Segment Information (Details of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Segment Reporting Information [Line Items]
Revenue	$ 3,264		$ 3,126
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	3,271		3,133
Operating Segments [Member] | Marsh Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,457		1,395
Operating Segments [Member] | Guy Carpenter Reinsurance Group [Member]
Segment Reporting Information [Line Items]
Revenue	382		376
Operating Segments [Member] | Mercer Consulting Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,061		1,041
Operating Segments [Member] | Oliver Wyman Group Consulting Group [Member]
Segment Reporting Information [Line Items]
Revenue	371		321
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,839	[1]	1,771	[1]
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,432	[2]	1,362	[2]
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(7)		(7)
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	$ 7		$ 7

[1] Includes interest income on fiduciary funds of $6 million and $8 million in 2014 and 2013, respectively, and equity method income of $1 million in 2013, respectively.

[2] Includes inter-segment revenue of $7 million in both 2014 and 2013 and interest income on fiduciary funds of $1 million in both 2014 and 2013.