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SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details) - USD ($) $ in Millions | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
--- | --- | --- | ---
Segment Reporting Information [Line Items] |  |  |
Assets | $ 2,135.5 | $ 3,147.2 | $ 13,102.9
U.S. Iron Ore [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 1,476.4 | 1,464.9 | 1,537.9
Asia Pacific Iron Ore [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 202.5 | 306.2 | 1,176.8
Total Segment Assets [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 1,678.9 | 1,771.1 | 2,714.7
Corporate [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 441.7 | 666.2 | 204.2
Discontinued Operations [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Disposal Group, Including Discontinued Operation, Assets | $ 14.9 | $ 709.9 | $ 10,184.0

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