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Financial Instruments - Summary of Assets and Liabilities Measured at Fair Value by Level within Fair Value Hierarchy (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds $ 79.3 $ 69.5 $ 60.8
Derivative financial assets 12.9 11.7  
Derivative financial liabilities 21.8 127.6  
Borrowings 1,526.9 1,845.8  
Financial liabilities not measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Borrowings 1,689.8 1,952.4  
Investments - listed [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Investments 42.4 47.9  
Environmental trust funds [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 7.4 7.2  
Environmental trust funds [member] | Financial assets not measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 71.9 62.3  
Trade receivables from provisional copper concentrate sales [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Trade receivables from provisional copper sales 23.7 22.8  
Asanko redeemable preference shares [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Redeemable preference shares 92.6 95.5  
Warrants [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 12.9 11.7  
Oil derivative contracts [member] | Financial liabilities measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 15.1 0.0  
Oil derivative contracts [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 0.0 1.1  
Copper derivative contracts [member] | Financial liabilities measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 14.0 0.0  
Gold derivative contracts [member] | Financial liabilities measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 0.0 127.3  
Gold derivative contracts [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 27.3 0.0  
Foreign Currency Derivative Contracts [Member] | Financial liabilities measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 0.0 0.3  
Foreign Currency Derivative Contracts [Member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 86.0 0.0  
Loan Advanced Contractor [Member] | Financial assets not measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Loan advanced - contractor 68.4 0.0  
Level 1 [member] | Financial liabilities not measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Borrowings 1,156.3 1,700.4  
Level 1 [member] | Investments - listed [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Investments 42.4 47.9  
Level 2 [member] | Environmental trust funds [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 7.4 7.2  
Level 2 [member] | Environmental trust funds [member] | Financial assets not measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 71.9 62.3  
Level 2 [member] | Trade receivables from provisional copper concentrate sales [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Trade receivables from provisional copper sales 23.7 22.8  
Level 2 [member] | Warrants [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 12.9 11.7  
Level 2 [member] | Oil derivative contracts [member] | Financial liabilities measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 15.1 0.0  
Level 2 [member] | Oil derivative contracts [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 0.0 1.1  
Level 2 [member] | Copper derivative contracts [member] | Financial liabilities measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 14.0 0.0  
Level 2 [member] | Gold derivative contracts [member] | Financial liabilities measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 0.0 127.3  
Level 2 [member] | Gold derivative contracts [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 27.3 0.0  
Level 2 [member] | Foreign Currency Derivative Contracts [Member] | Financial liabilities measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 0.0 0.3  
Level 2 [member] | Foreign Currency Derivative Contracts [Member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 86.0 0.0  
Level 3 [Member] | Financial liabilities not measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Borrowings 533.5 252.0  
Level 3 [Member] | Asanko redeemable preference shares [member] | Financial assets measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Redeemable preference shares 92.6 95.5  
Level 3 [Member] | Loan Advanced Contractor [Member] | Financial assets not measured at fair value [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Loan advanced - contractor $ 68.4 $ 0.0