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ACQUISITIONS - Narrative (Details)
$ in Millions
3 Months Ended
Dec. 09, 2020
USD ($)
Mar. 13, 2020
USD ($)
Mar. 31, 2021
USD ($)
Mar. 31, 2020
USD ($)
Business Acquisition [Line Items]        
Acquisition-related costs     $ 13 $ 42
ArcelorMittal USA        
Business Acquisition [Line Items]        
Acquisition-related costs     $ 2  
Consideration transferred $ 2,600      
AK Steel        
Business Acquisition [Line Items]        
Exchange ratio   0.400    
Consideration transferred   $ 1,535