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, 2016
Dec. 31, 2015
Loss Contingencies [Line Items]    
Environmental $ 2.8 $ 3.6
Mine Reclamation and Closing Liability, current and noncurrent 204.0 230.4
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent 206.8 234.0
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current 12.9 2.8
Environmental Loss Contingency And Mine Reclamation And Closing Liability Noncurrent 193.9 231.2
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent 187.8 214.0
Asia Pacific Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent 16.2 16.4
LTV Steel Mining Company [Member] | Previously Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent $ 25.5 $ 24.1