Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Revenue	$ 4,770	$ 4,583	$ 15,698	$ 14,683
Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Revenue	2,838	2,670	9,700	9,036
Fiduciary interest income	40	4	57	12
Risk and Insurance Services | Total International
Disaggregation of Revenue [Line Items]
Revenue	1,019	986	3,531	3,433
Risk and Insurance Services | EMEA
Disaggregation of Revenue [Line Items]
Revenue	589	600	2,176	2,233
Risk and Insurance Services | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	312	281	1,015	902
Risk and Insurance Services | Latin America
Disaggregation of Revenue [Line Items]
Revenue	118	105	340	298
Risk and Insurance Services | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,451	1,366	4,263	3,894
Consulting
Disaggregation of Revenue [Line Items]
Revenue	1,951	1,925	6,045	5,690
Consulting | Wealth
Disaggregation of Revenue [Line Items]
Revenue	561	613	1,775	1,861
Consulting | Health
Disaggregation of Revenue [Line Items]
Revenue	451	449	1,562	1,398
Consulting | Career
Disaggregation of Revenue [Line Items]
Revenue	272	253	679	618
Marsh & Guy Carpenter | Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Revenue	2,798	2,666	9,643	9,024
Marsh | Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Revenue	2,470	2,352	7,794	7,327
Guy Carpenter | Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Revenue	328	314	1,849	1,697
Mercer | Consulting
Disaggregation of Revenue [Line Items]
Revenue	1,284	1,315	4,016	3,877
Oliver Wyman Group | Consulting
Disaggregation of Revenue [Line Items]
Revenue	$ 667	$ 610	$ 2,029	$ 1,813