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RELATED PARTIES (Narrative) (Details) $ in Millions | 6 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jun. 30, 2019 USD ($) Facility | Dec. 31, 2017 USD ($) | Dec. 31, 2018
Segment Reporting Information [Line Items] |  |  |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00% |  |
Mining and Pelletizing [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Number of mines (in number of facilities) | Facility | 4 |  |
Empire |  |  |
Segment Reporting Information [Line Items] |  |  |
Noncontrolling Interest, Ownership Percentage by Parent |  | 100.00% |
Purchase of Noncontrolling Interest | $ |  | $ 132.7 |
Other Liabilities [Member] | Empire |  |  |
Segment Reporting Information [Line Items] |  |  |
Noncontrolling Interest Purchase, Installment Amount | $ | $ 44.2 |  |
Hibbing [Member] | Mining and Pelletizing [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Number of mines (in number of facilities) | Facility | 1 |  |
Hibbing [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Ownership interest, equity method investment | 23.00% |  | 23.00%

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