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) - Hibbing [Member] - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
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 $ 6.5 $ 8.7