Segment Information (Selected Information and Details for MMC's Operating Segments) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jan. 01, 2024	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Revenue		$ 6,221	$ 5,876	$ 12,694	$ 11,800
Operating Income/(Loss)		1,642	1,457	3,567	3,183
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		6,238	5,894	12,725	11,831
Operating Income/(Loss)		1,707	1,545	3,704	3,351
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		4,022	3,722	8,295	7,628
Operating Income/(Loss)		1,297	1,157	2,862	2,552
Interest on fiduciary funds		125	108	247	199
Equity method investment income (loss)		10	12	10	11
Total Segments | Consulting
Segment Reporting Information [Line Items]
Revenue		2,216	2,172	4,430	4,203
Operating Income/(Loss)		410	388	842	799
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue		(17)	(18)	(31)	(31)
Operating Income/(Loss)		(65)	(88)	(137)	(168)
Intersegment Eliminations | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue		4	5	5	5
Intersegment Eliminations | Consulting
Segment Reporting Information [Line Items]
Revenue		$ 13	$ 13	$ 26	$ 26