Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 4,583	$ 3,968	$ 14,683	$ 12,808
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,670	2,291	9,036	7,805
Fiduciary interest income	4	8	12	40
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	986	883	3,433	2,960
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	600	536	2,233	1,887
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	281	254	902	790
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	105	93	298	283
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,366	1,126	3,894	3,271
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,925	1,696	5,690	5,074
Consulting Segment | Wealth
Disaggregation of Revenue [Line Items]
Revenue	613	566	1,861	1,719
Consulting Segment | Health
Disaggregation of Revenue [Line Items]
Revenue	449	430	1,398	1,348
Consulting Segment | Career
Disaggregation of Revenue [Line Items]
Revenue	253	220	618	549
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,666	2,283	9,024	7,765
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,352	2,009	7,327	6,231
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	314	274	1,697	1,534
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,315	1,216	3,877	3,616
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 610	$ 480	$ 1,813	$ 1,458