Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Revenue		$ 3,503	$ 3,336
Operating Income (Loss)		809	733
Operating Segments
Segment Reporting Information [Line Items]
Revenue		3,515	3,346
Operating Income (Loss)		854	780
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]	1,989	1,868
Operating Income (Loss)		613	535
Interest on fiduciary funds (less than)		8	6
Equity method income		2	1
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]	1,526	1,478
Operating Income (Loss)		241	245
Interest on fiduciary funds (less than)		1	1
Equity method income		4	4
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue		(12)	(10)
Operating Income (Loss)		(45)	(47)
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue		0	1
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue		$ 12	$ 9

[1] Includes inter-segment revenue of $1 million in 2016, interest income on fiduciary funds of $8 million and $6 million in 2017 and 2016, respectively, and equity method income of $2 million in 2017 and $1 million in 2016, respectively.

[2] Includes inter-segment revenue of $12 million and $9 million in 2017 and 2016, respectively, interest income on fiduciary funds of $1 million in both 2017 and 2016, and equity method income of $4 million in both 2017 and 2016.