Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015
Segment Reporting Information [Line Items]
Revenue	$ 3,135		$ 3,115		$ 9,847		$ 9,555
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,145		3,124		9,880		9,585
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1,632	[1]	1,584	[1]	5,350	[2]	5,137	[2]
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	1,370		1,322		4,427		4,230
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	262		262		923		907
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,513	[3]	1,540	[3]	4,530	[4]	4,448	[4]
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,109		1,090		3,227		3,173
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	404		450		1,303		1,275
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (10)		$ (9)		$ (33)		$ (30)

[1] Includes inter-segment revenue of $1 million and $0 million in 2016 and 2015, respectively, interest income on fiduciary funds of $8 million and $6 million in 2016 and 2015, respectively, and equity method income of $4 million in 2016 and $2 million in 2015, respectively.

[2] Includes inter-segment revenue of $5 million in both 2016 and in 2015, interest income on fiduciary funds of $20 million and $16 million in 2016 and 2015, respectively, and equity method income of $11 million and $4 million in 2016 and 2015, respectively.

[3] Includes inter-segment revenue of $9 million in both 2016 and 2015, interest income on fiduciary funds of $1 million in both 2016 and 2015, and equity method income of $5 million in both 2016 and 2015.

[4] Includes inter-segment revenue of $28 million and $25 million in 2016 and 2015, respectively, interest income on fiduciary funds of $2 million and $3 million in 2016 and 2015, respectively, and equity method income of $14 million and $13 million in 2016 and 2015, respectively.