Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenue	$ 4,583	$ 3,968	$ 14,683	$ 12,808
Operating Income (Loss)	740	540	3,326	2,495
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,670	2,291	9,036	7,805
Interest on fiduciary funds	4	8	12	40
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,925	1,696	5,690	5,074
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,595	3,987	14,726	12,879
Operating Income (Loss)	807	611	3,522	2,698
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,670	2,291	9,036	7,805
Operating Income (Loss)	403	333	2,413	1,883
Interest on fiduciary funds	4	8	12	40
Equity method income (loss)	8	(6)	25	7
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,925	1,696	5,690	5,074
Operating Income (Loss)	404	278	1,109	815
Equity method income (loss)			0	4
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(12)	(19)	(43)	(71)
Operating Income (Loss)	(67)	(71)	(196)	(203)
Intersegment Eliminations \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue			5	5
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 11	$ 19	$ 38	$ 66

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

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