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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, 2019
Dec. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Disclosure of fair value measurement of assets and liability [line items]        
Environmental trust funds $ 69.5 $ 60.8 [1] $ 55.5 $ 55.5
Trade receivables from provisional copper sales 23.6 15.3 [1]    
Redeemable preference shares 95.5 132.9 [1]    
Derivative financial assets 11.7 9.3    
Borrowings 1,845.8 1,906.8 [1]   $ 1,782.4 [1]
Environmental trust funds [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Environmental trust funds 7.2 6.5 [1]    
Financial assets at amortised cost, class [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Environmental trust funds 62.3 54.3 [1]    
Borrowings 1,952.4 1,897.0 [1]    
Investments - listed [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Investments 47.9 93.0 [1]    
Investments - unlisted [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Investments [1]   0.1    
Trade receivables from provisional copper concentrate sales [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Trade receivables from provisional copper sales 22.8 15.3 [1]    
Warrants [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Derivative financial assets 11.7 9.3 [1]    
Oil derivative contracts [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Derivative financial assets 1.1 4.7 [1]    
Copper derivative contracts [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Derivative financial assets [1]   1.2    
Gold derivative contracts [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Derivative financial assets [1]   2.4    
Derivative financial liabilities 127.3 13.9 [1]    
Foreign exchange derivatives [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Derivative financial liabilities 0.3 8.7 [1]    
Level 1 [member] | Financial assets at amortised cost, class [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Borrowings 1,700.4 839.6 [1]    
Level 1 [member] | Investments - listed [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Investments 47.9 93.0 [1]    
Level 2 [member] | Environmental trust funds [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Environmental trust funds 7.2 6.5 [1]    
Level 2 [member] | Financial assets at amortised cost, class [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Environmental trust funds 62.3 54.3 [1]    
Level 2 [member] | Trade receivables from provisional copper concentrate sales [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Trade receivables from provisional copper sales 22.8 15.3 [1]    
Level 2 [member] | Warrants [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Derivative financial assets 11.7 9.3 [1]    
Level 2 [member] | Oil derivative contracts [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Derivative financial assets 1.1 4.7 [1]    
Level 2 [member] | Copper derivative contracts [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Derivative financial assets [1]   1.2    
Level 2 [member] | Gold derivative contracts [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Derivative financial assets [1]   2.4    
Derivative financial liabilities 127.3 13.9 [1]    
Level 2 [member] | Foreign exchange derivatives [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Derivative financial liabilities 0.3 8.7 [1]    
Level 3 [Member]        
Disclosure of fair value measurement of assets and liability [line items]        
Redeemable preference shares 95.5 132.9 [1]    
Level 3 [Member] | Financial assets at amortised cost, class [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Borrowings $ 252.0 1,057.4 [1]    
Level 3 [Member] | Investments - unlisted [member]        
Disclosure of fair value measurement of assets and liability [line items]        
Investments [1]   $ 0.1    
[1] Refer note 42.