Segment Information (Details Of Operating Segment Revenue) (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
Risk and Insurance Services [Member]
Revenue	1,620	[1]	1,459	[1]	3,254	[2]	2,951	[2]
Marsh [Member]
Revenue	1,362		1,214		2,654		2,389
Guy Carpenter [Member]
Revenue	258		245		600		562
Total Consulting [Member]
Revenue	1,319		1,168		2,580		2,323
Mercer [Member]
Revenue	945		838		1,867		1,687
Oliver Wyman Group [Member]
Revenue	374		330		713		636
Total Operating Segments [Member]
Revenue	2,939		2,627		5,834		5,274
Corporate / Eliminations [Member]
Revenue	$ (11)		$ (21)		$ (22)		$ (33)

[1] 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.

[2] 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.