Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Revenue	$ 2,845		$ 2,806		$ 8,922		$ 8,618
Operating Income	378		310		1,423		1,247
Interest income on fiduciary funds	6		9		18		21
Equity method income	(1)		0		23		14
Risk and Insurance Services [Member]
Revenue	1,510	[1]	1,475	[1]	4,955	[2]	4,729	[2]
Operating Income	234		186		1,052		925
Inter-segment revenue	1		2		4		4
Interest income on fiduciary funds	10		14		31		36
Equity method income			1		10		10
Consulting [Member]
Revenue	1,346	[3]	1,339	[3]	4,000	[4]	3,919	[4]
Operating Income	193		161		524		441
Inter-segment revenue	10		6		29		26
Interest income on fiduciary funds	1		1		2		3
Total Operating Segments [Member]
Revenue	2,856		2,814		8,955		8,648
Operating Income	427		347		1,576		1,366
Corporate / Eliminations [Member]
Revenue	(11)		(8)		(33)		(30)
Operating Income	$ (49)		$ (37)		$ (153)		$ (119)

[1] Includes inter-segment revenue of $1 million and $2 million in 2012 and 2011, respectively, interest income on fiduciary funds of $10 million and $14 million in 2012 and 2011, respectively, and equity method income of $1 million in 2011.

[2] Includes inter-segment revenue of $4 million in both 2012 and 2011, interest income on fiduciary funds of $31 million and $36 million in 2012 and 2011, respectively, and equity method income of $10 million in both 2012 and 2011.

[3] Includes inter-segment revenue of $10 million and $6 million in 2012 and 2011, respectively, and interest income on fiduciary funds of $1 million in both 2012 and 2011.

[4] Includes inter-segment revenue of $29 million and $26 million in 2012 and 2011, respectively, and interest income on fiduciary funds of $2 million and $3 million in 2012 and 2011, respectively.