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
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2017
Hibbing [Member] | Other Noncurrent Assets [Member]              
Related Party Transaction [Line Items]              
Ownership interest, equity method investment 23.00%           23.00%
Hibbing [Member] | Other Noncurrent Liabilities [Member]              
Related Party Transaction [Line Items]              
Equity Method Investments $ 7.2           $ 11.0
Unrealized Net Gain (Loss) on Foreign Currency Translation [Member]              
Related Party Transaction [Line Items]              
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax $ (228.1) $ 0.0 $ 0.0 $ 0.0 $ 0.0 $ 0.0