Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Revenue	$ 5,876	$ 5,379	$ 11,800	$ 10,928
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,722	3,313	7,628	6,862
Risk and Insurance Services | Marsh
Segment Reporting Information [Line Items]
Revenue	3,038	2,778	5,782	5,324
Risk and Insurance Services | Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	576	522	1,647	1,521
Consulting
Segment Reporting Information [Line Items]
Revenue	2,172	2,084	4,203	4,094
Consulting | Mercer
Segment Reporting Information [Line Items]
Revenue	1,374	1,389	2,718	2,732
Consulting | Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	798	695	1,485	1,362
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,894	5,397	11,831	10,956
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,722	3,313	7,628	6,862
Operating Segments | Risk and Insurance Services | Marsh
Segment Reporting Information [Line Items]
Revenue	3,103	2,787	5,903	5,336
Operating Segments | Risk and Insurance Services | Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	619	526	1,725	1,526
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	2,172	2,084	4,203	4,094
Operating Segments | Consulting | Mercer
Segment Reporting Information [Line Items]
Revenue	1,374	1,389	2,718	2,732
Operating Segments | Consulting | Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	798	695	1,485	1,362
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (18)	$ (18)	$ (31)	$ (28)