Segment Information (Details Of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Revenue	$ 2,845		$ 2,806		$ 8,922		$ 8,618
Marsh [Member]
Revenue	1,260		1,221		4,069		3,875
Guy Carpenter [Member]
Revenue	250		254		886		854
Risk and Insurance Services [Member]
Revenue	1,510	[1]	1,475	[1]	4,955	[2]	4,729	[2]
Mercer [Member]
Revenue	995		975		2,912		2,842
Oliver Wyman Group [Member]
Revenue	351		364		1,088		1,077
Consulting [Member]
Revenue	1,346	[3]	1,339	[3]	4,000	[4]	3,919	[4]
Total Operating Segments [Member]
Revenue	2,856		2,814		8,955		8,648
Corporate / Eliminations [Member]
Revenue	$ (11)		$ (8)		$ (33)		$ (30)

[1] Includes inter-segment revenue of $1 million and $2 million in 2012 and 2011, respectively, interest income on fiduciary funds of $10 million and $14 million in 2012 and 2011, respectively, and equity method income of $1 million in 2011.

[2] Includes inter-segment revenue of $4 million in both 2012 and 2011, interest income on fiduciary funds of $31 million and $36 million in 2012 and 2011, respectively, and equity method income of $10 million in both 2012 and 2011.

[3] Includes inter-segment revenue of $10 million and $6 million in 2012 and 2011, respectively, and interest income on fiduciary funds of $1 million in both 2012 and 2011.

[4] Includes inter-segment revenue of $29 million and $26 million in 2012 and 2011, respectively, and interest income on fiduciary funds of $2 million and $3 million in 2012 and 2011, respectively.