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. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]      
Non-current liabilities $ (4,113.0) $ (312.0)  
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]      
Defined Benefit Plan, Accumulated Benefit Obligation 6,537.0 1,010.0  
ArcelorMittal USA [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Acquired through business combinations 3,294.0    
AK Steel [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Acquired through business combinations 949.0    
Pension Plan [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligations - beginning of year 1,021.0 906.0  
Service cost (excluding expense) 23.0 17.0 $ 19.0
Interest cost 64.0 35.0 30.0
Actuarial (gain) loss 162.0 112.0  
Benefits paid (146.0) (62.0)  
Participant contributions 0.0 0.0  
Acquired through business combinations 5,535.0 0.0  
Effect of settlement (94.0) 0.0  
Other 0.0 13.0  
Benefit obligations - end of year 6,565.0 1,021.0 906.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets — beginning of year 749.0 687.0  
Actual return on plan assets 472.0 98.0  
Participant contributions 0.0 0.0  
Employer contributions 50.0 16.0  
Benefits paid (146.0) (62.0)  
Acquired through business combinations 4,301.0 0.0  
Effect of settlement (94.0) 0.0  
Other 0.0 10.0  
Fair value of plan assets — end of year 5,332.0 749.0 687.0
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]      
Assets for Plan Benefits, Defined Benefit Plan 3.0 0.0  
Current liabilities (12.0) 0.0  
Non-current liabilities (1,224.0) (272.0)  
Total amount recognized (1,233.0) (272.0)  
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]      
Net actuarial loss 164.0 382.0  
Prior service cost (credit) 6.0 7.0  
Net amount recognized 170.0 389.0  
Funded (Unfunded) Status (1,233.0) (272.0)  
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligations - beginning of year 255.0 242.0  
Service cost (excluding expense) 8.0 2.0 2.0
Interest cost 19.0 10.0 8.0
Actuarial (gain) loss 14.0 19.0  
Benefits paid (89.0) (26.0)  
Participant contributions 22.0 6.0  
Acquired through business combinations 3,528.0 0.0  
Effect of settlement 0.0 0.0  
Other 0.0 2.0  
Benefit obligations - end of year 3,757.0 255.0 242.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets — beginning of year 260.0 240.0  
Actual return on plan assets 45.0 35.0  
Participant contributions 17.0 1.0  
Employer contributions 30.0 3.0  
Benefits paid (88.0) (19.0)  
Acquired through business combinations 519.0 0.0  
Effect of settlement 0.0 0.0  
Other 0.0 0.0  
Fair value of plan assets — end of year 783.0 260.0 $ 240.0
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]      
Assets for Plan Benefits, Defined Benefit Plan 54.0 49.0  
Current liabilities (139.0) (4.0)  
Non-current liabilities (2,889.0) (40.0)  
Total amount recognized (2,974.0) 5.0  
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]      
Net actuarial loss 56.0 73.0  
Prior service cost (credit) (6.0) (8.0)  
Net amount recognized 50.0 65.0  
Funded (Unfunded) Status $ (2,974.0) $ 5.0