Annual report pursuant to Section 13 and 15(d)

RELATED PARTIES (Narrative) (Details)

v3.8.0.1
RELATED PARTIES (Narrative) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Facility
Dec. 31, 2016
USD ($)
Segment Reporting Information [Line Items]    
Noncontrolling Interest, Ownership Percentage by Parent 100.00%  
Due to Related Parties, Current $ 12.3 $ 0.0
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ (105.0)  
U.S. Iron Ore [Member]    
Segment Reporting Information [Line Items]    
Number Of Mines | Facility 4  
Joint Venture Partners [Member] | U.S. Iron Ore [Member]    
Segment Reporting Information [Line Items]    
Number Of Mines | Facility 1  
Empire [Member]    
Segment Reporting Information [Line Items]    
Noncontrolling Interest, Ownership Percentage by Parent 100.00%  
Increase in Ownership Equity 21.00%  
Purchase of Noncontrolling Interest $ 132.7  
Stockholders' Equity, Period Increase (Decrease) (12.1)  
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests (116.7)  
Empire [Member] | Arcelor Mittal [Member]    
Segment Reporting Information [Line Items]    
Recorded Distributions of Partners' Equity Amounts 57.5 51.7
Paid Distributions of Partners' Equity Amounts 48.8 40.6
Subsequent Paid Distributions of Partners' Equity Amounts 8.7 $ 11.1
Other Liabilities [Member] | Empire [Member]    
Segment Reporting Information [Line Items]    
Noncontrolling Interest Purchase, Installment Amount 44.2  
Other Noncurrent Liabilities [Member] | Empire [Member]    
Segment Reporting Information [Line Items]    
Noncontrolling Interest Purchase, Installment Amount $ 44.2