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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) - Hibbing [Member] - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Other Noncurrent Assets [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 23.00% 23.00%
Other Noncurrent Liabilities [Member]    
Related Party Transaction [Line Items]    
Equity Method Investments $ 8.0 $ 11.0