Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Revenue	$ 4,651	$ 4,071
Operating Income (Loss)	1,070	938
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,911
Interest on fiduciary funds	23
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,762
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,673	4,096
Operating Income (Loss)	1,136	1,012
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,911	2,423
Operating Income (Loss)	854	733
Interest on fiduciary funds	23	23
Equity method income	(1)	2
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,762	1,673
Operating Income (Loss)	282	279
Interest on fiduciary funds	1	1
Equity method income	4	5
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(22)	(25)
Operating Income (Loss)	(66)	(74)
Intersegment Eliminations \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1	1
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 21	$ 24

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

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- Definition

Amount of income (loss) for proportionate share of equity method investee's income (loss).

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