Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Revenue	$ 5,924	$ 5,549
Operating Income (Loss)	1,726	1,445
Loss on deconsolidation of russian businesses	0	39
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,906	3,549
Interest on fiduciary funds	91	4
Consulting
Segment Reporting Information [Line Items]
Revenue	2,031	2,010
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,937	5,559
Operating Income (Loss)	1,806	1,513
Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,906	3,549
Operating Income (Loss)	1,395	1,121
Interest on fiduciary funds	91	4
Loss on deconsolidation of russian businesses		27
Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue	2,031	2,010
Operating Income (Loss)	411	392
Loss on deconsolidation of russian businesses		12
Operating Segments \| Small Individual Financial Advisory Business
Segment Reporting Information [Line Items]
Pre-tax loss on sale	19
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(13)	(10)
Operating Income (Loss)	(80)	(68)
Intersegment Eliminations \| Consulting
Segment Reporting Information [Line Items]
Revenue	$ 13	$ 10

Interest income earned on both U.S. and foreign fiduciary funds held in trust.

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Amount of gain (loss) from deconsolidation of subsidiary and derecognition of group of assets constituting transfer of business or nonprofit activity, excluding conveyance of oil and gas mineral rights and transfer of good or service in contract with customer.

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