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Dividends - Summary of Dividends (Detail)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
$ / shares
Dec. 31, 2018
R / shares
Dec. 31, 2017
USD ($)
$ / shares
Dec. 31, 2017
R / shares
Dec. 31, 2016
USD ($)
$ / shares
Dec. 31, 2016
R / shares
Dec. 31, 2015
R / shares
Disclosure of Dividends [line items]              
Total dividends $ 45.5   $ 62.8   $ 39.2    
Dividends per share - cents | $ / shares $ 0.06   $ 0.08   $ 0.05    
Final Dividend [Member]              
Disclosure of Dividends [line items]              
Dividends recognised as distribution to owners $ 34.7   $ 37.5   $ 10.6    
Dividends per share - cents | R / shares       R 0.50   R 0.60 R 0.21
Interim Dividend [Member]              
Disclosure of Dividends [line items]              
Dividends recognised as distribution to owners $ 10.8   $ 25.3   $ 28.6    
Dividends per share - cents | R / shares   R 0.20   R 0.40   R 0.50