Annual report pursuant to Section 13 and 15(d)

RELATED PARTIES (Narrative) (Details)

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RELATED PARTIES (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2002
Segment Reporting Information [Line Items]      
Accounts receivable and derivative assets related parties $ 149.8 $ 180.4  
Other current liabilities related parties 20.2 43.0  
U.S. Iron Ore [Member]
     
Segment Reporting Information [Line Items]      
Number of mines (in number of facilities) 5    
Eastern Canadian Iron Ore [Member]
     
Segment Reporting Information [Line Items]      
Number of mines (in number of facilities) 2    
Joint Venture Partners [Member] | U.S. Iron Ore [Member]
     
Segment Reporting Information [Line Items]      
Number of mines (in number of facilities) 3    
Joint Venture Partners [Member] | Eastern Canadian Iron Ore [Member]
     
Segment Reporting Information [Line Items]      
Number of mines (in number of facilities) 1    
Empire [Member]
     
Segment Reporting Information [Line Items]      
Long term accounts notes and loans receivable net noncurrent 19.3 26.5 120.0
Life of supply agreement (in years) 12 years    
Prior [Member] | Empire [Member]
     
Segment Reporting Information [Line Items]      
Percent ownership interest before transaction     46.70%
Current [Member] | Empire [Member]
     
Segment Reporting Information [Line Items]      
Percent ownership interest after transaction     79.00%
Other Non-current Assets [Member] | Empire [Member]
     
Segment Reporting Information [Line Items]      
Present value of other non-current assets $ 9.3 $ 16.5