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SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Segment Reporting Information [Line Items] |  |
Assets | $ 1,923.3 | $ 1,923.9
U.S. Iron Ore [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 1,467.2 | 1,372.5
Asia Pacific Iron Ore [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 139.4 | 155.1
Total Segment Assets [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 1,606.6 | 1,527.6
Corporate [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | $ 316.7 | $ 396.3

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