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RELATED PARTIES (Narrative) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2019 | Dec. 31, 2017 | Dec. 31, 2018
Segment Reporting Information [Line Items] |  |  |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00% |  |
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] |  |  |
Segment Reporting Information [Line Items] |  |  |
Ownership interest, equity method investment | 23.00% |  | 23.00%

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