Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Revenue	$ 5,924	$ 5,549
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,906	3,549
Risk and Insurance Services | Marsh
Segment Reporting Information [Line Items]
Revenue	2,744	2,546
Risk and Insurance Services | Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	1,071	999
Consulting
Segment Reporting Information [Line Items]
Revenue	2,031	2,010
Consulting | Mercer
Segment Reporting Information [Line Items]
Revenue	1,344	1,343
Consulting | Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	687	667
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,937	5,559
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,906	3,549
Operating Segments | Risk and Insurance Services | Marsh
Segment Reporting Information [Line Items]
Revenue	2,800	2,549
Operating Segments | Risk and Insurance Services | Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	1,106	1,000
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	2,031	2,010
Operating Segments | Consulting | Mercer
Segment Reporting Information [Line Items]
Revenue	1,344	1,343
Operating Segments | Consulting | Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	687	667
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (13)	$ (10)