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ACCOUNTING POLICIES (Intangible Assets) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Disclosure of detailed information about intangible assets [line items]      
Mining concession, period 50 years    
Mining concession, additional term 50 years    
Research and development expense $ 19.0 $ 16.3 $ 9.8
Aggregate cash-generating units for which amount of goodwill or intangible assets with indefinite useful lives is not significant      
Disclosure of detailed information about intangible assets [line items]      
Goodwill $ 662.3 662.3  
Information system projects      
Disclosure of detailed information about intangible assets [line items]      
Amortisation period (not exceeding) 3 years    
Mining assets      
Disclosure of detailed information about intangible assets [line items]      
Impairment loss recognised in profit or loss, property, plant and equipment $ 42.3    
Steel | Aggregate cash-generating units for which amount of goodwill or intangible assets with indefinite useful lives is not significant      
Disclosure of detailed information about intangible assets [line items]      
Goodwill $ 619.8 $ 619.8  
Mining      
Disclosure of detailed information about intangible assets [line items]      
Amortisation rates, intangible assets other than goodwill 13.00% 12.00% 7.00%
Mining | Aggregate cash-generating units for which amount of goodwill or intangible assets with indefinite useful lives is not significant      
Disclosure of detailed information about intangible assets [line items]      
Goodwill $ 42.5 $ 42.5  
Bottom of range | Trademarks      
Disclosure of detailed information about intangible assets [line items]      
Useful lives or amortisation rates, intangible assets other than goodwill 5 years    
Top of range | Trademarks      
Disclosure of detailed information about intangible assets [line items]      
Useful lives or amortisation rates, intangible assets other than goodwill 10 years