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RELATED PARTIES (Narrative) (Details) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) Facility
Segment Reporting Information [Line Items] |  |
Noncontrolling Interest, Ownership Percentage by Parent |  | 100.00%
Purchase of Noncontrolling Interest | $ (105.0) |
U.S. Iron Ore [Member] |  |
Segment Reporting Information [Line Items] |  |
Number of mines (in number of facilities) | Facility |  | 4
Joint Venture Partners [Member] | U.S. Iron Ore [Member] |  |
Segment Reporting Information [Line Items] |  |
Number of mines (in number of facilities) | Facility |  | 1
Empire |  |
Segment Reporting Information [Line Items] |  |
Noncontrolling Interest, Ownership Percentage by Parent |  | 100.00%
Purchase of Noncontrolling Interest |  | $ 132.7
Noncontrolling Interest Purchase, Installment Amount |  | 44.2
Other Noncurrent Liabilities [Member] | Empire |  |
Segment Reporting Information [Line Items] |  |
Noncontrolling Interest Purchase, Installment Amount |  | $ 44.2

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