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, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 14,378
Fair value of plan assets at end of year	17,085	$ 14,378
Amounts recognized in the consolidated balance sheets:
Non-current assets	1,632	1,688
Non-current liabilities	(2,336)	(1,911)
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	31	34	$ 76
Interest cost	487	463	497
Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	0	1	1
Interest cost	3	3	4
U.S. | Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	5,529	6,221
Interest cost	241	235	264
Employee contributions	0	0
Plan combination	64	0
Actuarial (gain) loss	753	(502)
Benefits paid	(265)	(425)
Benefit obligation at end of year	6,322	5,529	6,221
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	4,062	4,787
Actual return on plan assets	834	(330)
Employer contributions	35	30
Employee contributions	0	0
Benefits paid	(265)	(425)
Other	49	0
Fair value of plan assets at end of year	4,715	4,062	4,787
Net funded status	(1,607)	(1,467)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(29)	(28)
Non-current liabilities	(1,578)	(1,439)
Net amount recognized in consolidated balance sheet	(1,607)	(1,467)
Amounts recognized in other comprehensive (loss) income:
Net actuarial (loss) gain	(2,114)	(1,896)
Total recognized accumulated other comprehensive (loss) income	(2,114)	(1,896)
Cumulative employer contributions in excess of (less than) net periodic cost	507	429
Net amount recognized in consolidated balance sheet	(1,607)	(1,467)
Accumulated benefit obligation	6,322	5,529
U.S. | Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	32	36
Interest cost	1	1	2
Employee contributions	4	4
Plan combination	0	0
Actuarial (gain) loss	1	(1)
Benefits paid	(7)	(8)
Benefit obligation at end of year	31	32	36
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	1	2
Actual return on plan assets	0	0
Employer contributions	4	3
Employee contributions	4	4
Benefits paid	(7)	(8)
Other	0	0
Fair value of plan assets at end of year	2	1	2
Net funded status	(29)	(31)
Amounts recognized in the consolidated balance sheets:
Current liabilities	(1)	(2)
Non-current liabilities	(28)	(29)
Net amount recognized in consolidated balance sheet	(29)	(31)
Amounts recognized in other comprehensive (loss) income:
Net actuarial (loss) gain	4	6
Total recognized accumulated other comprehensive (loss) income	4	6
Cumulative employer contributions in excess of (less than) net periodic cost	(33)	(37)
Net amount recognized in consolidated balance sheet	(29)	(31)
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	8,969	10,053
Service cost	31	34	76
Interest cost	246	228	233
Employee contributions	2	2
Plan combination	915	0
Actuarial (gain) loss	1,339	(450)
Plan amendments	(1)	44
Effect of settlement	(25)	(162)
Benefits paid	(364)	(290)
Foreign currency changes	209	(491)
Other	0	1
Benefit obligation at end of year	11,321	8,969	10,053
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	10,306	11,388
Plan combination	683	0
Actual return on plan assets	1,367	(141)
Effect of settlement	(25)	(162)
Employer contributions	87	82
Employee contributions	2	2
Benefits paid	(364)	(290)
Foreign currency changes	257	(573)
Fair value of plan assets at end of year	12,313	10,306	11,388
Net funded status	992	1,337
Amounts recognized in the consolidated balance sheets:
Non-current assets	1,632	1,687
Current liabilities	(6)	(5)
Non-current liabilities	(634)	(345)
Net amount recognized in consolidated balance sheet	992	1,337
Amounts recognized in other comprehensive (loss) income:
Prior service credit	(2)	(2)
Net actuarial (loss) gain	(3,055)	(2,568)
Total recognized accumulated other comprehensive (loss) income	(3,057)	(2,570)
Cumulative employer contributions in excess of (less than) net periodic cost	4,049	3,907
Net amount recognized in consolidated balance sheet	992	1,337
Accumulated benefit obligation	11,079	8,752
Non-U.S. | Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	57	68
Service cost	0	1	1
Interest cost	2	2	2
Employee contributions	0	0
Plan combination	0	0
Actuarial (gain) loss	3	(8)
Plan amendments	0	0
Effect of settlement	0	0
Benefits paid	(3)	(3)
Foreign currency changes	2	(3)
Other	0	0
Benefit obligation at end of year	61	57	68
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	3	3
Employee contributions	0	0
Benefits paid	(3)	(3)
Foreign currency changes	0	0
Fair value of plan assets at end of year	0	0	$ 0
Net funded status	(61)	(57)
Amounts recognized in the consolidated balance sheets:
Non-current assets	0	0
Current liabilities	(3)	(3)
Non-current liabilities	(58)	(54)
Net amount recognized in consolidated balance sheet	(61)	(57)
Amounts recognized in other comprehensive (loss) income:
Prior service credit	11	12
Net actuarial (loss) gain	(5)	(1)
Total recognized accumulated other comprehensive (loss) income	6	11
Cumulative employer contributions in excess of (less than) net periodic cost	(67)	(68)
Net amount recognized in consolidated balance sheet	(61)	(57)
Accumulated benefit obligation	$ 0	$ 0