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SEGMENT REPORTING SEGMENT REPORTING (Summary of Assets by Segment) (Details) - USD ($) $ in Millions | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | ---
Segment Reporting Information [Line Items] |  |  |
Assets | $ 1,923.9 | $ 2,135.5 | $ 3,147.2
U.S. Iron Ore [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 1,372.5 | 1,476.4 | 1,464.9
Asia Pacific Iron Ore [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 155.1 | 202.5 | 306.2
Total Segment Assets [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 1,527.6 | 1,678.9 | 1,771.1
Corporate [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 396.3 | 441.7 | 666.2
Discontinued Operations [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Disposal Group, Including Discontinued Operation, Assets | $ 0.0 | $ 14.9 | $ 709.9

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