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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