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SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details) - USD ($) $ in Millions | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Segment Reporting Information [Line Items] |  |
Assets | $ 2,271.5 | $ 3,164.0
U.S. Iron Ore [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 1,498.3 | 1,464.9
Asia Pacific Iron Ore [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 210.5 | 274.6
All Other Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 26.3 | 147.0
Total Segment Assets [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 1,735.1 | 1,886.5
Corporate [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 395.0 | 546.8
Discontinued Operations [Member] |  |
Segment Reporting Information [Line Items] |  |
Disposal Group, Including Discontinued Operation, Assets | $ 141.4 | $ 730.7

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