SEGMENT REPORTING - Segment Assets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total assets	$ 18,066	$ 16,771
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total assets	276	683
Operating Segments
Segment Reporting Information [Line Items]
Total assets	17,790	16,088
Operating Segments | Steelmaking
Segment Reporting Information [Line Items]
Total assets	17,439	15,849
Operating Segments | Other Businesses
Segment Reporting Information [Line Items]
Total assets	$ 351	$ 239