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, 2017
Dec. 31, 2016
Loss Contingencies [Line Items]    
Environmental $ 2.8 $ 2.8
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current 12.8 12.9
Environmental Loss Contingency And Mine Reclamation And Closing Liability Noncurrent 198.2 193.9
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent 190.9 187.8
Asia Pacific Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent 17.3 16.2
North American Coal [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent 208.2 204.0
Eastern Canadian Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent $ 211.0 $ 206.8