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Dividends - Summary of Dividends (Parenthetical) (Detail)
 12 Months Ended
Dec. 31, 2020
$ / shares
Dec. 31, 2020
R / shares
Dec. 31, 2019
$ / shares
Dec. 31, 2019
R / shares
Dec. 31, 2018
$ / shares
Dec. 31, 2018
R / shares
Dec. 31, 2017
R / shares
Disclosure of Dividends [line items]              
Dividends declared per share | $ / shares $ 0.16   $ 0.05   $ 0.06    
Final Dividend [Member]              
Disclosure of Dividends [line items]              
Dividends declared per share       R 1   R 0.20 R 0.50
Dividend declared date Feb. 12, 2020 Feb. 12, 2020          
Interim Dividend [Member]              
Disclosure of Dividends [line items]              
Dividends declared per share   R 1.60   R 0.60   R 0.20  
Approved dividend [member]              
Disclosure of Dividends [line items]              
Final dividend approved by the Board, Value   R 3.20          
Dividend approved date Feb. 17, 2021 Feb. 17, 2021