Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Revenue	$ 5,017	$ 4,189	$ 10,100	$ 8,840
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,141	2,603	6,366	5,514
Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,650	2,161	4,975	4,222
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	488	433	1,383	1,260
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,892	1,616	3,765	3,378
Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,274	1,149	2,562	2,400
Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	618	467	1,203	978
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,033	4,219	10,131	8,892
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	3,141	2,603	6,366	5,514
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,652	2,168	4,981	4,244
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	489	435	1,385	1,270
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,892	1,616	3,765	3,378
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,274	1,149	2,562	2,400
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	618	467	1,203	978
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (16)	$ (30)	$ (31)	$ (52)