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Dividends - Summary of Dividends (Parenthetical) (Detail)
 12 Months Ended
Dec. 31, 2019
$ / shares
Dec. 31, 2019
R / shares
Dec. 31, 2018
$ / shares
Dec. 31, 2018
R / shares
Dec. 31, 2017
$ / shares
Dec. 31, 2017
R / shares
Disclosure of Dividends [line items]            
Dividends declared per share | $ / shares $ 5   $ 6   $ 8  
Final Dividend [Member]            
Disclosure of Dividends [line items]            
Dividends declared per share   R 20   R 50   R 60
Dividend declared date Feb. 15, 2019 Feb. 15, 2019        
Interim Dividend [Member]            
Disclosure of Dividends [line items]            
Dividends declared per share   R 60   R 20   R 40
Approved dividend [member]            
Disclosure of Dividends [line items]            
Final dividend approved by the Board, Value   R 100        
Dividend approved date Feb. 12, 2020 Feb. 12, 2020