SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION - Non-cash Investing and Financing Activities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
ArcelorMittal USA [Member]
Noncash or Part Noncash Acquisitions [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given, Pre-existing Relationship	$ 237	$ 0	$ 0
ArcelorMittal USA [Member] | Common Stock [Member]
Noncash or Part Noncash Acquisitions [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given	990	0	0
ArcelorMittal USA [Member] | Series B Preferred Stock
Noncash or Part Noncash Acquisitions [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given	738	0	0
AK Steel [Member] | Common Stock [Member]
Noncash or Part Noncash Acquisitions [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given	618	0	0
Share-based Payment Arrangement [Member] | AK Steel [Member]
Noncash or Part Noncash Acquisitions [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given	$ 4	$ 0	$ 0