Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) (USD $) In Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Revenue	$ 2,928		$ 2,606		$ 5,812		$ 5,241
Operating Income (Loss)	465		(50)		937		375
Interest income on fiduciary funds	5		3		12		7
Equity method income	(4)		18		14		17
Risk and Insurance Services [Member] | Inter Segment Revenue [Member]
Inter-segment revenue	1		1
Consulting [Member]
Revenue	1,319	[1]	1,168	[1]	2,580	[2]	2,323	[2]
Operating Income (Loss)	152	[1]	(275)		280		(159)
Inter-segment revenue	10		20		20		31
Interest income on fiduciary funds	1		1		2		2
Total Operating Segments [Member]
Revenue	2,939		2,627		5,834		5,274
Operating Income (Loss)	508		(17)		1,019		446
Corporate / Eliminations [Member]
Revenue	(11)		(21)		(22)		(33)
Operating Income (Loss)	(43)		(33)		(82)		(71)
Risk and Insurance Services [Member]
Revenue	1,620	[3]	1,459	[3]	3,254	[4]	2,951	[4]
Operating Income (Loss)	356		258		739		605
Inter-segment revenue					2		2
Interest income on fiduciary funds	10		11
Equity method income	$ 9		$ 8		$ 7		$ 8

[1] Includes inter-segment revenue of $10 million and $20 million in 2011 and 2010, respectively, and interest income on fiduciary funds of $1 million in both 2011 and 2010.

[2] Includes inter-segment revenue of $20 million and $31 million in 2011 and 2010, respectively, and interest income on fiduciary funds of $2 million in both 2011 and 2010.

[3] Includes inter-segment revenue of $1 million in both 2011 and 2010, respectively, interest income on fiduciary funds of $10 million and $11 million in 2011 and 2010, respectively, and equity method income of $7 million in both 2011 and 2010.

[4] Includes inter-segment revenue of $2 in both 2011 and 2010, respectively, interest income on fiduciary funds of $22 million in both 2011 and 2010, respectively, and equity method income of $9 million and $8 million in 2011 and 2010, respectively.