Segment Information (Details Of Operating Segment Revenue) (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
Risk and Insurance Services [Member]
Revenue	1,475	[1]	1,327	[1]	4,729	[2]	4,278	[2]
Marsh [Member]
Revenue	1,221		1,092		3,875		3,481
Guy Carpenter [Member]
Revenue	254		235		854		797
Mercer [Member]
Revenue	975		881		2,842		2,568
Oliver Wyman Group [Member]
Revenue	364		322		1,077		958
Consulting [Member]
Revenue	1,339	[3]	1,203	[3]	3,919	[4]	3,526	[4]
Total Operating Segments [Member]
Revenue	2,814		2,530		8,648		7,804
Corporate / Eliminations [Member]
Revenue	$ (8)		$ (6)		$ (30)		$ (39)

[1] 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 .

[2] 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.

[3] 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.

[4] 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.