Annual report pursuant to Section 13 and 15(d)

PENSION AND OTHER POSTRETIREMENT BENEFITS - Obligations and Funded Status (Details)

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PENSION AND OTHER POSTRETIREMENT BENEFITS - Obligations and Funded Status (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 09, 2020
Mar. 13, 2020
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Amounts recognized in accumulated other comprehensive loss (income):          
Accumulated benefit obligation     $ 6,013 $ 6,537  
AK Steel          
Change in benefit obligations:          
Acquired through business combinations   $ 949      
ArcelorMittal USA          
Change in benefit obligations:          
Acquired through business combinations $ 3,294        
Pension Plan          
Change in benefit obligations:          
Benefit obligations — beginning of year     6,565 1,021  
Service cost     56 23 $ 17
Interest cost     103 64 35
Plan amendments     0 0  
Actuarial loss (gain)     (131) 162  
Benefits paid     (456) (146)  
Participant contributions     0 0  
Acquired through business combinations     0 5,535  
Effect of settlement     (101) (94)  
Benefit obligations — end of year     6,036 6,565 1,021
Change in plan assets:          
Fair value of plan assets — beginning of year     5,332 749  
Actual return on plan assets     668 472  
Participant contributions     0 0  
Employer contributions     155 50  
Benefits paid     (454) (146)  
Acquired through business combinations     0 4,301  
Effect of settlement     (95) (94)  
Fair value of plan assets — end of year     5,606 5,332 749
Funded status     (430) (1,233)  
Amounts recognized in Statements of Financial Position:          
Non-current assets     153 3  
Current liabilities     (5) (12)  
Non-current liabilities     (578) (1,224)  
Total amount recognized     (430) (1,233)  
Amounts recognized in accumulated other comprehensive loss (income):          
Net actuarial loss (gain)     (286) 164  
Prior service cost (credit)     5 6  
Net amount recognized     (281) 170  
Other Postretirement Benefits Plan          
Change in benefit obligations:          
Benefit obligations — beginning of year     3,757 255  
Service cost     51 8 2
Interest cost     74 19 10
Plan amendments     8 0  
Actuarial loss (gain)     (456) 14  
Benefits paid     (227) (89)  
Participant contributions     47 22  
Acquired through business combinations     0 3,528  
Effect of settlement     0 0  
Benefit obligations — end of year     3,254 3,757 255
Change in plan assets:          
Fair value of plan assets — beginning of year     783 260  
Actual return on plan assets     29 45  
Participant contributions     26 17  
Employer contributions     139 30  
Benefits paid     (165) (88)  
Acquired through business combinations     0 519  
Effect of settlement     0 0  
Fair value of plan assets — end of year     812 783 $ 260
Funded status     (2,442) (2,974)  
Amounts recognized in Statements of Financial Position:          
Non-current assets     71 54  
Current liabilities     (130) (139)  
Non-current liabilities     (2,383) (2,889)  
Total amount recognized     (2,442) (2,974)  
Amounts recognized in accumulated other comprehensive loss (income):          
Net actuarial loss (gain)     (392) 56  
Prior service cost (credit)     4 (6)  
Net amount recognized     $ (388) $ 50