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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Investments in Affiliates (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 09, 2020 | Dec. 31, 2019
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Other Noncurrent Assets [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures | $ (105) |  |
Other Noncurrent Liabilities [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures |  |  | $ (18)
Combined Metals of Chicago, LLC [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest, equity method investment | 40.00% |  |
Spartan Steel Coating, LLC [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest, equity method investment | 48.00% |  |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity | $ 33 |  |
Hibbing [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest, equity method investment | 85.30% | 62.30% | 23.00%

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- Definition Difference between amount at which an investment accounted for under the equity method of accounting is carried (reported) on the balance sheet and amount of underlying equity in net assets the reporting Entity has in the investee. + References 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)(3) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References 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 + Details
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