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Share-Based Payments - Summary of Information Relating to the Options and Equity - Settled Instruments (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of fair value of equity instruments granted [Line Items]      
Number of instruments 18,290,651 9,062,261 3,472,100
Weighted average share price during the year on the Johannesburg Stock Exchange (US$) $ 3.76 $ 4.29 $ 3.55
Restricted Shares ("PVRS") awarded [Member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Number of instruments 18,279,130 8,531,650 2,446,922
Price (US$) $ 0.00 $ 0.00 $ 0.00
Contractual life (years) 0 years 0 years 0 years
4.28 - 6.06 [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Number of instruments 0 0 448,296
Price (US$) $ 0.00 $ 0.00 $ 5.03
Contractual life (years) 0 years 0 years 2 months 19 days
4.28 - 6.06 [member] | Bottom of range [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Range of exercise price for outstanding equity instruments $ 4.28    
4.28 - 6.06 [member] | Top of range [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Range of exercise price for outstanding equity instruments $ 6.06    
6.07 - 7.84 [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Number of instruments 0 3,835 33,641
Price (US$) $ 0.00 $ 6.79 $ 5.86
Contractual life (years) 0 years 6 months 7 months 6 days
6.07 - 7.84 [member] | Bottom of range [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Range of exercise price for outstanding equity instruments $ 6.07    
6.07 - 7.84 [member] | Top of range [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Range of exercise price for outstanding equity instruments $ 7.84    
7.85 - 9.62 [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Number of instruments 0 515,255 531,720
Price (US$) $ 0.00 $ 7.37 $ 6.84
Contractual life (years) 0 years 4 months 2 days 1 year 4 months 6 days
7.85 - 9.62 [member] | Bottom of range [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Range of exercise price for outstanding equity instruments $ 7.85    
7.85 - 9.62 [member] | Top of range [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Range of exercise price for outstanding equity instruments $ 9.62    
9.63 - 11.40 [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Number of instruments 11,521 11,521 11,521
Price (US$) $ 9.42 $ 8.44 $ 7.84
Contractual life (years) 0 years 1 year 2 years 4 days
9.63 - 11.40 [member] | Bottom of range [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Range of exercise price for outstanding equity instruments $ 9.63    
9.63 - 11.40 [member] | Top of range [member]      
Disclosure of fair value of equity instruments granted [Line Items]      
Range of exercise price for outstanding equity instruments $ 11.40