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, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at end of year	$ 19,074
Amounts recognized in the consolidated balance sheets:
Non-current assets	1,768	$ 1,632
Non-current liabilities	(2,662)	(2,336)
Pension  Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	36	31	$ 34
Interest cost	421	487	463
Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	0	0	1
Interest cost	3	3	3
U.S. | Pension  Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	6,322	5,529
Interest cost	213	241	235
Employee contributions	0	0
Plan combination	0	64
Actuarial (gain) loss	650	753
Benefits paid	(271)	(265)
Benefit obligation at end of year	6,914	6,322	5,529
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	4,715	4,062
Actual return on plan assets	591	834
Employer contributions	65	35
Employee contributions	0	0
Benefits paid	(271)	(265)
Other	0	49
Fair value of plan assets at end of year	5,100	4,715	4,062
Net funded status	(1,814)	(1,607)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(30)	(29)
Non-current liabilities	(1,784)	(1,578)
Net amount recognized in consolidated balance sheet	(1,814)	(1,607)
Amounts recognized in other comprehensive (loss) income:
Net actuarial (loss) gain	(2,446)	(2,114)
Total recognized accumulated other comprehensive (loss) income	(2,446)	(2,114)
Cumulative employer contributions in excess of (less than) net periodic cost	632	507
Net amount recognized in consolidated balance sheet	(1,814)	(1,607)
Accumulated benefit obligation	6,914	6,322
U.S. | Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	31	32
Interest cost	1	1	1
Employee contributions	4	4
Plan combination	0	0
Actuarial (gain) loss	1	1
Benefits paid	(6)	(7)
Benefit obligation at end of year	31	31	32
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	2	1
Actual return on plan assets	0	0
Employer contributions	3	4
Employee contributions	4	4
Benefits paid	(6)	(7)
Other	(1)	0
Fair value of plan assets at end of year	2	2	1
Net funded status	(29)	(29)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(1)	(1)
Non-current liabilities	(28)	(28)
Net amount recognized in consolidated balance sheet	(29)	(29)
Amounts recognized in other comprehensive (loss) income:
Net actuarial (loss) gain	3	4
Total recognized accumulated other comprehensive (loss) income	3	4
Cumulative employer contributions in excess of (less than) net periodic cost	(32)	(33)
Net amount recognized in consolidated balance sheet	(29)	(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	11,321	8,969
Service cost	36	31	34
Interest cost	208	246	228
Employee contributions	2	2
Plan combination	0	915
Actuarial (gain) loss	1,273	1,339
Plan amendments	11	(1)
Effect of settlement	(13)	(25)
Special termination benefits	1	0
Benefits paid	(402)	(364)
Foreign currency changes	561	209
Benefit obligation at end of year	12,998	11,321	8,969
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	12,313	10,306
Plan combination	0	683
Actual return on plan assets	1,415	1,367
Effect of settlement	(13)	(25)
Employer contributions	78	87
Employee contributions	2	2
Benefits paid	(402)	(364)
Foreign currency changes	635	257
Fair value of plan assets at end of year	14,028	12,313	10,306
Net funded status	1,030	992
Amounts recognized in the consolidated balance sheets:
Non-current assets	1,764	1,632
Current liabilities	(7)	(6)
Non-current liabilities	(727)	(634)
Net amount recognized in consolidated balance sheet	1,030	992
Amounts recognized in other comprehensive (loss) income:
Prior service credit	(13)	(2)
Net actuarial (loss) gain	(3,467)	(3,055)
Total recognized accumulated other comprehensive (loss) income	(3,480)	(3,057)
Cumulative employer contributions in excess of (less than) net periodic cost	4,510	4,049
Net amount recognized in consolidated balance sheet	1,030	992
Accumulated benefit obligation	12,736	11,079
Non-U.S. | Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	61	57
Service cost	0	0	1
Interest cost	2	2	2
Employee contributions	0	0
Plan combination	0	0
Actuarial (gain) loss	10	3
Plan amendments	0	0
Effect of settlement	0	0
Special termination benefits	0	0
Benefits paid	(2)	(3)
Foreign currency changes	2	2
Benefit obligation at end of year	73	61	57
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	(73)	(61)
Amounts recognized in the consolidated balance sheets:
Non-current assets	0	0
Current liabilities	(3)	(3)
Non-current liabilities	(70)	(58)
Net amount recognized in consolidated balance sheet	(73)	(61)
Amounts recognized in other comprehensive (loss) income:
Prior service credit	9	11
Net actuarial (loss) gain	(16)	(5)
Total recognized accumulated other comprehensive (loss) income	(7)	6
Cumulative employer contributions in excess of (less than) net periodic cost	(66)	(67)
Net amount recognized in consolidated balance sheet	(73)	(61)
Accumulated benefit obligation	$ 0	$ 0