Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTIES (Narrative) (Details)

v3.7.0.1
RELATED PARTIES (Narrative) (Details)
$ in Millions
3 Months Ended
Mar. 31, 2017
USD ($)
Facility
Mar. 31, 2016
USD ($)
Dec. 31, 2016
USD ($)
Segment Reporting Information [Line Items]      
Due to Related Parties, Current     $ 8.7
Trade Accounts Receivable [Member]      
Segment Reporting Information [Line Items]      
Due from Related Parties, Current $ 9.1   46.9
Other Current Assets [Member]      
Segment Reporting Information [Line Items]      
Due from Related Parties, Current $ 53.8   $ 26.8
U.S. Iron Ore [Member]      
Segment Reporting Information [Line Items]      
Number of mines (in number of facilities) | Facility 4    
Joint Venture Partners [Member] | U.S. Iron Ore [Member]      
Segment Reporting Information [Line Items]      
Number of mines (in number of facilities) | Facility 2    
Empire [Member] | Arcelor Mittal [Member]      
Segment Reporting Information [Line Items]      
Subsequent Paid Distributions of Partners' Equity Amounts $ 8.7 $ 11.1  
Recorded Distributions of Partners' Equity Amounts   $ 17.0