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Stated Capital - Schedule of Stated Capital (Parenthetical) (Detail)
$ / shares in Units, $ in Millions, R in Billions
 12 Months Ended
Feb. 12, 2020
USD ($)
$ / shares
shares
Feb. 12, 2020
ZAR (R)
shares
Dec. 31, 2020
USD ($)
shares
Dec. 31, 2019
USD ($)
shares
Dec. 31, 2018
USD ($)
Disclosure of classes of share capital [line items]          
Shares issued during the period | shares 41,431,635 41,431,635      
Percentage of placing shares issued 5.00% 5.00%      
Proceeds from placing $ 249.0 R 3.7 $ 249.0 $ 0.0 $ 0.0
Investors price per share | $ / shares $ 90.20        
Ordinary shares [member]          
Disclosure of classes of share capital [line items]          
Shares issued during the period | shares     41,431,635 0  
Proceeds from placing | $     $ 249.0 $ 0.0  
No Par Value Per Share | $ / shares $ 0