Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting Information [Line Items]
Revenue	$ 12,261		$ 11,924		$ 11,526
Fixed Assets, Net	828		809		804
United States [Member]
Segment Reporting Information [Line Items]
Revenue	5,485		5,300		5,131
Fixed Assets, Net	494		494		505
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenue	1,979		1,960		1,922
Fixed Assets, Net	121		121		133
Europe [Member]
Segment Reporting Information [Line Items]
Revenue	1,943		1,879		1,906
Fixed Assets, Net	64		63		65
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Revenue	1,396		1,346		1,287
Fixed Assets, Net	72		62		37
Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Revenue	1,494		1,478		1,320
Fixed Assets, Net	77		69		64
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	12,297		11,963		11,566
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	6,596	[1]	6,350	[1]	6,079	[1]
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	5,701	[2]	5,613	[2]	5,487	[2]
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(36)	[3]	(39)	[3]	(40)	[3]
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	5		5		4
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	31		34		36
Marsh Insurance Group [Member] | Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	5,461		5,265		5,031
Guy Carpenter Reinsurance Group [Member] | Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,135		1,085		1,048
Mercer Consulting Group [Member] | Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	4,241		4,147		4,004
Oliver Wyman Group Consulting Group [Member] | Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	$ 1,460		$ 1,466		$ 1,483

[1] Includes inter-segment revenue of $5 million in both 2013 and 2012 and $4 million in 2011, interest income on fiduciary funds of $27 million, $39 million and $47 million in 2013, 2012 and 2011, respectively, and equity method income of $8 million, $11 million and $14 million in 2013, 2012 and 2011, respectively.

[2] Includes inter-segment revenue of $31 million, $34 million and $36 million in 2013, 2012 and 2011, respectively, and interest income on fiduciary funds of $5 million in 2013, and $4 million in both 2012 and 2011.

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