Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Revenue		$ 4,071	$ 4,000
Operating Income (Loss)		938	908
Equity method income		2	7
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Interest on fiduciary funds		23
Operating Segments
Segment Reporting Information [Line Items]
Revenue		4,096	4,012
Operating Income (Loss)		1,012	963
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]	2,423	2,344
Operating Income (Loss)	[1]	733	716
Interest on fiduciary funds		23	13
Equity method income		2	(1)
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]	1,673	1,668
Operating Income (Loss)	[2]	279	247
Interest on fiduciary funds		1	1
Equity method income		5	3
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue		(25)	(12)
Operating Income (Loss)		(74)	(55)
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		$ 24	$ 11

[1] Includes inter-segment revenue of $1 million in 2019 and 2018, respectively, interest income on fiduciary funds of $23 million and $13 million in 2019 and 2018, respectively, and equity method income (loss) of $2 million and $(1) million in 2019 and 2018, respectively

[2] Includes inter-segment revenue of $24 million and $11 million in 2019 and 2018, respectively, interest income on fiduciary funds of $1 million in 2019 and 2018, respectively, and equity method income of $5 million and $3 million in 2019 and in 2018, respectively.