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RELATED PARTIES (Narrative) (Details) $ in Millions | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2016 USD ($) | Jun. 30, 2017 Facility | Jun. 30, 2016 USD ($) | Mar. 31, 2017 USD ($) | Mar. 31, 2016 USD ($)
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 |  | 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 | $ 24.4 |  | $ 41.4 |  |
Paid Distributions of Partners' Equity Amounts | $ 17.0 |  | $ 17.0 |  |

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