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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Revenue Reimbursement) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Freight and Reimbursement Revenue | $ 160.1 | $ 221.4 | $ 174.8
Mining and Pelletizing [Member] |  |  |
Freight and Reimbursement Revenue | 160.1 | 221.4 | 174.8
Mining and Pelletizing [Member] | Freight Revenue [Member] |  |  |
Freight and Reimbursement Revenue | 160.1 | 166.7 | 106.8
Mining and Pelletizing [Member] | Co-venturer [Member] |  |  |
Freight and Reimbursement Revenue | 0.0 | $ 54.7 | $ 68.0
Calculated under Revenue Guidance in Effect before Topic 606 [Member] |  |  |
Freight and Reimbursement Revenue | $ 156.2 |  |

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