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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) T
Site Contingency [Line Items] |  |
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) |  | $ (9.2)
Proved Developed and Undeveloped Reserves, Net, Period Increase (Decrease) | T |  | 115,000,000
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent | $ 200.1 | $ 206.8
Accrual for Environmental Loss Contingencies | 2.9 | 2.8
Mine Reclamation and Closing Liability, current and noncurrent | 197.2 | 204.0
U.S. Iron Ore [Member] |  |
Site Contingency [Line Items] |  |
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) | (26.2) |
Asia Pacific Iron Ore [Member] |  |
Site Contingency [Line Items] |  |
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) | 10.1 |
Previously Owned Or Operating Facilities [Member] | LTV Steel Mining Company [Member] |  |
Site Contingency [Line Items] |  |
Mine Reclamation and Closing Liability, current and noncurrent | $ 28.6 | 25.5
Selenium [Domain] |  |
Site Contingency [Line Items] |  |
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) |  | $ (29.6)

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- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 + Details
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