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RELATED PARTIES (Narrative) (Details) $ in Millions | 6 Months Ended |  |  |
--- | --- | --- | --- | ---
Jun. 30, 2018 USD ($) Facility | Jan. 01, 2018 USD ($) | Dec. 31, 2017 USD ($) | Mar. 31, 2017
Segment Reporting Information [Line Items] |  |  |  |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00% |  |  |
Partnership distribution payable | $ 44.2 | $ 44.2 | $ 44.2 |
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% |  |  | 79.00%
Purchase of Noncontrolling Interest | $ 132.7 |  |  |
Other Noncurrent Liabilities [Member] | Empire |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Noncontrolling Interest Purchase, Installment Amount | $ 44.2 |  |  |

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