Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenue	$ 3,341		$ 3,135		$ 10,339		$ 9,847
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,350		3,145		10,373		9,880
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1,763	[1]	1,632	[1]	5,668	[2]	5,350	[2]
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	1,490		1,370		4,713		4,427
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	273		262		955		923
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,587	[3]	1,513	[3]	4,705	[4]	4,530	[4]
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,149		1,109		3,335		3,227
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	438		404		1,370		1,303
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (9)		$ (10)		$ (34)		$ (33)

[1] Includes inter-segment revenue of $1 million in both 2017 and 2016, interest income on fiduciary funds of $11 million and $8 million in 2017 and 2016, respectively, and equity method income of $2 million and $4 million in 2017 and 2016, respectively.

[2] Includes inter-segment revenue of $5 million in both 2017 and 2016, interest income on fiduciary funds of $28 million and $20 million in 2017 and 2016, respectively, and equity method income of $11 million in both 2017 and 2016.

[3] Includes inter-segment revenue of $8 million and $9 million in 2017 and 2016, respectively, interest income on fiduciary funds of $2 million in 2017 and $1 million in 2016, respectively, and equity method income of $3 million in 2017 and $5 million in 2016, respectively.

[4] Includes inter-segment revenue of $29 million and $28 million in 2017 and 2016, respectively, interest income on fiduciary funds of $3 million in 2017 and $2 million in 2016, respectively, and equity method income of $12 million in 2017 and $14 million in 2016, respectively.