Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Revenue	$ 5,549	$ 5,083
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,549	3,225
Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,546	2,325
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	999	895
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	2,010	1,873
Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,343	1,288
Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	667	585
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,559	5,098
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,549	3,225
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,549	2,329
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	1,000	896
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	2,010	1,873
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,343	1,288
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	667	585
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (10)	$ (15)