Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Revenue	$ 6,473	$ 5,924
Operating Income/(Loss)	1,925	1,726
Total Segments
Segment Reporting Information [Line Items]
Revenue	6,487	5,937
Operating Income/(Loss)	1,997	1,806
Total Segments | Discontinued Operations, Disposed of by Sale | Small Individual Financial Advisory Business
Segment Reporting Information [Line Items]
Net gain includes in revenue consolidated statements of income	21
Total Segments | Small Individual Financial Advisory Business
Segment Reporting Information [Line Items]
Net gain includes in revenue consolidated statements of income		(19)
Total Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	4,273	3,906
Operating Income/(Loss)	1,565	1,395
Interest on fiduciary funds	122	91
Total Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	2,214	2,031
Operating Income/(Loss)	432	411
Total Segments | Consulting | Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	789	687
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(14)	(13)
Operating Income/(Loss)	(72)	(80)
Intersegment Eliminations | Consulting
Segment Reporting Information [Line Items]
Revenue	$ 13	$ 13