Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Segment Reporting Information [Line Items]
Revenue	$ 3,264		$ 3,126
Operating Income (Loss)	673		607
Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Interest income on fiduciary funds	6		8
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Interest income on fiduciary funds	1		1
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	3,271		3,133
Operating Income (Loss)	718		655
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,839	[1]	1,771	[1]
Operating Income (Loss)	493		468
Interest income on fiduciary funds	6		8
Equity method income			1
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,432	[2]	1,362	[2]
Operating Income (Loss)	225		187
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(7)		(7)
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	7		7
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ (45)		$ (48)

[1] Includes interest income on fiduciary funds of $6 million and $8 million in 2014 and 2013, respectively, and equity method income of $1 million in 2013, respectively.

[2] Includes inter-segment revenue of $7 million in both 2014 and 2013 and interest income on fiduciary funds of $1 million in both 2014 and 2013.