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RELATED PARTIES (Narrative) (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 USD ($) Facility | Dec. 31, 2017 USD ($) installment
Segment Reporting Information [Line Items] |  |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00% |
Due to Related Parties, Current | $ 1.8 | $ 12.3
Acquisition of Noncontrolling Interest |  | $ (105.0)
Mining and Pelletizing [Member] |  |
Segment Reporting Information [Line Items] |  |
Number Of Mines | Facility | 4 |
Joint Venture Partners [Member] | Mining and Pelletizing [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%
Distribution of Noncontrolling Interest |  | $ 132.7
Noncontrolling Interest Purchase, Number Of Installment Payments | installment |  | 3
Stockholders' Equity, Period Increase (Decrease) |  | $ (12.1)
Acquisition of Noncontrolling Interest |  | $ (116.7)
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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