Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTIES (Narrative) (Details)

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RELATED PARTIES (Narrative) (Details)
$ in Millions
6 Months Ended
Jun. 30, 2015
USD ($)
Facility
Dec. 31, 2014
USD ($)
Segment Reporting Information [Line Items]    
Due from Related Parties, Current $ 18.4 $ 127.6
Due to Related Parties, Current $ 18.3 $ 11.8
U.S. Iron Ore [Member]    
Segment Reporting Information [Line Items]    
Number of mines (in number of facilities) | Facility 5  
Joint Venture Partners [Member] | U.S. Iron Ore [Member]    
Segment Reporting Information [Line Items]    
Number of mines (in number of facilities) | Facility 3  
Empire [Member] | Arcelor Mittal [Member]    
Segment Reporting Information [Line Items]    
Recorded Distributions of Partners' Equity Amounts $ 31.7  
Paid Distributions of Partners' Equity Amounts $ 17.1