Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Revenue		$ 3,336	$ 3,215
Interest Income, Fiduciary Assets		6	5
Consulting Segment
Segment Reporting Information [Line Items]
Interest Income, Fiduciary Assets		1	1
Operating Segments
Segment Reporting Information [Line Items]
Revenue		3,346	3,224
Operating Segments | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue		1,493	1,434
Operating Segments | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue		375	369
Operating Segments | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue		1,039	1,037
Operating Segments | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue		439	384
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]	1,868	1,803
Interest Income, Fiduciary Assets		6	5
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]	1,478	1,421
Interest Income, Fiduciary Assets		1	1
Corporate and Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue		(10)	(9)
Intersegment Eliminations | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue		1	1
Intersegment Eliminations | Consulting Segment
Segment Reporting Information [Line Items]
Revenue		$ 9	$ 8

[1] Includes inter-segment revenue of $1 million in both 2016 and 2015, interest income on fiduciary funds of $6 million in 2016 and $5 million in 2015, and equity method income of $1 million in 2016 and $2 million in 2015.

[2] Includes inter-segment revenue of $9 million in 2016 and $8 million in 2015, interest income on fiduciary funds of $1 million in both 2016 and 2015, and equity method income of $4 million in 2016 and $3 million in 2015.