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Dividends - Summary of Dividends (Parenthetical) (Detail)
 12 Months Ended
Dec. 31, 2018
$ / shares
Dec. 31, 2018
R / shares
Dec. 31, 2017
$ / shares
Dec. 31, 2017
R / shares
Dec. 31, 2016
$ / shares
Dec. 31, 2016
R / shares
Dec. 31, 2015
R / shares
Disclosure of Dividends [line items]              
Dividends declared per share | $ / shares $ 0.06   $ 0.08   $ 0.05    
Final Dividend [Member]              
Disclosure of Dividends [line items]              
Dividends declared per share       R 0.50   R 0.60 R 0.21
Dividend declared date Feb. 13, 2018 Feb. 13, 2018          
Interim Dividend [Member]              
Disclosure of Dividends [line items]              
Dividends declared per share   R 0.20   R 0.40   R 0.50  
Approved dividend [member]              
Disclosure of Dividends [line items]              
Final dividend approved by the Board, Value   R 0.20          
Dividend approved date Feb. 13, 2019 Feb. 13, 2019