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GOODWILL AND INTANGIBLE ASSETS AND LIABILITIES - Goodwill (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Goodwill [Line Items]    
Goodwill $ 144.0 $ 2.1
Steel and Manufacturing [Member]    
Goodwill [Line Items]    
Goodwill 141.9  
Mining and Pelletizing [Member]    
Goodwill [Line Items]    
Goodwill $ 2.1 $ 2.1