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Goodwill - Key assumptions calculation of fair value less costs of disposal (Parenthetical) (Detail) - Richards Bay Minerals [member]
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of key assumptions calculation of fair value less costs of disposal [Line Items]    
Increase in the discount rate applied to post-tax cash flows 8.70% 9.00%
5% decrease in the titanium slag price [member] | Titanium [member]    
Disclosure of key assumptions calculation of fair value less costs of disposal [Line Items]    
Percentage of decrease in the titanium slag price 5.00%  
1% increase in the discount rate applied to post-tax cash flows [member]    
Disclosure of key assumptions calculation of fair value less costs of disposal [Line Items]    
Increase in the discount rate applied to post-tax cash flows 1.00%  
10% strengthening of South African rand [member]    
Disclosure of key assumptions calculation of fair value less costs of disposal [Line Items]    
Strengthening percentage of the South African rand 10.00%