SEGMENT REPORTING - Segment Capital Additions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Capital additions	$ 167	$ 73	$ 329	$ 231
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Capital additions	0	0	18	1
Steelmaking
Segment Reporting Information [Line Items]
Capital additions	146	66	279	220
Other Businesses
Segment Reporting Information [Line Items]
Capital additions	$ 21	$ 7	$ 32	$ 10