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Environmental And Mine Closure Obligations (Summary Of Mine Closure Obligations) (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Loss Contingencies [Line Items]    
Environmental $ 16.6 $ 15.5
Total mine closure 225.4 211.4
Total environmental and mine closure obligations 242.0 226.9
Less current portion 15.7 13.7
Long term environmental and mine closure obligations 226.3 213.2
Previously Owned Or Operating Facilities [Member] | LTV Steel Mining Company [Member]
   
Loss Contingencies [Line Items]    
Total mine closure 17.1 16.5
Owned Or Operating Facilities [Member] | U.S. Iron Ore [Member]
   
Loss Contingencies [Line Items]    
Total mine closure 79.7 74.3
Owned Or Operating Facilities [Member] | Eastern Canadian Iron Ore [Member]
   
Loss Contingencies [Line Items]    
Total mine closure 73.2 68.0
Owned Or Operating Facilities [Member] | Asia Pacific Iron Ore [Member]
   
Loss Contingencies [Line Items]    
Total mine closure 17.3 16.3
Owned Or Operating Facilities [Member] | North American Coal [Member]
   
Loss Contingencies [Line Items]    
Total mine closure $ 38.1 $ 36.3