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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended |
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Dec. 31, 2017 | Mar. 31, 2018
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current | $ 3.6 | $ 21.3
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent | 200.1 | 202.5
U.S. Iron Ore [Member] |  |
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) | (26.2) |
Asia Pacific Iron Ore [Member] |  |
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current |  | $ 17.7
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) | $ (10.1) |

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- Definition Environmental Loss Contingency And Mine Reclamation And Closing Liability Current + References No definition available. + Details
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- Definition Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent + References No definition available. + Details
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- Definition Increase (decrease) in the accrual for environmental loss contingencies during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.Y.Q2) -URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751 + Details
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