Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTIES - Narrative (Details)

v3.21.2
RELATED PARTIES - Narrative (Details) - Hibbing Taconite Company
Jun. 30, 2021
Dec. 09, 2020
Jun. 30, 2020
Related Party Transaction [Line Items]      
Equity Method Investment, Ownership Percentage 85.30%   23.00%
U. S. Steel      
Related Party Transaction [Line Items]      
Equity Method Investment, Ownership Percentage 14.70%    
Arcelor Mittal      
Related Party Transaction [Line Items]      
Equity Method Investment, Ownership Percentage   62.30%