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Goodwill - Key assumptions calculation of fair value less costs of disposal (Parenthetical) (Detail) - Richards Bay Minerals
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of key assumptions calculation of fair value less costs of disposal [Line Items]    
Fair value less costs of disposal, post-tax discount rate 8.60% 8.60%
5% decrease in the titanium slag price | Titanium    
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    
Disclosure of key assumptions calculation of fair value less costs of disposal [Line Items]    
Fair value less costs of disposal, post-tax discount rate 1.00%  
10% strengthening of the South African rand    
Disclosure of key assumptions calculation of fair value less costs of disposal [Line Items]    
Strengthening percentage of the South African rand 10.00%