Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Revenue	$ 5,083	$ 4,651
Operating Income (Loss)	1,358	1,070
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,225	2,911
Interest on fiduciary funds	5	23
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,873	1,762
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,098	4,673
Operating Income (Loss)	1,421	1,136
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,225	2,911
Operating Income (Loss)	1,060	854
Interest on fiduciary funds	5	23
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,873	1,762
Operating Income (Loss)	361	282
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	(15)	(22)
Operating Income (Loss)	(63)	(66)
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	$ 15	$ 21