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RELATED PARTIES (Narrative) (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 USD ($) Facility | Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items] |  |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00% |
Due to Related Parties, Current | $ 12.3 | $ 0.0
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests | $ (105.0) |
U.S. Iron Ore [Member] |  |
Segment Reporting Information [Line Items] |  |
Number Of Mines | Facility | 4 |
Joint Venture Partners [Member] | U.S. Iron Ore [Member] |  |
Segment Reporting Information [Line Items] |  |
Number Of Mines | Facility | 1 |
Empire [Member] |  |
Segment Reporting Information [Line Items] |  |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00% |
Increase in Ownership Equity | 21.00% |
Purchase of Noncontrolling Interest | $ 132.7 |
Stockholders' Equity, Period Increase (Decrease) | (12.1) |
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests | (116.7) |
Empire [Member] | Arcelor Mittal [Member] |  |
Segment Reporting Information [Line Items] |  |
Recorded Distributions of Partners' Equity Amounts | 57.5 | 51.7
Paid Distributions of Partners' Equity Amounts | 48.8 | 40.6
Subsequent Paid Distributions of Partners' Equity Amounts | 8.7 | $ 11.1
Other Liabilities [Member] | Empire [Member] |  |
Segment Reporting Information [Line Items] |  |
Noncontrolling Interest Purchase, Installment Amount | 44.2 |
Other Noncurrent Liabilities [Member] | Empire [Member] |  |
Segment Reporting Information [Line Items] |  |
Noncontrolling Interest Purchase, Installment Amount | $ 44.2 |

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