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Financial Instruments - Summary of Carrying Amounts and Fair Values of Financial Assets and Financial Liabilities (Detail) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
 [1]
Disclosure of detailed information about financial instruments [line items]          
- Borrowings $ 1,845.8 $ 1,906.8 [1]   $ 1,782.4 [1]  
- Trade and other payables 385.3 307.6      
- Finance lease liabilities 447.4 111.5      
- Environmental trust funds 69.5 60.8 [1] $ 55.5 55.5  
- Trade and other receivables 137.1 138.6 [1] 171.8 171.8  
- Investments 155.1 235.3      
- Cash and cash equivalents 515.0 219.7 [1] $ 393.8 $ 393.8 [1] $ 471.0
- Warrants 11.7 9.3      
Fair Value [member] | Financial liabilities at fair value [member]          
Disclosure of detailed information about financial instruments [line items]          
- Copper derivative contracts 127.6 22.6      
Total 127.6 22.6      
Fair Value [member] | Financial liabilities not measured at fair value [member]          
Disclosure of detailed information about financial instruments [line items]          
- Borrowings 1,952.4 1,897.0      
- Trade and other payables 385.3 307.6      
- Finance lease liabilities 332.9 88.6      
Total 2,670.6 2,293.2      
Fair Value [member] | Financial assets at fair value [member]          
Disclosure of detailed information about financial instruments [line items]          
- Environmental trust funds 7.2 6.5      
- Trade receivables from provisional copper sales 22.8 15.3      
- Investments 47.9 93.1      
Asanko redeemable preference shares 95.5 132.9      
- Warrants 11.7 9.3      
- Gold and oil derivative contracts 1.1 8.3      
Total 186.2 265.4      
Fair Value [member] | Financial assets not measured at fair value [member]          
Disclosure of detailed information about financial instruments [line items]          
- Environmental trust funds 62.3 54.3      
- Trade receivables from provisional copper sales   49.6      
- Trade and other receivables 51.4        
- Cash and cash equivalents 515.0 219.7      
Total 628.7 323.6      
Carrying amount [member] | Financial liabilities at fair value [member]          
Disclosure of detailed information about financial instruments [line items]          
- Copper derivative contracts 127.6 22.6      
Total 127.6 22.6      
Carrying amount [member] | Financial liabilities at fair value [member] | Fair value through profit or loss [member]          
Disclosure of detailed information about financial instruments [line items]          
- Copper derivative contracts 127.6 22.6      
Total 127.6 22.6      
Carrying amount [member] | Financial liabilities not measured at fair value [member]          
Disclosure of detailed information about financial instruments [line items]          
- Borrowings 1,845.8 1,906.8      
- Trade and other payables 385.3 307.6      
- Finance lease liabilities 332.9 88.6      
Total 2,564.0 2,303.0      
Carrying amount [member] | Financial liabilities not measured at fair value [member] | Other financial liabilities measured at amortised cost, category [member]          
Disclosure of detailed information about financial instruments [line items]          
- Borrowings 1,845.8        
- Trade and other payables 385.3        
- Finance lease liabilities 332.9        
Total 2,564.0        
Carrying amount [member] | Financial assets at fair value [member]          
Disclosure of detailed information about financial instruments [line items]          
- Environmental trust funds 7.2 6.5      
- Trade receivables from provisional copper sales 22.8 15.3      
- Investments 47.9 93.1      
Asanko redeemable preference shares 95.5 132.9      
- Warrants 11.7 9.3      
- Gold and oil derivative contracts 1.1 8.3      
Total 186.2 265.4      
Carrying amount [member] | Financial assets at fair value [member] | Fair value through profit or loss [member]          
Disclosure of detailed information about financial instruments [line items]          
- Environmental trust funds 7.2 6.5      
- Trade receivables from provisional copper sales 22.8 15.3      
- Warrants 11.7 9.3      
- Gold and oil derivative contracts 1.1 8.3      
Total 42.8 39.4      
Carrying amount [member] | Financial assets at fair value [member] | FVOCI [member]          
Disclosure of detailed information about financial instruments [line items]          
- Investments 47.9 93.1      
Asanko redeemable preference shares 95.5 132.9      
Total 143.4 226.0      
Carrying amount [member] | Financial assets not measured at fair value [member]          
Disclosure of detailed information about financial instruments [line items]          
- Environmental trust funds 62.3 54.3      
- Trade receivables from provisional copper sales   49.6      
- Trade and other receivables 51.4        
- Cash and cash equivalents 515.0 219.7      
Total 628.7 323.6      
Carrying amount [member] | Financial assets not measured at fair value [member] | Amortised cost [member]          
Disclosure of detailed information about financial instruments [line items]          
- Environmental trust funds 62.3 54.3      
- Trade receivables from provisional copper sales   49.6      
- Trade and other receivables 51.4        
- Cash and cash equivalents 515.0 219.7      
Total $ 628.7 $ 323.6      
[1] Refer note 42.