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)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
Sep. 30, 2019
USD ($)
unit
Sep. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Schedule of Equity Method Investments [Line Items]        
Number of Reporting Units | unit   2    
Gain on foreign currency translation $ 228.1 $ 0.0 $ 228.1  
Hibbing [Member]        
Schedule of Equity Method Investments [Line Items]        
Ownership interest, equity method investment   23.00%   23.00%
Hibbing [Member] | Other Noncurrent Liabilities [Member]        
Schedule of Equity Method Investments [Line Items]        
Equity Method Investments   $ 14.0   $ 15.4