Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenue	$ 3,968	$ 3,504	$ 12,388	$ 11,238
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,991	3,519	12,461	11,277
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,206	1,863	7,203	6,303
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	1,923	1,643	5,850	5,106
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	283	220	1,353	1,197
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,785	1,656	5,258	4,974
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,280	1,175	3,695	3,504
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	505	481	1,563	1,470
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (23)	$ (15)	$ (73)	$ (39)