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Segment Report - Schedule of Segment Report (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of operating segments [line items]      
Percentage of voting equity interests acquired     30.00%
Tax rate included in South Deep 30.00% 30.00%  
Share of loss of associates after taxation included in other income $ (1.3) $ (2.3) [1] $ (5.7) [1]
Profit on disposal of investments 0.0 2.3 [1] 0.1 [1]
Other income/cost [member]      
Disclosure of operating segments [line items]      
Share of loss of associates after taxation included in other income (1.3)    
Corporate related costs $ 9.0    
Other income [member]      
Disclosure of operating segments [line items]      
Share of loss of associates after taxation included in other income   (2.3)  
Profit on disposal of investments   2.3  
Corporate related costs   $ 23.1  
Cost [member]      
Disclosure of operating segments [line items]      
Share of loss of associates after taxation included in other income     (5.7)
Profit on disposal of investments     0.1
Corporate related costs     $ 6.2
[1] As Restated - Refer note 40 for further details.