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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

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