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SEGMENT REPORTING (Summary of Assets by Segment) (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Segment Reporting Information [Line Items] |  |  |
Assets | $ 3,503.8 | $ 3,529.6 | $ 2,953.4
Mining and Pelletizing [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 1,643.1 | 1,694.1 | 1,500.6
Metallics [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 913.6 | 265.9 | 13.4
Total Segment Assets [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 2,556.7 | 1,960.0 | 1,514.0
Corporate [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 940.1 | 1,557.2 | 1,300.6
Discontinued Operations [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets of discontinued operations | $ 7.0 | $ 12.4 | $ 138.8

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