Annual report pursuant to Section 13 and 15(d)

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Revenue Reimbursement) (Details)

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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Revenue Reimbursement) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Freight and venture partners' cost reimbursements $ 180.9 $ 278.0 $ 259.0
U.S. Iron Ore [Member]      
Freight and venture partners' cost reimbursements 157.3 271.0 259.5
U.S. Iron Ore [Member] | Freight Revenue [Member]      
Freight and venture partners' cost reimbursements 105.3 163.0 177.3
U.S. Iron Ore [Member] | Co-venturer [Member]      
Freight and venture partners' cost reimbursements $ 52.0 $ 108.0 $ 82.2