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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds $ 60.8 $ 55.5 $ 44.5
Trade receivables from provisional copper sales 23.4 46.6  
Redeemable preference shares 132.9    
Derivative financial assets 9.3 5.5  
Derivative financial liabilities 22.6 3.3  
Environmental trust funds [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 6.5 7.3  
Investments - listed [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Investments 93.0 99.0  
Investments - unlisted [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Investments 0.1 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 15.3 21.2  
Warrants [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 9.3 5.5  
Oil derivative contracts [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 4.7 14.1  
Copper derivative contracts [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 1.2    
Derivative financial liabilities 0.0 3.3  
Gold derivative contracts [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 2.4 10.9  
Derivative financial liabilities 13.9    
Foreign exchange derivatives [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 8.7    
Level 1 [member] | Investments - listed [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Investments 93.0 99.0  
Level 2 [member] | Environmental trust funds [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Environmental trust funds 6.5 7.3  
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 15.3 21.2  
Level 2 [member] | Warrants [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 9.3 5.5  
Level 2 [member] | Oil derivative contracts [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 4.7 14.1  
Level 2 [member] | Copper derivative contracts [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 1.2    
Derivative financial liabilities   3.3  
Level 2 [member] | Gold derivative contracts [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial assets 2.4 10.9  
Derivative financial liabilities 13.9    
Level 2 [member] | Foreign exchange derivatives [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Derivative financial liabilities 8.7    
Level 3 [Member]      
Disclosure of fair value measurement of assets and liability [line items]      
Redeemable preference shares 132.9    
Level 3 [Member] | Investments - unlisted [member]      
Disclosure of fair value measurement of assets and liability [line items]      
Investments $ 0.1 $ 0.1