Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenue	$ 4,583	$ 3,968	$ 14,683	$ 12,808
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,670	2,291	9,036	7,805
Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,352	2,009	7,327	6,231
Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	314	274	1,697	1,534
Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,925	1,696	5,690	5,074
Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,315	1,216	3,877	3,616
Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	610	480	1,813	1,458
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,595	3,987	14,726	12,879
Operating Segments | Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Revenue	2,670	2,291	9,036	7,805
Operating Segments | Risk and Insurance Services Segment | Marsh Insurance Group
Segment Reporting Information [Line Items]
Revenue	2,355	2,015	7,336	6,259
Operating Segments | Risk and Insurance Services Segment | Guy Carpenter Reinsurance Group
Segment Reporting Information [Line Items]
Revenue	315	276	1,700	1,546
Operating Segments | Consulting Segment
Segment Reporting Information [Line Items]
Revenue	1,925	1,696	5,690	5,074
Operating Segments | Consulting Segment | Mercer Consulting Group
Segment Reporting Information [Line Items]
Revenue	1,315	1,216	3,877	3,616
Operating Segments | Consulting Segment | Oliver Wyman Group Consulting Group
Segment Reporting Information [Line Items]
Revenue	610	480	1,813	1,458
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (12)	$ (19)	$ (43)	$ (71)