RELATED PARTIES - Narrative (Details) - Hibbing Taconite Company	Sep. 30, 2021	Dec. 09, 2020	Sep. 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%