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, 2024	Dec. 31, 2023	Dec. 31, 2022
Change in plan assets:
Fair value of plan assets at beginning of year	$ 13,452
Fair value of plan assets at end of year	12,575	$ 13,452
Amounts recognized in the consolidated balance sheets:
Non-current assets	1,914	2,051
Non-current liabilities	(840)	(779)
Pension  Benefits
Change in benefit obligation:
Service cost	23	23	$ 28
Interest cost	579	599	389
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	4,690	4,876
Interest cost	250	260	193
Employee contributions	0	0
Plan combination	62	0
Actuarial loss (gain)	(107)	(20)
Benefits paid	(301)	(426)
Benefit obligation at end of year	4,594	4,690	4,876
Change in plan assets:
Fair value of plan assets at beginning of year	4,234	4,276
Actual return on plan assets	95	351
Employer contributions	34	33
Employee contributions	0	0
Benefits paid	(301)	(426)
Fair value of plan assets at end of year	4,062	4,234	4,276
Net funded status	(532)	(456)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(37)	(31)
Non-current liabilities	(495)	(425)
Net amount recognized in consolidated balance sheets	(532)	(456)
Amounts recognized in other comprehensive loss:
Prior service (cost)	(1)	(1)
Net actuarial (loss) gain	(1,427)	(1,347)	(1,419)
Total recognized accumulated other comprehensive (loss) income, December 31	(1,428)	(1,348)
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	896	892
Net amount recognized in consolidated balance sheets	(532)	(456)
Accumulated benefit obligation	4,577	4,690
U.S. | Post-retirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	20	22
Interest cost	1	1	1
Employee contributions	3	3
Plan combination	0	0
Actuarial loss (gain)	2	2
Benefits paid	(9)	(8)
Benefit obligation at end of year	17	20	22
Change in plan assets:
Fair value of plan assets at beginning of year	2	2
Actual return on plan assets	0	0
Employer contributions	5	5
Employee contributions	4	3
Benefits paid	(9)	(8)
Fair value of plan assets at end of year	2	2	2
Net funded status	(15)	(18)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(1)	(1)
Non-current liabilities	(14)	(17)
Net amount recognized in consolidated balance sheets	(15)	(18)
Amounts recognized in other comprehensive loss:
Prior service (cost)	0	0
Net actuarial (loss) gain	2	4	8
Total recognized accumulated other comprehensive (loss) income, December 31	2	4
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	(17)	(22)
Net amount recognized in consolidated balance sheets	(15)	(18)
Accumulated benefit obligation	0	0
Non-U.S. | Pension  Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	7,521	6,886
Service cost	23	23	28
Interest cost	329	339	196
Employee contributions	3	3
Actuarial loss (gain)	(423)	226
Plan amendments	(2)	0
Effect of settlement	(16)	(15)
Benefits paid	(365)	(352)
Foreign currency changes	(236)	411
Benefit obligation at end of year	6,834	7,521	6,886
Change in plan assets:
Fair value of plan assets at beginning of year	9,308	8,764
Actual return on plan assets	(259)	358
Effect of settlement	(16)	(15)
Employer contributions	59	78
Employee contributions	3	3
Benefits paid	(365)	(352)
Foreign currency changes	(235)	472
Fair value of plan assets at end of year	8,495	9,308	8,764
Net funded status	1,661	1,787
Amounts recognized in the consolidated balance sheets:
Non-current assets	1,913	2,050
Current liabilities	(8)	(7)
Non-current liabilities	(244)	(256)
Net amount recognized in consolidated balance sheets	1,661	1,787
Amounts recognized in other comprehensive loss:
Prior service (cost)	(14)	(17)	(16)
Net actuarial (loss) gain	(3,519)	(3,219)	(2,610)
Total recognized accumulated other comprehensive (loss) income, December 31	(3,533)	(3,236)
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	5,194	5,023
Net amount recognized in consolidated balance sheets	1,661	1,787
Accumulated benefit obligation	6,725	7,396
Non-U.S. | Post-retirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	40	48
Service cost	0	0	0
Interest cost	2	2	2
Employee contributions	0	0
Actuarial loss (gain)	9	(10)
Plan amendments	0	0
Effect of settlement	0	0
Benefits paid	(3)	(2)
Foreign currency changes	(2)	2
Benefit obligation at end of year	46	40	48
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Effect of settlement	0	0
Employer contributions	3	2
Employee contributions	0	0
Benefits paid	(3)	(2)
Foreign currency changes	0	0
Fair value of plan assets at end of year	0	0	0
Net funded status	(46)	(40)
Amounts recognized in the consolidated balance sheets:
Non-current assets	0	0
Current liabilities	(3)	(3)
Non-current liabilities	(43)	(37)
Net amount recognized in consolidated balance sheets	(46)	(40)
Amounts recognized in other comprehensive loss:
Prior service (cost)	1	3	5
Net actuarial (loss) gain	3	17	$ 6
Total recognized accumulated other comprehensive (loss) income, December 31	4	20
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	(50)	(60)
Net amount recognized in consolidated balance sheets	(46)	(40)
Accumulated benefit obligation	$ 0	$ 0