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ACCOUNTING POLICIES (Intangible assets) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of detailed information about intangible assets [line items]      
Mining concession, period 50 years    
Mining concession, additional term 50 years    
Research and development expense $ 9.8 $ 9.2 $ 6.2
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 P10Y    
Cash-generating units      
Disclosure of detailed information about intangible assets [line items]      
Goodwill $ 662.3    
Steel | Cash-generating units      
Disclosure of detailed information about intangible assets [line items]      
Goodwill $ 619.8    
Mining      
Disclosure of detailed information about intangible assets [line items]      
Amortisation rates, intangible assets other than goodwill 7.00% 7.00% 10.00%
Mining | Cash-generating units      
Disclosure of detailed information about intangible assets [line items]      
Goodwill $ 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 P5Y    
Top of range | Trademarks      
Disclosure of detailed information about intangible assets [line items]      
Useful lives or amortisation rates, intangible assets other than goodwill P10Y