Segment Information (Selected Information And Details For MMC's Operating Segments) (Details) (USD $) In Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Revenue	$ 2,806		$ 2,524		$ 8,618		$ 7,765
Operating Income	310		239		1,247		614
Interest income on fiduciary funds	9		6		21		13
Equity method income			(4)		14		13
Risk and Insurance Services [Member] | Inter Segment Revenue [Member]
Inter-segment revenue	2		4
Consulting [Member]
Revenue	1,339	[1]	1,203	[1]	3,919	[2]	3,526	[2]
Operating Income	161		138		441		(21)
Inter-segment revenue	6		2		26		33
Interest income on fiduciary funds	1		1		3		3
Total Operating Segments [Member]
Revenue	2,814		2,530		8,648		7,804
Operating Income	347		280		1,366		726
Corporate / Eliminations [Member]
Revenue	(8)		(6)		(30)		(39)
Operating Income	(37)		(41)		(119)		(112)
Risk and Insurance Services [Member]
Revenue	1,475	[3]	1,327	[3]	4,729	[4]	4,278	[4]
Operating Income	186		142		925		747
Inter-segment revenue					4		6
Interest income on fiduciary funds	14		11		36		33
Equity method income	$ 1		$ 1		$ 10		$ 9

[1] Includes inter-segment revenue of $6 million and $2 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 $26 million and $33 million in 2011 and 2010, respectively, and interest income on fiduciary funds of $3 million in both 2011 and 2010.

[3] Includes inter-segment revenue of $2 million and $4 million in 2011 and 2010, respectively, interest income on fiduciary funds of $14 million and $11 million in 2011 and 2010, respectively, and equity method income of $1 million in both 2011 and 2010 .

[4] Includes inter-segment revenue of $4 million and $6 million in 2011 and 2010, respectively, interest income on fiduciary funds of $36 million and $33 million in 2011 and 2010, respectively, and equity method income of $10 million and $9 million in 2011 and 2010, respectively.