Annual report [Section 13 and 15(d), not S-K Item 405]

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, 2024
Dec. 31, 2023
Dec. 31, 2022
Amounts recognized in Statements of Financial Position:      
Pension and OPEB assets $ 427 $ 329  
Non-current liabilities (751) (821)  
Amounts recognized in accumulated other comprehensive loss (income):      
Accumulated benefit obligation 4,236 4,557  
Pension Benefits      
Change in benefit obligations:      
Benefit obligations — beginning of year 4,571 4,646  
Service cost 28 31 $ 45
Interest cost 218 235 144
Plan amendments 0 3  
Actuarial loss (gain) (165) 116  
Benefits paid (429) (436)  
Participant contributions 0 0  
Acquired through business combinations 35 0  
Foreign currency exchange (1) 0  
Effect of settlement (9) (24)  
Other 0 0  
Benefit obligations — end of year 4,248 4,571 4,646
Change in plan assets:      
Fair value of plan assets — beginning of year 4,282 4,338  
Actual return on plan assets 254 375  
Participant contributions 0 0  
Employer contributions 119 29  
Benefits paid (429) (436)  
Acquired through business combinations 21 0  
Defined Benefit Plan, Plan Assets, Foreign Currency Translation Gain (Loss) (1) 0  
Effect of settlement (9) (24)  
Fair value of plan assets — end of year 4,237 4,282 4,338
Funded status (11) (289)  
Amounts recognized in Statements of Financial Position:      
Pension and OPEB assets 239 137  
Current liabilities (7) (18)  
Non-current liabilities (243) (408)  
Total amount recognized (11) (289)  
Amounts recognized in accumulated other comprehensive loss (income):      
Net actuarial gain (400) (304)  
Prior service cost (credit) 89 106  
Net amount recognized (311) (198)  
Pension Benefits | Stelco Holdings Inc      
Change in benefit obligations:      
Benefit obligations — end of year 14    
OPEB      
Change in benefit obligations:      
Benefit obligations — beginning of year 1,036 1,233  
Service cost 8 10 35
Interest cost 50 64 72
Plan amendments 7 8  
Actuarial loss (gain) 162 (158)  
Benefits paid (155) (163)  
Participant contributions 36 42  
Acquired through business combinations 0 0  
Foreign currency exchange 0 0  
Effect of settlement 0 0  
Other 3 0  
Benefit obligations — end of year 1,147 1,036 1,233
Change in plan assets:      
Fair value of plan assets — beginning of year 739 728  
Actual return on plan assets 46 67  
Participant contributions 36 42  
Employer contributions 76 65  
Benefits paid (155) (163)  
Acquired through business combinations 0 0  
Defined Benefit Plan, Plan Assets, Foreign Currency Translation Gain (Loss) 0 0  
Effect of settlement 0 0  
Fair value of plan assets — end of year 742 739 $ 728
Funded status (405) (297)  
Amounts recognized in Statements of Financial Position:      
Pension and OPEB assets 188 192  
Current liabilities (85) (76)  
Non-current liabilities (508) (413)  
Total amount recognized (405) (297)  
Amounts recognized in accumulated other comprehensive loss (income):      
Net actuarial gain (1,720) (2,033)  
Prior service cost (credit) (109) (131)  
Net amount recognized $ (1,829) $ (2,164)