Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenue	$ 4,770	$ 4,583	$ 15,698	$ 14,683
Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	2,838	2,670	9,700	9,036
Risk and Insurance Services | Marsh
Segment Reporting Information [Line Items]
Revenue	2,470	2,352	7,794	7,327
Risk and Insurance Services | Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	328	314	1,849	1,697
Consulting
Segment Reporting Information [Line Items]
Revenue	1,951	1,925	6,045	5,690
Consulting | Mercer
Segment Reporting Information [Line Items]
Revenue	1,284	1,315	4,016	3,877
Consulting | Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	667	610	2,029	1,813
Operating Segments
Segment Reporting Information [Line Items]
Revenue	4,789	4,595	15,745	14,726
Operating Segments | Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	2,838	2,670	9,700	9,036
Operating Segments | Risk and Insurance Services | Marsh
Segment Reporting Information [Line Items]
Revenue	2,497	2,355	7,833	7,336
Operating Segments | Risk and Insurance Services | Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	341	315	1,867	1,700
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenue	1,951	1,925	6,045	5,690
Operating Segments | Consulting | Mercer
Segment Reporting Information [Line Items]
Revenue	1,284	1,315	4,016	3,877
Operating Segments | Consulting | Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	667	610	2,029	1,813
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (19)	$ (12)	$ (47)	$ (43)