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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, 2022
Dec. 31, 2021
Dec. 31, 2020
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds $ 98.8 $ 88.1 $ 79.3
Derivative financial liabilities 0.0 6.8  
Borrowings 1,079.3 1,078.1  
Financial liabilities not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Borrowings 1,089.6 1,191.6  
Investments – listed | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Investments – listed 34.5 30.9  
Environmental trust funds | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 2.9 2.9  
Environmental trust funds | Financial assets not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 95.9 85.2  
Trade receivables from provisional copper sales | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Trade receivables from provisional copper sales 29.6 25.8  
Asanko redeemable preference shares | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Asanko redeemable preference shares 60.3 94.5  
Oil derivative contracts | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Oil derivative contracts 0.0 5.1  
Loan advanced – contractor | Financial assets not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Loan advanced – contractor 23.4 27.3  
Foreign currency derivative contracts | Financial liabilities measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 0.0 6.8  
Level 1 | Financial liabilities not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Borrowings 1,006.1 1,108.1  
Level 1 | Investments – listed | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Investments – listed 34.5 30.9  
Level 1 | Environmental trust funds | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 0.0 0.0  
Level 1 | Environmental trust funds | Financial assets not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 0.0 0.0  
Level 1 | Trade receivables from provisional copper sales | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Trade receivables from provisional copper sales 0.0 0.0  
Level 1 | Asanko redeemable preference shares | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Asanko redeemable preference shares 0.0 0.0  
Level 1 | Oil derivative contracts | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Oil derivative contracts 0.0 0.0  
Level 1 | Loan advanced – contractor | Financial assets not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Loan advanced – contractor 0.0  
Level 1 | Foreign currency derivative contracts | Financial liabilities measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 0.0 0.0  
Level 2 | Financial liabilities not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Borrowings 0.0 0.0  
Level 2 | Investments – listed | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Investments – listed 0.0 0.0  
Level 2 | Environmental trust funds | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 2.9 2.9  
Level 2 | Environmental trust funds | Financial assets not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 95.9 85.2  
Level 2 | Trade receivables from provisional copper sales | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Trade receivables from provisional copper sales 29.6 25.8  
Level 2 | Asanko redeemable preference shares | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Asanko redeemable preference shares 0.0 0.0  
Level 2 | Oil derivative contracts | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Oil derivative contracts 0.0 5.1  
Level 2 | Loan advanced – contractor | Financial assets not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Loan advanced – contractor 0.0 0.0  
Level 2 | Foreign currency derivative contracts | Financial liabilities measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 0.0 6.8  
Level 3 | Financial liabilities not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Borrowings 83.5 83.5  
Level 3 | Investments – listed | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Investments – listed 0.0 0.0  
Level 3 | Environmental trust funds | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 0.0 0.0  
Level 3 | Environmental trust funds | Financial assets not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 0.0 0.0  
Level 3 | Trade receivables from provisional copper sales | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Trade receivables from provisional copper sales 0.0 0.0  
Level 3 | Asanko redeemable preference shares | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Asanko redeemable preference shares 60.3 94.5  
Level 3 | Oil derivative contracts | Financial assets measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Oil derivative contracts 0.0 0.0  
Level 3 | Loan advanced – contractor | Financial assets not measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Loan advanced – contractor 23.4 27.3  
Level 3 | Foreign currency derivative contracts | Financial liabilities measured at fair value      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities $ 0.0 $ 0.0