Annual report pursuant to Section 13 and 15(d)

ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details)

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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Site Contingency [Line Items]    
Payments for Environmental Liabilities $ 8.2 $ 2.4
Accrual for Environmental Loss Contingencies 8.4 15.7
Mine Reclamation and Closing Liability, current and noncurrent 312.6 249.4
Total environmental and mine closure obligations 321.0 265.1
Current Fiscal Year End Date --12-31  
U.S. Iron Ore [Member]
    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 5  
Eastern Canadian Iron Ore [Member]
    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 2  
Asia Pacific Iron Ore [Member]
    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 1  
North American Coal [Member]
    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 4  
LTV Steel Mining Company [Member]
    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 1  
Rio Tinto Mine Site [Member]
    
Site Contingency [Line Items]    
Accrual for Environmental Loss Contingencies 5.3 11.5
Loss Contingency, Range of Possible Loss, Maximum 12.2  
Rio Tinto Mine Site [Member] | Rio Tinto Working Group RTWG [Member]
    
Site Contingency [Line Items]    
Accrual for Environmental Loss Contingencies $ 29.0