
Current portion of silicosis settlement costs (4.0)   (4.6)  
Non-current portion of silicosis settlement costs $ 14.3   $ 16.6