Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenue	$ 3,968	$ 3,968	$ 12,808	$ 12,388
Operating Income (Loss)	540	467	2,495	2,085
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,291	2,206	7,805	7,203
Interest on fiduciary funds	8	31	40	80
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,696	1,785	5,074	5,258
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,987	3,991	12,879	12,461
Operating Income (Loss)	611	535	2,698	2,342
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,291	2,206	7,805	7,203
Operating Income (Loss)	333	218	1,883	1,468
Interest on fiduciary funds	8	31	40	80
Equity method income	(6)	10	7	16
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,696	1,785	5,074	5,258
Operating Income (Loss)	278	317	815	874
Interest on fiduciary funds	1	1	1	2
Equity method income			4	10
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(19)	(23)	(71)	(73)
Operating Income (Loss)	(71)	(68)	(203)	(257)
Intersegment Eliminations \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue		3	5	7
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 19	$ 20	$ 66	$ 66

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

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