Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Segment Reporting Information [Line Items]
Revenue	$ 3,495		$ 3,376		$ 6,998		$ 6,712
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,508		3,389		7,023		6,735
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	1,916	[1]	1,850	[1]	3,905	[2]	3,718	[2]
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	1,621		1,564		3,223		3,057
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	295		286		682		661
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,592	[3]	1,539	[3]	3,118	[4]	3,017	[4]
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,109		1,079		2,186		2,118
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	483		460		932		899
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (13)		$ (13)		$ (25)		$ (23)

[1] Includes inter-segment revenue of $4 million and $3 million in 2017 and 2016, respectively, interest income on fiduciary funds of $9 million and $6 million in 2017 and 2016, respectively, and equity method income of $7 million and $6 million in 2017 and 2016, respectively.

[2] Includes inter-segment revenue of $4 million in both 2017 and 2016, interest income on fiduciary funds of $17 million and $12 million in 2017 and 2016, respectively, and equity method income of $9 million and $7 million in 2017 and 2016, respectively.

[3] Includes inter-segment revenue of $9 million and $10 million in 2017 and 2016, respectively, interest income on fiduciary funds of less than $1 million in both 2017 and 2016, and equity method income of $5 million in both 2017 and 2016.

[4] Includes inter-segment revenue of $21 million and $19 million in 2017 and 2016, respectively, interest income on fiduciary funds of $1 million in both 2017 and 2016, and equity method income of $9 million in both 2017 and 2016.