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Provisions - Summary of Silicosis Settlement Costs (Detail)
R in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2019
ZAR (R)
Dec. 31, 2018
ZAR (R)
Jan. 01, 2018
USD ($)
Statement [LineItems]          
Balance at the beginning of the year $ 25.1 $ 31.9      
Changes in estimates (1.6) (4.5)      
Unwinding of provision recognised as finance expense 1.3 2.0      
Payment (4.6) 0.0      
Translation 1.0 (4.3)      
Balance at end of the year 21.2 25.1      
Current portion of silicosis settlement costs (4.6) 0.0 [1]     $ 0.0
Non-current portion of silicosis settlement costs 16.6 25.1      
Silicosis [Member]          
Statement [LineItems]          
Balance at end of the year 21.2 25.1 R 297.1 R 367.8  
Current portion of silicosis settlement costs (4.6) 0.0      
Non-current portion of silicosis settlement costs $ 16.6 $ 25.1      
[1] Refer note 42.