Segment Information (Details of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Segment Reporting Information [Line Items]
Revenue	$ 3,215		$ 3,264
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	3,224		3,271
Operating Segments [Member] | Marsh Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,434		1,457
Operating Segments [Member] | Guy Carpenter Reinsurance Group [Member]
Segment Reporting Information [Line Items]
Revenue	369		382
Operating Segments [Member] | Mercer Consulting Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,037		1,061
Operating Segments [Member] | Oliver Wyman Group Consulting Group [Member]
Segment Reporting Information [Line Items]
Revenue	384		371
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,803	[1]	1,839	[1]
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,421	[2]	1,432	[2]
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(9)		(7)
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1		0
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	$ 8		$ 7

[1] Includes inter-segment revenue of $1 million and $0 million in 2015 and 2014, respectively, interest income on fiduciary funds of $5 million and $6 million in 2015 and 2014, respectively, and equity method income of $2 million and $0 million in 2015 and 2014, respectively.

[2] Includes inter-segment revenue of $8 million and $7 million in 2015 and 2014, respectively, interest income on fiduciary funds of $1 million in both 2015 and 2014, and equity method income of $3 million in 2015 and $0 million in 2014.