Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Revenue	$ 4,189	$ 4,349	$ 8,840	$ 8,420
Operating Income (Loss)	885	680	1,955	1,618
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,603	2,574	5,514	4,997
Interest on fiduciary funds	9	26	32	49
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,616	1,800	3,378	3,473
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,219	4,374	8,892	8,470
Operating Income (Loss)	951	795	2,087	1,807
Operating Segments \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,603	2,574	5,514	4,997
Operating Income (Loss)	696	517	1,550	1,250
Interest on fiduciary funds	9	26	32	49
Equity method income	14	4	13	6
Operating Segments \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,616	1,800	3,378	3,473
Operating Income (Loss)	255	278	537	557
Interest on fiduciary funds	1	1	1	2
Equity method income		5	4	10
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(30)	(25)	(52)	(50)
Operating Income (Loss)	(66)	(115)	(132)	(189)
Intersegment Eliminations \| Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	4	3	5	4
Intersegment Eliminations \| Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 26	$ 22	$ 47	$ 46

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

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