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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Loss Contingencies [Line Items]    
Environmental $ 2.9 $ 2.8
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current 10.8 12.9
Environmental Loss Contingency And Mine Reclamation And Closing Liability Noncurrent 205.4 193.9
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent 195.0 187.8
Asia Pacific Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent 18.3 16.2
North American Coal [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent 213.3 204.0
Eastern Canadian Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent $ 216.2 $ 206.8