Segment Information (Details Of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Revenue	$ 3,026		$ 2,928		$ 6,077		$ 5,812
Marsh [Member]
Revenue	1,421		1,362		2,809		2,654
Guy Carpenter [Member]
Revenue	277		258		636		600
Risk and Insurance Services [Member]
Revenue	1,698	[1]	1,620	[1]	3,445	[2]	3,254	[2]
Mercer [Member]
Revenue	960		945		1,917		1,867
Oliver Wyman Group [Member]
Revenue	381		374		737		713
Consulting [Member]
Revenue	1,341	[3]	1,319	[3]	2,654	[4]	2,580	[4]
Total Operating Segments [Member]
Revenue	3,039		2,939		6,099		5,834
Corporate / Eliminations [Member]
Revenue	$ (13)		$ (11)		$ (22)		$ (22)

[1] Includes inter-segment revenue of $2 million and $1 million in 2012 and 2011, respectively, interest income on fiduciary funds of $10 million in both 2012 and 2011, and equity method income of $9 million and $7 million in 2012 and 2011, respectively.

[2] Includes inter-segment revenue of $3 million and $2 million in 2012 and 2011, respectively, interest income on fiduciary funds of $21 million and $22 million in 2012 and 2011, respectively, and equity method income of $10 million and $9 million in 2012 and 2011, respectively.

[3] Includes inter-segment revenue of $11 million and $10 million in 2012 and 2011, respectively, and interest income on fiduciary funds of $1 million in 2011.

[4] Includes inter-segment revenue of $19 million and $20 million in 2012 and 2011, respectively, and interest income on fiduciary funds of $1 million and $2 million in 2012 and 2011, respectively.