Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTIES (Narrative) (Details)

v3.19.1
RELATED PARTIES (Narrative) (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
Facility
Dec. 31, 2017
USD ($)
Dec. 31, 2018
Segment Reporting Information [Line Items]      
Noncontrolling Interest, Ownership Percentage by Parent 100.00%    
Mining and Pelletizing [Member]      
Segment Reporting Information [Line Items]      
Number of mines (in number of facilities) | Facility 4    
Empire      
Segment Reporting Information [Line Items]      
Noncontrolling Interest, Ownership Percentage by Parent   100.00%  
Purchase of Noncontrolling Interest | $   $ 132.7  
Other Liabilities [Member] | Empire      
Segment Reporting Information [Line Items]      
Noncontrolling Interest Purchase, Installment Amount | $ $ 44.2    
Hibbing [Member] | Mining and Pelletizing [Member]      
Segment Reporting Information [Line Items]      
Number of mines (in number of facilities) | Facility 1    
Hibbing [Member] | Investments in Ventures [Member]      
Segment Reporting Information [Line Items]      
Ownership interest, equity method investment 23.00%   23.00%