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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Loss Contingencies [Line Items]    
Total environmental and mine closure obligations $ 265.1 $ 226.9
Payments related to environmental and mine closure liabilities 2.4 1.9
Environmental remediation 15.7 15.5
Mine Reclamation and Closing Liability, current and noncurrent 249.4 211.4
U.S. Iron Ore [Member]
   
Loss Contingencies [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 4  
Eastern Canadian Iron Ore [Member]
   
Loss Contingencies [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 2  
North American Coal [Member]
   
Loss Contingencies [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 6  
LTV Steel Mining Company [Member]
   
Loss Contingencies [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 1  
Asia Pacific Iron Ore [Member]
   
Loss Contingencies [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 1  
Rio Tinto Mine Site [Member]
   
Loss Contingencies [Line Items]    
Environmental remediation 11.5 10.0
Anticipated share of total settlement cost 12.0  
Rio Tinto Working Group RTWG [Member] | Rio Tinto Mine Site [Member]
   
Loss Contingencies [Line Items]    
Environmental remediation $ 25.0