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Share-Based Payments - Summary of Information Relating to the Options and Equity - Settled Instruments (Detail) - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of fair value of equity instruments granted [line items]      
Weighted average share price during the year on the Johannesburg Stock Exchange (US$) $ 4.82 $ 3.46 $ 3.76