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, 2023	Dec. 31, 2022	Dec. 31, 2021
Change in plan assets:
Fair value of plan assets at beginning of year	$ 13,042
Fair value of plan assets at end of year	13,452	$ 13,042
Amounts recognized in the consolidated balance sheets:
Non-current assets	2,051	2,127
Non-current liabilities	(779)	(921)
Pension  Benefits
Change in benefit obligation:
Service cost	23	28	$ 38
Interest cost	599	389	341
Post-retirement Benefits
Change in benefit obligation:
Service cost	0	0	1
Interest cost	3	3	2
U.S. | Pension  Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	4,876	6,594
Interest cost	260	193	184
Employee contributions	0	0
Actuarial (gain) loss	(20)	(1,625)
Benefits paid	(426)	(286)
Benefit obligation at end of year	4,690	4,876	6,594
Change in plan assets:
Fair value of plan assets at beginning of year	4,276	5,537
Actual return on plan assets	351	(1,005)
Employer contributions	33	30
Employee contributions	0	0
Benefits paid	(426)	(286)
Fair value of plan assets at end of year	4,234	4,276	5,537
Net funded status	(456)	(600)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(31)	(31)
Non-current liabilities	(425)	(569)
Net amount recognized in consolidated balance sheet	(456)	(600)
Amounts recognized in other comprehensive loss:
Prior service (cost)	(1)	(1)
Net actuarial (loss) gain	(1,347)	(1,419)
Total recognized accumulated other comprehensive (loss) income, December 31	(1,348)	(1,420)
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	892	820
Net amount recognized in consolidated balance sheet	(456)	(600)
Accumulated benefit obligation	4,690	4,876
U.S. | Post-retirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	22	28
Interest cost	1	1	1
Employee contributions	3	3
Actuarial (gain) loss	2	(3)
Benefits paid	(8)	(7)
Benefit obligation at end of year	20	22	28
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	4
Employee contributions	3	3
Benefits paid	(8)	(7)
Fair value of plan assets at end of year	2	2	2
Net funded status	(18)	(20)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(1)	(1)
Non-current liabilities	(17)	(19)
Net amount recognized in consolidated balance sheet	(18)	(20)
Amounts recognized in other comprehensive loss:
Prior service (cost)	0	0
Net actuarial (loss) gain	4	8
Total recognized accumulated other comprehensive (loss) income, December 31	4	8
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	(22)	(28)
Net amount recognized in consolidated balance sheet	(18)	(20)
Accumulated benefit obligation	0	0
Non-U.S. | Pension  Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	6,886	12,057
Service cost	23	28	38
Interest cost	339	196	157
Employee contributions	3	3
Plan combination	0	2
Actuarial (gain) loss	226	(3,953)
Effect of settlement	(15)	(22)
Benefits paid	(352)	(342)
Foreign currency changes	411	(1,083)
Benefit obligation at end of year	7,521	6,886	12,057
Change in plan assets:
Fair value of plan assets at beginning of year	8,764	13,855
Plan combination	0	1
Actual return on plan assets	358	(3,609)
Effect of settlement	(15)	(22)
Employer contributions	78	139
Employee contributions	3	3
Benefits paid	(352)	(342)
Foreign currency changes	472	(1,261)
Fair value of plan assets at end of year	9,308	8,764	13,855
Net funded status	1,787	1,878
Amounts recognized in the consolidated balance sheets:
Non-current assets	2,050	2,127
Current liabilities	(7)	(6)
Non-current liabilities	(256)	(243)
Net amount recognized in consolidated balance sheet	1,787	1,878
Amounts recognized in other comprehensive loss:
Prior service (cost)	(17)	(16)
Net actuarial (loss) gain	(3,219)	(2,610)
Total recognized accumulated other comprehensive (loss) income, December 31	(3,236)	(2,626)
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	5,023	4,504
Net amount recognized in consolidated balance sheet	1,787	1,878
Accumulated benefit obligation	7,396	6,776
Non-U.S. | Post-retirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	48	68
Service cost	0	0	1
Interest cost	2	2	1
Employee contributions	0	0
Plan combination	0	0
Actuarial (gain) loss	(10)	(14)
Effect of settlement	0	0
Benefits paid	(2)	(2)
Foreign currency changes	2	(6)
Benefit obligation at end of year	40	48	68
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	2	2
Employee contributions	0	0
Benefits paid	(2)	(2)
Foreign currency changes	0	0
Fair value of plan assets at end of year	0	0	$ 0
Net funded status	(40)	(48)
Amounts recognized in the consolidated balance sheets:
Non-current assets	0	0
Current liabilities	(3)	(3)
Non-current liabilities	(37)	(45)
Net amount recognized in consolidated balance sheet	(40)	(48)
Amounts recognized in other comprehensive loss:
Prior service (cost)	3	5
Net actuarial (loss) gain	17	6
Total recognized accumulated other comprehensive (loss) income, December 31	20	11
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	(60)	(59)
Net amount recognized in consolidated balance sheet	(40)	(48)
Accumulated benefit obligation	$ 0	$ 0