BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Revenue Reimbursement) (Details) - USD ($) $ in Millions |
12 Months Ended | ||
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Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
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| Freight and venture partners' cost reimbursements | $ 241.0 | $ 195.5 | $ 180.9 |
| U.S. Iron Ore [Member] | Ìý | Ìý | Ìý |
| Freight and venture partners' cost reimbursements | 221.4 | 174.8 | 157.3 |
| U.S. Iron Ore [Member] | Freight Revenue [Member] | Ìý | Ìý | Ìý |
| Freight and venture partners' cost reimbursements | 166.7 | 106.8 | 105.3 |
| U.S. Iron Ore [Member] | Co-venturer [Member] | Ìý | Ìý | Ìý |
| Freight and venture partners' cost reimbursements | $ 54.7 | $ 68.0 | $ 52.0 |
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- Definition Repayment received or receivable for expenses incurred on behalf of a client or customer, other than those reimbursements received by landlords from tenants. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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