Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Segment Reporting Information [Line Items]
Revenue	$ 11,526		$ 10,550		$ 9,831
Fixed Assets, Net	804		822		850
Risk and Insurance Services [Member]
Segment Reporting Information [Line Items]
Revenue	6,301	[1]	5,764	[1]	5,284	[2]
Marsh [Member]
Segment Reporting Information [Line Items]
Revenue	5,253		4,781		4,363
Guy Carpenter [Member]
Segment Reporting Information [Line Items]
Revenue	1,048		983		921
Mercer [Member]
Segment Reporting Information [Line Items]
Revenue	3,782		3,478		3,327
Oliver Wyman Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,483		1,357		1,282
Consulting [Member]
Segment Reporting Information [Line Items]
Revenue	5,265	[3]	4,835	[3]	4,609	[4]
Total Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	11,566		10,599		9,893
Corporate / Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(40)	[2]	(49)	[2]	(62)	[2]
United States [Member]
Segment Reporting Information [Line Items]
Revenue	5,131		4,708		4,436
Fixed Assets, Net	505		511		515
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenue	1,922		1,720		1,708
Fixed Assets, Net	133		132		147
Continental Europe [Member]
Segment Reporting Information [Line Items]
Revenue	1,906		1,809		1,798
Fixed Assets, Net	65		69		74
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Revenue	1,287		1,067		862
Fixed Assets, Net	37		43		41
Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Revenue	1,320		1,295		1,089
Fixed Assets, Net	$ 64		$ 67		$ 73

[1] Includes inter-segment revenue of $4 million, $7 million and $18 million in 2011, 2010 and 2009 respectively, interest income on fiduciary funds of $47 million, $45 million and $54 million in 2011, 2010, and 2009, respectively, and equity method income of $14 million, $12 million and $13 million in 2011, 2010, and 2009, respectively.

[2] Includes results of corporate advisory and restructuring business.

[3] Includes inter-segment revenue of $36 million, $43 million and $45 million in 2011, 2010 and 2009, respectively, and interest income on fiduciary funds of $4 million in 2011, 2010, and 2009.

[4] Corporate assets primarily include insurance recoverables, pension related assets, the owned portion of the Company headquarters building and intercompany eliminations. Also includes assets of discontinued operations and the corporate advisory and restructuring business of $1.6 billion in 2009.