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, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	$ 1,688	$ 1,693
Noncurrent liabilities	(1,911)	(1,888)
Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	34	76	$ 178
Interest cost	463	497	537
Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	1	1	2
Interest cost	3	4	5
United States | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	6,221	5,894
Service cost	0	0	106
Interest cost	235	264	264
Employee contributions	0	0
Actuarial loss (gain)	(502)	538
Benefits paid	(425)	(475)
Benefit obligation at end of year	5,529	6,221	5,894
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	4,787	4,365
Actual return on plan assets	(330)	812
Employer/Company contributions	30	85
Employee contributions	0	0
Benefits paid	(425)	(475)
Fair value of plan assets at end of year	4,062	4,787	4,365
Funded status	(1,467)	(1,434)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(28)	(27)
Noncurrent liabilities	(1,439)	(1,407)
Net asset (liability) recognized in consolidated balance sheet, December 31	(1,467)	(1,434)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Net actuarial (loss) gain	(1,896)	(1,766)
Total recognized accumulated other comprehensive (loss) income, December 31	(1,896)	(1,766)
Cumulative employer contributions in excess (less than) net periodic cost	429	332
Net asset (liability) recognized in consolidated balance sheet, December 31	(1,467)	(1,434)
Accumulated benefit obligation at December 31	5,529	6,221
United States | Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	36	37
Service cost	0	0	0
Interest cost	1	2	2
Employee contributions	4	3
Actuarial loss (gain)	(1)	3
Benefits paid	(8)	(9)
Benefit obligation at end of year	32	36	37
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/Company contributions	3	6
Employee contributions	4	3
Benefits paid	(8)	(9)
Fair value of plan assets at end of year	1	2	2
Funded status	(31)	(34)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(2)	(2)
Noncurrent liabilities	(29)	(32)
Net asset (liability) recognized in consolidated balance sheet, December 31	(31)	(34)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Net actuarial (loss) gain	6	6
Total recognized accumulated other comprehensive (loss) income, December 31	6	6
Cumulative employer contributions in excess (less than) net periodic cost	(37)	(40)
Net asset (liability) recognized in consolidated balance sheet, December 31	(31)	(34)
Accumulated benefit obligation at December 31	0	0
Foreign Plan | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	10,053	9,670
Service cost	34	76	72
Interest cost	228	233	273
Employee contributions	2	7
Plan amendments	44	0
Actuarial loss (gain)	(450)	(149)
Effect of settlement	(162)	(211)
Effect of curtailment	0	(1)
Benefits paid	(290)	(291)
Foreign currency charges	(491)	703
Other	1	16
Benefit obligation at end of year	8,969	10,053	9,670
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	11,388	10,017
Actual return on plan assets	(141)	875
Effect of settlement	(162)	(211)
Employer/Company contributions	82	229
Employee contributions	2	7
Foreign currency changes	(573)	749
Benefits paid	(290)	(291)
Other	0	13
Fair value of plan assets at end of year	10,306	11,388	10,017
Funded status	1,337	1,335
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	1,687	1,684
Current liabilities	(5)	(6)
Noncurrent liabilities	(345)	(343)
Net asset (liability) recognized in consolidated balance sheet, December 31	1,337	1,335
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	(2)	43
Net actuarial (loss) gain	(2,568)	(2,646)
Total recognized accumulated other comprehensive (loss) income, December 31	(2,570)	(2,603)
Cumulative employer contributions in excess (less than) net periodic cost	3,907	3,938
Net asset (liability) recognized in consolidated balance sheet, December 31	1,337	1,335
Accumulated benefit obligation at December 31	8,752	9,783
Foreign Plan | Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	68	81
Service cost	1	1	2
Interest cost	2	2	3
Employee contributions	0	0
Plan amendments	0	(17)
Actuarial loss (gain)	(8)	0
Effect of settlement	0	0
Effect of curtailment	0	0
Benefits paid	(3)	(3)
Foreign currency charges	(3)	4
Other	0	0
Benefit obligation at end of year	57	68	81
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/Company contributions	3	3
Employee contributions	0	0
Foreign currency changes	0	0
Benefits paid	(3)	(3)
Other	0	0
Fair value of plan assets at end of year	0	0	$ 0
Funded status	(57)	(68)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	0	0
Current liabilities	(3)	(4)
Noncurrent liabilities	(54)	(64)
Net asset (liability) recognized in consolidated balance sheet, December 31	(57)	(68)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	12	15
Net actuarial (loss) gain	(1)	(10)
Total recognized accumulated other comprehensive (loss) income, December 31	11	5
Cumulative employer contributions in excess (less than) net periodic cost	(68)	(73)
Net asset (liability) recognized in consolidated balance sheet, December 31	(57)	(68)
Accumulated benefit obligation at December 31	$ 0	$ 0