Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Revenue	$ 6,473	$ 5,924
Total Segments
Segment Reporting Information [Line Items]
Revenue	6,487	5,937
Total Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	4,273	3,906
Total Segments | Risk and Insurance Services | Marsh
Segment Reporting Information [Line Items]
Revenue	3,081	2,800
Total Segments | Risk and Insurance Services | Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	1,192	1,106
Total Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	2,214	2,031
Total Segments | Consulting | Mercer
Segment Reporting Information [Line Items]
Revenue	1,425	1,344
Total Segments | Consulting | Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	789	687
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (14)	$ (13)