Segment Information (Details Of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Revenue	$ 3,051		$ 2,884
Marsh [Member]
Revenue	1,388		1,292
Guy Carpenter [Member]
Revenue	359		342
Risk and Insurance Services [Member]
Revenue	1,747	[1]	1,634	[1]
Mercer [Member]
Revenue	957		922
Oliver Wyman Group [Member]
Revenue	356		339
Consulting [Member]
Revenue	1,313	[2]	1,261	[2]
Total Operating Segments [Member]
Revenue	3,060		2,895
Corporate / Eliminations [Member]
Revenue	$ (9)		$ (11)

[1] Includes inter-segment revenue of $1 million in both 2012 and 2011, interest income on fiduciary funds of $11 million and $12 million in 2012 and 2011, respectively, and equity method income of $1 million and $2 million in 2012 and 2011, respectively.

[2] Includes inter-segment revenue of $8 million and $10 million in 2012 and 2011, respectively, and interest income on fiduciary funds of $1 million in both 2012 and 2011.