Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			6 Months Ended
	Apr. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Revenue		$ 5,379		$ 5,017	$ 10,928	$ 10,100
Operating Income (Loss)		1,364		1,228	2,809	2,586
Loss on deconsolidation of Russian businesses			$ 39		39	0
U.S. Affinity Business
Segment Reporting Information [Line Items]
Gain on disposal	$ 112
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue		3,313		3,141	6,862	6,366
Interest on fiduciary funds		13		3	17	8
Consulting Segment
Segment Reporting Information [Line Items]
Revenue		2,084		1,892	4,094	3,765
Operating Segments
Segment Reporting Information [Line Items]
Revenue		5,397		5,033	10,956	10,131
Operating Income (Loss)		1,442		1,294	2,955	2,715
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue		3,313		3,141	6,862	6,366
Operating Income (Loss)		967		950	2,088	2,010
Interest on fiduciary funds		13		3	17	8
Equity method income (loss)		7		14	8	17
Loss on deconsolidation of Russian businesses					27
Gain (loss) on consolidation of business				51		53
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue		2,084		1,892	4,094	3,765
Operating Income (Loss)		475		344	867	705
Loss on deconsolidation of Russian businesses					12
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue		(18)		(16)	(28)	(31)
Operating Income (Loss)		(78)		(66)	(146)	(129)
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue		5		4	5	4
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue		13		$ 12	23	$ 27
Gain on disposal		$ 112			$ 112