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 6 Months Ended
Jun. 30, 2016
USD ($)
Jun. 30, 2017
Facility
Jun. 30, 2016
USD ($)
Mar. 31, 2017
USD ($)
Mar. 31, 2016
USD ($)
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 $ 24.4   $ 41.4    
Paid Distributions of Partners' Equity Amounts $ 17.0   $ 17.0