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Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTIES - Narrative (Details)

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