Segment Information (Selected Information and Details for MMC's Operating Segments) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 01, 2024	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Revenue		$ 5,697	$ 5,382	$ 18,391	$ 17,182
Operating Income/(Loss)		1,108	996	4,675	4,179
Discontinued Operations, Disposed of by Sale | U.S. Health And Benefits And U.K Pension Administration Businesses
Segment Reporting Information [Line Items]
Net gain includes in revenue consolidated statements of income	$ 21
Total Segments
Segment Reporting Information [Line Items]
Revenue		5,715	5,396	18,440	17,227
Operating Income/(Loss)		1,195	1,064	4,899	4,415
Total Segments | Discontinued Operations, Disposed of by Sale | U.S. Health And Benefits And U.K Pension Administration Businesses
Segment Reporting Information [Line Items]
Net gain includes in revenue consolidated statements of income				21
Total Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue		3,453	3,190	11,748	10,818
Operating Income/(Loss)		733	640	3,595	3,192
Interest on fiduciary funds		138	131	385	330
Equity method investment income (loss)		7	3	17	14
Total Segments | Consulting
Segment Reporting Information [Line Items]
Revenue		2,262	2,206	6,692	6,409
Operating Income/(Loss)		462	424	1,304	1,223
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue		(18)	(14)	(49)	(45)
Operating Income/(Loss)		(87)	(68)	(224)	(236)
Intersegment Eliminations | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue				5	5
Intersegment Eliminations | Consulting
Segment Reporting Information [Line Items]
Revenue		$ 18	$ 14	$ 44	$ 40