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Segment Report - Schedule of Segment Report (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of operating segments [line items]      
Tax rate included in South Deep 29.00% 29.00% 30.00%
Equity Accounted Joint Venture $ 172.0 $ 177.5  
Share of loss of associates after taxation included in other income 3.1 (13.1) $ (1.3)
Revenue from the sale of copper 165.1 169.2 177.8
Corporate and other [member] | Other income/cost [member]      
Disclosure of operating segments [line items]      
Share of loss of associates after taxation included in other income 3.1 (13.1) (1.3)
Corporate related costs 18.9 31.4 $ 9.0
Repurchase of notes, value 1,000.0    
Gain loss on repurchase of notes 5.0    
Asanko [member]      
Disclosure of operating segments [line items]      
Equity Accounted Joint Venture $ 89.9 $ 85.8