Quarterly report pursuant to Section 13 or 15(d)

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)

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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Schedule of Equity Method Investments [Line Items]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 32.8  
Land Improvements [Member] | Minimum    
Schedule of Equity Method Investments [Line Items]    
Property, Plant and Equipment, Useful Life 20 years  
Land Improvements [Member] | Maximum [Member]    
Schedule of Equity Method Investments [Line Items]    
Property, Plant and Equipment, Useful Life 45 years  
Buildings [Member] | Minimum    
Schedule of Equity Method Investments [Line Items]    
Property, Plant and Equipment, Useful Life 40 years  
Buildings [Member] | Maximum [Member]    
Schedule of Equity Method Investments [Line Items]    
Property, Plant and Equipment, Useful Life 45 years  
Mining and Pelletizing Equipment [Member] | Minimum    
Schedule of Equity Method Investments [Line Items]    
Property, Plant and Equipment, Useful Life 3 years  
Mining and Pelletizing Equipment [Member] | Maximum [Member]    
Schedule of Equity Method Investments [Line Items]    
Property, Plant and Equipment, Useful Life 20 years  
Steel and Manufacturing Equipment [Member] | Minimum    
Schedule of Equity Method Investments [Line Items]    
Property, Plant and Equipment, Useful Life 3 years  
Steel and Manufacturing Equipment [Member] | Maximum [Member]    
Schedule of Equity Method Investments [Line Items]    
Property, Plant and Equipment, Useful Life 20 years  
Sales [Member]    
Schedule of Equity Method Investments [Line Items]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 26.8 $ (5.5)