Annual report pursuant to Section 13 and 15(d)

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)

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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
employees
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Basis of Presentation and Significant Accounting Policies [Line Items]      
Entity Number of Employees | employees 25,000    
Foreign Currency Transaction Gain (Loss), Realized | $ $ 0 $ 0 $ 228
Asia Pacific Iron Ore [Member]      
Basis of Presentation and Significant Accounting Policies [Line Items]      
Foreign Currency Transaction Gain (Loss), Realized | $     $ 228
Workforce Subject to Collective Bargaining Arrangements [Member]      
Basis of Presentation and Significant Accounting Policies [Line Items]      
Entity Number of Employees | employees 18,500