Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Revenue		$ 4,071	$ 4,000
Operating Segments
Segment Reporting Information [Line Items]
Revenue		4,096	4,012
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	[1]	2,423	2,344
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue		1,753	1,703
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue		670	641
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	[2]	1,673	1,668
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue		1,155	1,171
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue		518	497
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenue		$ (25)	$ (12)

[1] Includes inter-segment revenue of $1 million in 2019 and 2018, respectively, interest income on fiduciary funds of $23 million and $13 million in 2019 and 2018, respectively, and equity method income (loss) of $2 million and $(1) million in 2019 and 2018, respectively

[2] Includes inter-segment revenue of $24 million and $11 million in 2019 and 2018, respectively, interest income on fiduciary funds of $1 million in 2019 and 2018, respectively, and equity method income of $5 million and $3 million in 2019 and in 2018, respectively.