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, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 19,074
Fair value of plan assets at end of year	19,394	$ 19,074
Amounts recognized in the consolidated balance sheets:
Non-current assets	2,270	1,768
Non-current liabilities	(1,632)	(2,662)
Pension  Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	38	36	$ 31
Interest cost	341	421	487
Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	1	0
Interest cost	2	3	3
U.S. | Pension  Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	6,914	6,322
Interest cost	184	213	241
Employee contributions	0	0
Actuarial (gain) loss	(227)	650
Plan amendments	1	0
Benefits paid	(278)	(271)
Benefit obligation at end of year	6,594	6,914	6,322
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	5,100	4,715
Actual return on plan assets	680	591
Employer contributions	35	65
Employee contributions	0	0
Benefits paid	(278)	(271)
Other	0	0
Fair value of plan assets at end of year	5,537	5,100	4,715
Net funded status	(1,057)	(1,814)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(31)	(30)
Non-current liabilities	(1,026)	(1,784)
Net amount recognized in consolidated balance sheet	(1,057)	(1,814)
Amounts recognized in other comprehensive loss:
Prior service (credit) cost	1	0
Net actuarial loss	1,777	2,446
Total recognized accumulated other comprehensive (loss) income	1,778	2,446
Cumulative employer contributions in excess of (less than) net periodic cost	721	632
Net amount recognized in consolidated balance sheet	(1,057)	(1,814)
Accumulated benefit obligation	6,594	6,914
U.S. | Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	31	31
Interest cost	1	1	1
Employee contributions	4	4
Actuarial (gain) loss	(1)	1
Plan amendments	0	0
Benefits paid	(7)	(6)
Benefit obligation at end of year	28	31	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	3	3
Employee contributions	4	4
Benefits paid	(7)	(6)
Other	0	(1)
Fair value of plan assets at end of year	2	2	2
Net funded status	(26)	(29)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(1)	(1)
Non-current liabilities	(25)	(28)
Net amount recognized in consolidated balance sheet	(26)	(29)
Amounts recognized in other comprehensive loss:
Prior service (credit) cost	0	0
Net actuarial loss	(3)	(3)
Total recognized accumulated other comprehensive (loss) income	(3)	(3)
Cumulative employer contributions in excess of (less than) net periodic cost	(29)	(32)
Net amount recognized in consolidated balance sheet	(26)	(29)
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	12,998	11,321
Service cost	38	36	31
Interest cost	157	208	246
Employee contributions	2	2
Actuarial (gain) loss	(617)	1,273
Plan amendments	7	11
Effect of settlement	(16)	(13)
Effect of curtailment	(2)	0
Special termination benefits	0	1
Benefits paid	(395)	(402)
Foreign currency changes	(115)	561
Benefit obligation at end of year	12,057	12,998	11,321
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	14,028	12,313
Actual return on plan assets	306	1,415
Effect of settlement	(16)	(13)
Employer contributions	95	78
Employee contributions	2	2
Benefits paid	(395)	(402)
Foreign currency changes	(165)	635
Fair value of plan assets at end of year	13,855	14,028	12,313
Net funded status	1,798	1,030
Amounts recognized in the consolidated balance sheets:
Non-current assets	2,269	1,764
Current liabilities	(6)	(7)
Non-current liabilities	(465)	(727)
Net amount recognized in consolidated balance sheet	1,798	1,030
Amounts recognized in other comprehensive loss:
Prior service (credit) cost	(18)	(13)
Net actuarial loss	(2,904)	(3,467)
Total recognized accumulated other comprehensive (loss) income	(2,922)	(3,480)
Cumulative employer contributions in excess of (less than) net periodic cost	4,720	4,510
Net amount recognized in consolidated balance sheet	1,798	1,030
Accumulated benefit obligation	11,830	12,736
Non-U.S. | Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	73	61
Service cost	1	0
Interest cost	1	2	2
Employee contributions	0	0
Actuarial (gain) loss	(4)	10
Plan amendments	0	0
Effect of settlement	0	0
Effect of curtailment	0	0
Special termination benefits	0	0
Benefits paid	(3)	(2)
Foreign currency changes	0	2
Benefit obligation at end of year	68	73	61
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
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	(68)	(73)
Amounts recognized in the consolidated balance sheets:
Non-current assets	0	0
Current liabilities	(3)	(3)
Non-current liabilities	(65)	(70)
Net amount recognized in consolidated balance sheet	(68)	(73)
Amounts recognized in other comprehensive loss:
Prior service (credit) cost	7	9
Net actuarial loss	(10)	(16)
Total recognized accumulated other comprehensive (loss) income	(3)	(7)
Cumulative employer contributions in excess of (less than) net periodic cost	(65)	(66)
Net amount recognized in consolidated balance sheet	(68)	(73)
Accumulated benefit obligation	$ 0	$ 0