Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Revenue	$ 11,924		$ 11,526		$ 10,550
Fixed Assets, Net	809		804		822
Marsh [Member]
Segment Reporting Information [Line Items]
Revenue	5,496		5,253		4,781
Guy Carpenter [Member]
Segment Reporting Information [Line Items]
Revenue	1,085		1,048		983
Risk and Insurance Services [Member]
Segment Reporting Information [Line Items]
Revenue	6,581	[1]	6,301	[1]	5,764	[1]
Mercer [Member]
Segment Reporting Information [Line Items]
Revenue	3,916		3,782		3,478
Oliver Wyman Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,466		1,483		1,357
Consulting [Member]
Segment Reporting Information [Line Items]
Revenue	5,382	[2]	5,265	[2]	4,835	[2]
Total Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	11,963		11,566		10,599
Corporate / Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(39)	[3]	(40)	[3]	(49)	[3]
United States [Member]
Segment Reporting Information [Line Items]
Revenue	5,300		5,131		4,708
Fixed Assets, Net	494		505		511
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenue	1,960		1,922		1,720
Fixed Assets, Net	121		133		132
Continental Europe [Member]
Segment Reporting Information [Line Items]
Revenue	1,879		1,906		1,809
Fixed Assets, Net	63		65		69
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Revenue	1,346		1,287		1,067
Fixed Assets, Net	62		37		43
Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Revenue	1,478		1,320		1,295
Fixed Assets, Net	$ 69		$ 64		$ 67

[1] Includes inter-segment revenue of $5 million, $4 million and $7 million in 2012, 2011 and 2010, respectively, interest income on fiduciary funds of $39 million, $47 million and $45 million in 2012, 2011 and 2010, respectively, and equity method income of $11 million, $14 million and $12 million in 2012, 2011 and 2010, respectively.

[2] Includes inter-segment revenue of $34 million, $36 million and $43 million in 2012, 2011 and 2010, respectively, and interest income on fiduciary funds of $4 million in 2012, 2011 and 2010.

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