Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2015		Sep. 30, 2014		Sep. 30, 2015		Sep. 30, 2014
Segment Reporting Information [Line Items]
Revenue	$ 3,115		$ 3,141		$ 9,555		$ 9,705
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	3,124		3,151		9,585		9,733
Operating Segments [Member] | Marsh Insurance Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,322		1,343		4,230		4,295
Operating Segments [Member] | Guy Carpenter Reinsurance Group [Member]
Segment Reporting Information [Line Items]
Revenue	262		267		907		945
Operating Segments [Member] | Mercer Consulting Group [Member]
Segment Reporting Information [Line Items]
Revenue	1,090		1,112		3,173		3,244
Operating Segments [Member] | Oliver Wyman Group Consulting Group [Member]
Segment Reporting Information [Line Items]
Revenue	450		429		1,275		1,249
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,584	[1]	1,610	[1]	5,137	[2]	5,240	[2]
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	1,540	[3]	1,541	[3]	4,448	[4]	4,493	[4]
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(9)		(10)		(30)		(28)
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue	0		0		5		3
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue	$ 9		$ 10		$ 25		$ 25

[1] Includes inter-segment revenue of $0 million in both 2015 and 2014, interest income on fiduciary funds of $6 million in both 2015 and 2014, and equity method income of $2 million and $4 million in 2015 and 2014, respectively.

[2] Includes inter-segment revenue of $5 million and $3 million in 2015 and 2014, respectively, interest income on fiduciary funds of $16 million and $18 million in 2015 and 2014, respectively, and equity method income of $4 million in 2015 and $10 million in 2014.

[3] Includes inter-segment revenue of $9 million in 2015 and $10 million in 2014, interest income on fiduciary funds of $1 million in 2015 and $2 million in 2014, and equity method income of $5 million in 2015 and $0 million in 2014.

[4] Includes inter-segment revenue of $25 million in both 2015 and 2014, interest income on fiduciary funds of $3 million and $5 million in 2015 and 2014, respectively, and equity method income of $13 million in 2015 and $0 million in 2014.