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ACCOUNTING POLICIES (Intangible assets) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about intangible assets [line items]      
Mining concession, period 50 years    
Mining concession, additional term 50 years    
Research and development expense $ 8.3 $ 10.0 $ 8.9
Information system projects      
Disclosure of detailed information about intangible assets [line items]      
Amortisation period (not exceeding) 3 years    
Customer relationships and other contractual rights      
Disclosure of detailed information about intangible assets [line items]      
Useful lives or amortisation rates, intangible assets other than goodwill 10 years    
Cash-generating units      
Disclosure of detailed information about intangible assets [line items]      
Goodwill $ 662.3 662.3  
Steel | Cash-generating units      
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 4.00% 5.00% 8.00%
Mining | Cash-generating units      
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