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RELATED PARTIES (Narrative) (Details) $ in Millions | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) Facility | Sep. 30, 2016 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2016 | Mar. 31, 2016 USD ($)
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 |  |  |  |
Tilden [Member] |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Increase in Ownership Equity | 15.00% |  |  |  |  |  |
Purchase of Noncontrolling Interest | $ 105.0 |  |  |  |  |  |
Stockholders' Equity, Period Increase (Decrease) | 89.1 |  |  |  |  |  |
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests | $ 15.9 |  | $ (105.0) |  |  |  |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00% |  | 100.00% |  |  | 85.00% |
Empire [Member] |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Increase in Ownership Equity | 21.00% |  |  |  |  |  |
Purchase of Noncontrolling Interest | $ 132.7 |  |  |  |  |  |
Noncontrolling Interest Purchase, Installment Amount | 44.2 |  |  |  |  |  |
Stockholders' Equity, Period Increase (Decrease) | 16.0 |  |  |  |  |  |
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests | $ 116.7 |  | $ (132.7) |  |  |  |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00% |  | 100.00% |  |  | 79.00% |
Empire [Member] | Other Current Liabilities [Member] |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Noncontrolling Interest Purchase, Installment Amount | $ 44.2 |  |  |  |  |  |
Empire [Member] | Other Noncurrent Liabilities [Member] |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Noncontrolling Interest Purchase, Installment Amount | $ 44.2 |  |  |  |  |  |
Empire [Member] | Arcelor Mittal [Member] |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Subsequent Paid Distributions of Partners' Equity Amounts |  |  |  |  | $ 8.7 |  | $ 11.1
Recorded Distributions of Partners' Equity Amounts |  | $ 7.4 |  | $ 48.8 |  |  |
Paid Distributions of Partners' Equity Amounts |  | $ 41.4 |  | $ 41.4 |  |  |

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