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Annual report pursuant to Section 13 and 15(d)

ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details)

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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Loss Contingencies [Line Items] Ìý Ìý
Environmental $ 2.9 $ 2.8
Mine Reclamation and Closing Liability, current and noncurrent 197.2 204.0
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent 200.1 206.8
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current 3.6 12.9
Environmental Loss Contingency And Mine Reclamation And Closing Liability Noncurrent 196.5 193.9
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member] Ìý Ìý
Loss Contingencies [Line Items] Ìý Ìý
Mine Reclamation and Closing Liability, current and noncurrent 168.4 187.8
Asia Pacific Iron Ore [Member] | Owned Or Operating Facilities [Member] Ìý Ìý
Loss Contingencies [Line Items] Ìý Ìý
Mine Reclamation and Closing Liability, current and noncurrent 28.8 16.2
LTV Steel Mining Company [Member] | Previously Owned Or Operating Facilities [Member] Ìý Ìý
Loss Contingencies [Line Items] Ìý Ìý
Mine Reclamation and Closing Liability, current and noncurrent $ 28.6 $ 25.5