Annual report pursuant to Section 13 and 15(d)

SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION - Non-cash Investing and Financing Activities (Details)

v3.20.4
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