Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTIES (Narrative) (Details)

v3.8.0.1
RELATED PARTIES (Narrative) (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
Facility
Sep. 30, 2016
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2016
Mar. 31, 2016
USD ($)
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     1        
Tilden [Member]              
Segment Reporting Information [Line Items]              
Increase in Ownership Equity 15.00%            
Purchase of Noncontrolling Interest $ 105.0            
Stockholders' Equity, Period Increase (Decrease) 89.1            
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ 15.9   $ (105.0)        
Noncontrolling Interest, Ownership Percentage by Parent 100.00%   100.00%     85.00%  
Empire [Member]              
Segment Reporting Information [Line Items]              
Increase in Ownership Equity 21.00%            
Purchase of Noncontrolling Interest $ 132.7            
Noncontrolling Interest Purchase, Installment Amount 44.2            
Stockholders' Equity, Period Increase (Decrease) 16.0            
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ 116.7   $ (132.7)        
Noncontrolling Interest, Ownership Percentage by Parent 100.00%   100.00%     79.00%  
Empire [Member] | Other Current Liabilities [Member]              
Segment Reporting Information [Line Items]              
Noncontrolling Interest Purchase, Installment Amount $ 44.2            
Empire [Member] | Other Noncurrent Liabilities [Member]              
Segment Reporting Information [Line Items]              
Noncontrolling Interest Purchase, Installment Amount $ 44.2            
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   $ 7.4   $ 48.8      
Paid Distributions of Partners' Equity Amounts   $ 41.4   $ 41.4