Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Revenue	$ 5,549	$ 5,083
Operating Income (Loss)	1,445	1,358
Loss on deconsolidation of Russian businesses	(39)	0
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,549	3,225
Interest on fiduciary funds	4	5
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	2,010	1,873
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,559	5,098
Operating Income (Loss)	1,513	1,421
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,549	3,225
Operating Income (Loss)	1,121	1,060
Interest on fiduciary funds	4	5
Loss on deconsolidation of Russian businesses	(27)
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	2,010	1,873
Operating Income (Loss)	392	361
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(10)	(15)
Operating Income (Loss)	(68)	(63)
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	10	$ 15
Loss on deconsolidation of Russian businesses	$ (12)

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

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