RELATED PARTIES - Summary Of Ownership Interests (Details) - Hibbing Taconite Company [Member]	Sep. 30, 2020
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	23.00%
Arcelor Mittal [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	62.30%
U. S. Steel [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	14.70%

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant