Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Revenue		$ 3,503	$ 3,336
Operating Segments
Segment Reporting Information [Line Items]
Revenue		3,515	3,346
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]	1,989	1,868
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue		1,602	1,493
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue		387	375
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]	1,526	1,478
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue		1,077	1,039
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue		449	439
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue		$ (12)	$ (10)

[1] Includes inter-segment revenue of $1 million in 2016, interest income on fiduciary funds of $8 million and $6 million in 2017 and 2016, respectively, and equity method income of $2 million in 2017 and $1 million in 2016, respectively.

[2] Includes inter-segment revenue of $12 million and $9 million in 2017 and 2016, respectively, interest income on fiduciary funds of $1 million in both 2017 and 2016, and equity method income of $4 million in both 2017 and 2016.