Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2022	Mar. 31, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenue	$ 4,770		$ 4,583	$ 15,698	$ 14,683
Operating Income (Loss)	791		740	3,600	3,326
Loss on deconsolidation of russian businesses		$ 39		39	0
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	2,838		2,670	9,700	9,036
Interest on fiduciary funds	40		4	57	12
Consulting
Segment Reporting Information [Line Items]
Revenue	1,951		1,925	6,045	5,690
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,789		4,595	15,745	14,726
Operating Income (Loss)	879		807	3,834	3,522
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	2,838		2,670	9,700	9,036
Operating Income (Loss)	529		403	2,617	2,413
Interest on fiduciary funds	40		4	57	12
Equity method income (loss)			8	9	25
Gain (loss) on consolidation of business					52
Loss on deconsolidation of russian businesses				27
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	1,951		1,925	6,045	5,690
Operating Income (Loss)	350		404	1,217	1,109
Loss on deconsolidation of russian businesses				12
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(19)		(12)	(47)	(43)
Operating Income (Loss)	(88)		(67)	(234)	(196)
Intersegment Eliminations | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue				5	5
Intersegment Eliminations | Consulting
Segment Reporting Information [Line Items]
Revenue	$ 19		$ 11	42	$ 38
Gain on disposal				$ 112