v3.25.4
Retirement Benefits (Schedules Providing Information Concerning MMC's Defined Benefit Pension Plans and Postretirement Benefit Plans) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Change in plan assets:      
Fair value of plan assets at beginning of year $ 12,575    
Fair value of plan assets at end of year 13,072 $ 12,575  
Amounts recognized in the consolidated balance sheets:      
Non-current assets 2,140 1,914  
Non-current liabilities (786) (840)  
Pension  Benefits      
Change in benefit obligation:      
Service cost 28 23 $ 23
Interest cost 592 579 599
Post-retirement Benefits      
Change in benefit obligation:      
Service cost 0 0 0
Interest cost 3 3 3
U.S. | Pension  Benefits      
Change in benefit obligation:      
Benefit obligation at beginning of year 4,594 4,690  
Service cost 1 0 0
Interest cost 253 250 260
Employee contributions 0 0  
Plan combination (17) 62  
Actuarial (gain) loss 63 (107)  
Effect of settlement (4) 0  
Benefits paid (309) (301)  
Benefit obligation at end of year 4,581 4,594 4,690
Change in plan assets:      
Fair value of plan assets at beginning of year 4,062 4,234  
Actual return on plan assets 323 95  
Effect of settlement (4) 0  
Employer contributions 38 34  
Employee contributions 0 0  
Benefits paid (309) (301)  
Fair value of plan assets at end of year 4,110 4,062 4,234
Net funded status (471) (532)  
Amounts recognized in the consolidated balance sheets:      
Current liabilities (34) (37)  
Non-current liabilities (437) (495)  
Net amount recognized in consolidated balance sheets (471) (532)  
Amounts recognized in other comprehensive loss:      
Prior service (cost) (1) (1)  
Net actuarial (loss) gain (1,436) (1,427) (1,347)
Total recognized accumulated other comprehensive (loss) income, December 31 (1,437) (1,428)  
Cumulative employer contributions in excess of (less than) net benefit (credit) cost 966 896  
Net amount recognized in consolidated balance sheets (471) (532)  
Accumulated benefit obligation 4,576 4,577  
U.S. | Post-retirement Benefits      
Change in benefit obligation:      
Benefit obligation at beginning of year 17 20  
Service cost 0 0 0
Interest cost 1 1 1
Employee contributions 3 3  
Plan combination 0 0  
Actuarial (gain) loss 3 2  
Effect of settlement 0 0  
Benefits paid (8) (9)  
Benefit obligation at end of year 16 17 20
Change in plan assets:      
Fair value of plan assets at beginning of year 2 2  
Actual return on plan assets 0 0  
Effect of settlement 0 0  
Employer contributions 5 5  
Employee contributions 3 4  
Benefits paid (8) (9)  
Fair value of plan assets at end of year 2 2 2
Net funded status (14) (15)  
Amounts recognized in the consolidated balance sheets:      
Current liabilities 0 (1)  
Non-current liabilities (14) (14)  
Net amount recognized in consolidated balance sheets (14) (15)  
Amounts recognized in other comprehensive loss:      
Prior service (cost) 0 0  
Net actuarial (loss) gain (1) 2 4
Total recognized accumulated other comprehensive (loss) income, December 31 (1) 2  
Cumulative employer contributions in excess of (less than) net benefit (credit) cost (13) (17)  
Net amount recognized in consolidated balance sheets (14) (15)  
Accumulated benefit obligation 0 0  
Non-U.S. | Pension  Benefits      
Change in benefit obligation:      
Benefit obligation at beginning of year 6,834 7,521  
Service cost 27 23 23
Interest cost 339 329 339
Employee contributions 3 3  
Plan combination 3 0  
Actuarial (gain) loss (244) (423)  
Plan amendments 0 (2)  
Effect of settlement (28) (16)  
Benefits paid (372) (365)  
Foreign currency changes 539 (236)  
Benefit obligation at end of year 7,101 6,834 7,521
Change in plan assets:      
Fair value of plan assets at beginning of year 8,495 9,308  
Plan combination 4 0  
Actual return on plan assets 180 (259)  
Effect of settlement (28) (16)  
Employer contributions 44 59  
Employee contributions 3 3  
Benefits paid (372) (365)  
Foreign currency changes 657 (235)  
Fair value of plan assets at end of year 8,983 8,495 9,308
Net funded status 1,882 1,661  
Amounts recognized in the consolidated balance sheets:      
Non-current assets 2,140 1,913  
Current liabilities (10) (8)  
Non-current liabilities (248) (244)  
Net amount recognized in consolidated balance sheets 1,882 1,661  
Amounts recognized in other comprehensive loss:      
Prior service (cost) (14) (14) (17)
Net actuarial (loss) gain (3,897) (3,519) (3,219)
Total recognized accumulated other comprehensive (loss) income, December 31 (3,911) (3,533)  
Cumulative employer contributions in excess of (less than) net benefit (credit) cost 5,793 5,194  
Net amount recognized in consolidated balance sheets 1,882 1,661  
Accumulated benefit obligation 6,995 6,725  
Non-U.S. | Post-retirement Benefits      
Change in benefit obligation:      
Benefit obligation at beginning of year 46 40  
Service cost 0 0 0
Interest cost 2 2 2
Employee contributions 0 0  
Plan combination 0 0  
Actuarial (gain) loss (1) 9  
Plan amendments 0 0  
Effect of settlement 0 0  
Benefits paid (3) (3)  
Foreign currency changes 3 (2)  
Benefit obligation at end of year 47 46 40
Change in plan assets:      
Fair value of plan assets at beginning of year 0 0  
Plan combination 0 0  
Actual return on plan assets 0 0  
Effect of settlement 0 0  
Employer contributions 3 3  
Employee contributions 0 0  
Benefits paid (3) (3)  
Foreign currency changes 0 0  
Fair value of plan assets at end of year 0 0 0
Net funded status (47) (46)  
Amounts recognized in the consolidated balance sheets:      
Non-current assets 0 0  
Current liabilities (2) (3)  
Non-current liabilities (45) (43)  
Net amount recognized in consolidated balance sheets (47) (46)  
Amounts recognized in other comprehensive loss:      
Prior service (cost) 0 1 3
Net actuarial (loss) gain 3 3 $ 17
Total recognized accumulated other comprehensive (loss) income, December 31 3 4  
Cumulative employer contributions in excess of (less than) net benefit (credit) cost (50) (50)  
Net amount recognized in consolidated balance sheets (47) (46)  
Accumulated benefit obligation $ 0 $ 0