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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
Freight and venture partners' cost reimbursements | $ 241.0 | $ 195.5 | $ 180.9
U.S. Iron Ore [Member] |  |  |
Freight and venture partners' cost reimbursements | 221.4 | 174.8 | 157.3
U.S. Iron Ore [Member] | Freight Revenue [Member] |  |  |
Freight and venture partners' cost reimbursements | 166.7 | 106.8 | 105.3
U.S. Iron Ore [Member] | Co-venturer [Member] |  |  |
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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 605 -SubTopic 45 -Section 45 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=68075638&loc=d3e60722-111653 + Details
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