Annual report pursuant to Section 13 and 15(d)

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Investments in Affiliates (Details)

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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
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%