Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Revenue	$ 3,051		$ 2,884
Operating Income	527		472
Interest income on fiduciary funds	6		7
Equity method income	20		18
Risk and Insurance Services [Member]
Revenue	1,747	[1]	1,634	[1]
Operating Income	417		383
Inter-segment revenue	1		1
Interest income on fiduciary funds	11		12
Equity method income	1		2
Consulting [Member]
Revenue	1,313	[2]	1,261	[2]
Operating Income	159		128
Inter-segment revenue	8		10
Interest income on fiduciary funds	1		1
Total Operating Segments [Member]
Revenue	3,060		2,895
Operating Income	576		511
Corporate / Eliminations [Member]
Revenue	(9)		(11)
Operating Income	$ (49)		$ (39)

[1] Includes inter-segment revenue of $1 million in both 2012 and 2011, interest income on fiduciary funds of $11 million and $12 million in 2012 and 2011, respectively, and equity method income of $1 million and $2 million in 2012 and 2011, respectively.

[2] Includes inter-segment revenue of $8 million and $10 million in 2012 and 2011, respectively, and interest income on fiduciary funds of $1 million in both 2012 and 2011.