Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Revenue	$ 3,026		$ 2,928		$ 6,077		$ 5,812
Operating Income	518		465		1,045		937
Interest income on fiduciary funds	6		5		12		12
Equity method income	4		(4)		24		14
Risk and Insurance Services [Member]
Revenue	1,698	[1]	1,620	[1]	3,445	[2]	3,254	[2]
Operating Income	401		356		818		739
Inter-segment revenue	2		1		3		2
Interest income on fiduciary funds	10		10		21		22
Equity method income	9		7		10		9
Consulting [Member]
Revenue	1,341	[3]	1,319	[3]	2,654	[4]	2,580	[4]
Operating Income	172		152		331		280
Inter-segment revenue	11		10		19		20
Interest income on fiduciary funds			1		1		2
Total Operating Segments [Member]
Revenue	3,039		2,939		6,099		5,834
Operating Income	573		508		1,149		1,019
Corporate / Eliminations [Member]
Revenue	(13)		(11)		(22)		(22)
Operating Income	$ (55)		$ (43)		$ (104)		$ (82)

[1] Includes inter-segment revenue of $2 million and $1 million in 2012 and 2011, respectively, interest income on fiduciary funds of $10 million in both 2012 and 2011, and equity method income of $9 million and $7 million in 2012 and 2011, respectively.

[2] Includes inter-segment revenue of $3 million and $2 million in 2012 and 2011, respectively, interest income on fiduciary funds of $21 million and $22 million in 2012 and 2011, respectively, and equity method income of $10 million and $9 million in 2012 and 2011, respectively.

[3] Includes inter-segment revenue of $11 million and $10 million in 2012 and 2011, respectively, and interest income on fiduciary funds of $1 million in 2011.

[4] Includes inter-segment revenue of $19 million and $20 million in 2012 and 2011, respectively, and interest income on fiduciary funds of $1 million and $2 million in 2012 and 2011, respectively.