Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Revenue	$ 5,017	$ 4,189	$ 10,100	$ 8,840
Operating Income (Loss)	1,228	885	2,586	1,955
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,141	2,603	6,366	5,514
Interest on fiduciary funds	3	9	8	32
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,892	1,616	3,765	3,378
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,033	4,219	10,131	8,892
Operating Income (Loss)	1,294	951	2,715	2,087
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,141	2,603	6,366	5,514
Operating Income (Loss)	950	696	2,010	1,550
Interest on fiduciary funds	3	9	8	32
Equity method income (loss)	14	14	17	13
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,892	1,616	3,765	3,378
Operating Income (Loss)	344	255	705	537
Equity method income (loss)			0	4
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(16)	(30)	(31)	(52)
Operating Income (Loss)	(66)	(66)	(129)	(132)
Intersegment Eliminations \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	4	4	4	5
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 12	$ 26	$ 27	$ 47

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

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