SEGMENT REPORTING - Segment Assets (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total assets	$ 17,215	$ 16,771
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total assets	649	683
Operating Segments
Segment Reporting Information [Line Items]
Total assets	16,566	16,088
Operating Segments | Steelmaking
Segment Reporting Information [Line Items]
Total assets	16,271	15,849
Operating Segments | Other Businesses
Segment Reporting Information [Line Items]
Total assets	$ 295	$ 239