Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Revenue	$ 5,876	$ 5,379	$ 11,800	$ 10,928
Operating Income (Loss)	1,457	1,364	3,183	2,809
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,722	3,313	7,628	6,862
Interest on fiduciary funds	108	13	199	17
Consulting
Segment Reporting Information [Line Items]
Revenue	2,172	2,084	4,203	4,094
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,894	5,397	11,831	10,956
Operating Income (Loss)	1,545	1,442	3,351	2,955
Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,722	3,313	7,628	6,862
Operating Income (Loss)	1,157	967	2,552	2,088
Interest on fiduciary funds	108	13	199	17
Equity method investment income	12	7	11	8
Loss on consolidation of business				27
Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue	2,172	2,084	4,203	4,094
Operating Income (Loss)	388	475	799	867
Operating Segments \| Mercer U.S. Affinity Business
Segment Reporting Information [Line Items]
Pre-tax loss on sale		112		112
Operating Segments \| Small Individual Financial Advisory Business
Segment Reporting Information [Line Items]
Pre-tax loss on sale			(17)
Operating Segments \| Oliver Wyman Group
Segment Reporting Information [Line Items]
Loss on consolidation of business				12
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(18)	(18)	(31)	(28)
Operating Income (Loss)	(88)	(78)	(168)	(146)
Intersegment Eliminations \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	5	5	5	5
Intersegment Eliminations \| Consulting
Segment Reporting Information [Line Items]
Revenue	$ 13	$ 13	$ 26	$ 23

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