Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenue	$ 3,968	$ 3,968	$ 12,808	$ 12,388
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,291	2,206	7,805	7,203
Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,009	1,902	6,231	5,795
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	274	273	1,534	1,328
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,696	1,785	5,074	5,258
Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,216	1,280	3,616	3,695
Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	480	505	1,458	1,563
Operating Segments
Segment Reporting Information [Line Items]
Revenue	3,987	3,991	12,879	12,461
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,291	2,206	7,805	7,203
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,015	1,923	6,259	5,850
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	276	283	1,546	1,353
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,696	1,785	5,074	5,258
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,216	1,280	3,616	3,695
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	480	505	1,458	1,563
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (19)	$ (23)	$ (71)	$ (73)