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RELATED PARTIES (Narrative) (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2017 USD ($) Facility | Mar. 31, 2016 USD ($) | Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items] |  |  |
Due to Related Parties, Current |  |  | $ 8.7
Trade Accounts Receivable [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Due from Related Parties, Current | $ 9.1 |  | 46.9
Other Current Assets [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Due from Related Parties, Current | $ 53.8 |  | $ 26.8
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 |  | $ 17.0 |

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