Annual report pursuant to Section 13 and 15(d)

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Reimbursements Revenue Disclosure) (Details)

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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Reimbursements Revenue Disclosure) (Details) (U.S. Iron Ore [Member], USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Reimbursement Revenue, by Type [Line Items]      
Reimbursement revenue $ 250.8 $ 224.3 $ 223.4
Freight [Member]
     
Reimbursement Revenue, by Type [Line Items]      
Reimbursement revenue 142.0 128.4 83.6
Venture Partners' Cost [Member]
     
Reimbursement Revenue, by Type [Line Items]      
Reimbursement revenue $ 108.8 $ 95.9 $ 139.8