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, 2022	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 19,394
Fair value of plan assets at end of year	13,042	$ 19,394
Amounts recognized in the consolidated balance sheets:
Non-current assets	2,127	2,270
Non-current liabilities	(921)	(1,632)
Pension  Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	28	38	$ 36
Interest cost	389	341	421
Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	0	1
Interest cost	3	2	3
U.S. | Pension  Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	6,594	6,914
Interest cost	193	184	213
Employee contributions	0	0
Actuarial gain	(1,625)	(227)
Plan amendments	0	1
Benefits paid	(286)	(278)
Benefit obligation at end of year	4,876	6,594	6,914
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	5,537	5,100
Actual return on plan assets	(1,005)	680
Employer contributions	30	35
Employee contributions	0	0
Benefits paid	(286)	(278)
Fair value of plan assets at end of year	4,276	5,537	5,100
Net funded status	(600)	(1,057)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(31)	(31)
Non-current liabilities	(569)	(1,026)
Net amount recognized in consolidated balance sheet	(600)	(1,057)
Amounts recognized in other comprehensive loss:
Prior service cost	(1)	(1)
Net actuarial loss	1,419	1,777
Total recognized accumulated other comprehensive (loss) income, December 31	(1,420)	(1,778)
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	820	721
Net amount recognized in consolidated balance sheet	(600)	(1,057)
Accumulated benefit obligation	4,876	6,594
U.S. | Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	28	31
Interest cost	1	1	1
Employee contributions	3	4
Actuarial gain	(3)	(1)
Plan amendments	0	0
Benefits paid	(7)	(7)
Benefit obligation at end of year	22	28	31
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	2	2
Actual return on plan assets	0	0
Employer contributions	4	3
Employee contributions	3	4
Benefits paid	(7)	(7)
Fair value of plan assets at end of year	2	2	2
Net funded status	(20)	(26)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(1)	(1)
Non-current liabilities	(19)	(25)
Net amount recognized in consolidated balance sheet	(20)	(26)
Amounts recognized in other comprehensive loss:
Prior service cost	0	0
Net actuarial loss	(8)	(3)
Total recognized accumulated other comprehensive (loss) income, December 31	8	3
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	(28)	(29)
Net amount recognized in consolidated balance sheet	(20)	(26)
Accumulated benefit obligation	0	0
Non-U.S. | Pension  Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	12,057	12,998
Service cost	28	38	36
Interest cost	196	157	208
Employee contributions	3	2
Plan combination	2	0
Actuarial gain	(3,953)	(617)
Plan amendments	0	7
Effect of settlement	(22)	(16)
Effect of curtailment	0	(2)
Benefits paid	(342)	(395)
Foreign currency changes	(1,083)	(115)
Benefit obligation at end of year	6,886	12,057	12,998
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	13,855	14,028
Plan combination	1	0
Actual return on plan assets	(3,609)	306
Effect of settlement	(22)	(16)
Employer contributions	139	95
Employee contributions	3	2
Benefits paid	(342)	(395)
Foreign currency changes	(1,261)	(165)
Fair value of plan assets at end of year	8,764	13,855	14,028
Net funded status	1,878	1,798
Amounts recognized in the consolidated balance sheets:
Non-current assets	2,127	2,269
Current liabilities	(6)	(6)
Non-current liabilities	(243)	(465)
Net amount recognized in consolidated balance sheet	1,878	1,798
Amounts recognized in other comprehensive loss:
Prior service cost	16	18
Net actuarial loss	(2,610)	(2,904)
Total recognized accumulated other comprehensive (loss) income, December 31	2,626	2,922
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	4,504	4,720
Net amount recognized in consolidated balance sheet	1,878	1,798
Accumulated benefit obligation	6,776	11,830
Non-U.S. | Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	68	73
Service cost	0	1
Interest cost	2	1	2
Employee contributions	0	0
Plan combination	0	0
Actuarial gain	(14)	(4)
Plan amendments	0	0
Effect of settlement	0	0
Effect of curtailment	0	0
Benefits paid	(2)	(3)
Foreign currency changes	(6)	0
Benefit obligation at end of year	48	68	73
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
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	3
Employee contributions	0	0
Benefits paid	(2)	(3)
Foreign currency changes	0	0
Fair value of plan assets at end of year	0	0	$ 0
Net funded status	(48)	(68)
Amounts recognized in the consolidated balance sheets:
Non-current assets	0	0
Current liabilities	(3)	(3)
Non-current liabilities	(45)	(65)
Net amount recognized in consolidated balance sheet	(48)	(68)
Amounts recognized in other comprehensive loss:
Prior service cost	(5)	(7)
Net actuarial loss	6	(10)
Total recognized accumulated other comprehensive (loss) income, December 31	(11)	3
Cumulative employer contributions in excess of (less than) net benefit (credit) cost	(59)	(65)
Net amount recognized in consolidated balance sheet	(48)	(68)
Accumulated benefit obligation	$ 0	$ 0