Quarterly report pursuant to Section 13 or 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
Mar. 31, 2018
Dec. 31, 2017
Loss Contingencies [Line Items]    
Environmental $ 3.1 $ 2.9
Mine Reclamation and Closing Liability, current and noncurrent 199.4 197.2
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent 202.5 200.1
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current 21.3 3.6
Environmental Loss Contingency And Mine Reclamation And Closing Liability Noncurrent 181.2 196.5
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent 170.7 168.4
Asia Pacific Iron Ore [Member]    
Loss Contingencies [Line Items]    
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current 17.7  
Asia Pacific Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent $ 28.7 $ 28.8