SEGMENT REPORTING - Summary of Assets by Segment (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Assets	$ 16,771	$ 3,504
Operating Segments [Member]
Segment Reporting Information [Line Items]
Assets	16,088	2,557
Steelmaking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Assets	15,849	2,557
Other businesses | Operating Segments [Member]
Segment Reporting Information [Line Items]
Assets	239	0
Corporate and Other Segments [Member]
Segment Reporting Information [Line Items]
Assets	$ 683	$ 947