SEGMENT REPORTING (Summary of Assets by Segment) (Details) - USD ($) $ in Millions |
Sep. 30, 2018 |
Jan. 01, 2018 |
Dec. 31, 2017 |
|---|---|---|---|
| Segment Reporting Information [Line Items] | Ìý | Ìý | Ìý |
| Assets | $ 3,125.0 | $ 2,990.2 | $ 2,953.4 |
| U.S. Iron Ore [Member] | Ìý | Ìý | Ìý |
| Segment Reporting Information [Line Items] | Ìý | Ìý | Ìý |
| Assets | 1,798.8 | Ìý | 1,500.6 |
| Corporate Segment [Member] | Ìý | Ìý | Ìý |
| Segment Reporting Information [Line Items] | Ìý | Ìý | Ìý |
| Assets | 1,310.1 | Ìý | 1,314.0 |
| Discontinued Operations, Held-for-sale [Member] | Ìý | Ìý | Ìý |
| Segment Reporting Information [Line Items] | Ìý | Ìý | Ìý |
| Assets | $ 16.1 | Ìý | $ 138.8 |
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- Definition Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available.
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