Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenue	$ 5,382	$ 4,770	$ 17,182	$ 15,698
Operating Income/(Loss)	996	791	4,179	3,600
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,396	4,789	17,227	15,745
Operating Income/(Loss)	1,064	879	4,415	3,834
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,190	2,838	10,818	9,700
Operating Income/(Loss)	640	529	3,192	2,617
Interest on fiduciary funds	131	40	330	57
Equity method investment income	3		14	9
Loss on consolidation of business				27
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	2,206	1,951	6,409	6,045
Operating Income/(Loss)	424	350	1,223	1,217
Operating Segments | Small Individual Financial Advisory Business
Segment Reporting Information [Line Items]
Pre-tax loss on sale			17
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(14)	(19)	(45)	(47)
Operating Income/(Loss)	(68)	(88)	(236)	(234)
Intersegment Eliminations | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue			5	5
Intersegment Eliminations | Consulting
Segment Reporting Information [Line Items]
Revenue	$ 14	$ 19	$ 40	$ 42