Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Revenue	$ 5,379	$ 5,017	$ 10,928	$ 10,100
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,313	3,141	6,862	6,366
Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,778	2,650	5,324	4,975
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	522	488	1,521	1,383
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	2,084	1,892	4,094	3,765
Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,389	1,274	2,732	2,562
Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	695	618	1,362	1,203
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,397	5,033	10,956	10,131
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,313	3,141	6,862	6,366
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,787	2,652	5,336	4,981
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	526	489	1,526	1,385
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	2,084	1,892	4,094	3,765
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,389	1,274	2,732	2,562
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	695	618	1,362	1,203
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (18)	$ (16)	$ (28)	$ (31)