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Segment Report - Schedule of Segment Report (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of operating segments [line items]      
Equity Accounted Joint Venture $ 233.3 $ 172.0  
Share of loss of associates after taxation included in other income (2.6) 3.1 $ (13.1)
Revenue from the sale of copper 144.1 165.1 169.2
Asanko Gold [member]      
Disclosure of operating segments [line items]      
Equity Accounted Joint Venture 88.9 89.9  
Asanko Gold - impairment (49.5)    
Mining assets [member] | Asanko Gold [member]      
Disclosure of operating segments [line items]      
Asanko Gold - impairment 49.5    
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 2.6 3.1 13.1
Corporate related costs 8.3 18.9 31.4
Repurchase of notes, value   1,000.0  
Gain loss on repurchase of notes   5.0  
Salaries VAT Income 23.9    
Asanko [member]      
Disclosure of operating segments [line items]      
Equity Accounted Joint Venture $ 88.9 $ 89.9 $ 85.8
South deep [member]      
Disclosure of operating segments [line items]      
Tax rate included in South Deep 29.00% 29.00% 29.00%