Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTIES (Narrative) (Details)

v3.10.0.1
RELATED PARTIES (Narrative) (Details)
$ in Millions
 6 Months Ended
Jun. 30, 2018
USD ($)
Facility
Jan. 01, 2018
USD ($)
Dec. 31, 2017
USD ($)
Mar. 31, 2017
Segment Reporting Information [Line Items]        
Noncontrolling Interest, Ownership Percentage by Parent 100.00%      
Partnership distribution payable $ 44.2 $ 44.2 $ 44.2  
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 1      
Empire        
Segment Reporting Information [Line Items]        
Noncontrolling Interest, Ownership Percentage by Parent 100.00%     79.00%
Purchase of Noncontrolling Interest $ 132.7      
Other Noncurrent Liabilities [Member] | Empire        
Segment Reporting Information [Line Items]        
Noncontrolling Interest Purchase, Installment Amount $ 44.2