Segment Information (Selected Information and Details for MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Revenue	$ 6,351	$ 5,697	$ 20,386	$ 18,391
Compensation and benefits	3,894	3,442	11,639	10,366
Depreciation and amortization expense	91	90	270	276
Identified intangible amortization expense	133	90	412	269
Other operating expenses	1,063	967	3,061	2,805
Operating income	1,170	1,108	5,004	4,675
Operating Segments
Segment Reporting Information [Line Items]
Revenue	6,372	5,715	20,444	18,440
Compensation and benefits	3,853	3,404	11,527	10,258
Depreciation and amortization expense	77	73	227	228
Identified intangible amortization expense	133	90	412	269
Other operating expenses	1,058	953	3,059	2,786
Operating income	1,251	1,195	5,219	4,899
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,907	3,453	13,294	11,748
Compensation and benefits	2,424	2,095	7,337	6,321
Depreciation and amortization expense	52	48	153	140
Identified intangible amortization expense	116	77	357	233
Other operating expenses	565	500	1,641	1,459
Operating income	750	733	3,806	3,595
Interest on fiduciary funds	109	138	311	385
Equity method investment income (loss)	5	7	20	17
Operating Segments | Risk and Insurance Services | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting Information [Line Items]
Net gain included in revenue consolidated statements of income			28
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	2,465	2,262	7,150	6,692
Compensation and benefits	1,429	1,309	4,190	3,937
Depreciation and amortization expense	25	25	74	88
Identified intangible amortization expense	17	13	55	36
Other operating expenses	493	453	1,418	1,327
Operating income	501	462	1,413	1,304
Operating Segments | Consulting | U.S. Health And Benefits And U.K Pension Administration Businesses
Segment Reporting Information [Line Items]
Net gain included in revenue consolidated statements of income				21
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(21)	(18)	(58)	(49)
Compensation and benefits	41	38	112	108
Depreciation and amortization expense	14	17	43	48
Identified intangible amortization expense	0	0	0	0
Other operating expenses	5	14	2	19
Operating income	(81)	(87)	(215)	(224)
Intersegment Eliminations | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue			5	5
Intersegment Eliminations | Consulting
Segment Reporting Information [Line Items]
Revenue	$ 20	$ 18	$ 53	$ 44