Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation of Revenue [Line Items]
Revenue	$ 5,379	$ 5,017	$ 10,928	$ 10,100
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	3,313	3,141	6,862	6,366
Fiduciary interest income	13	3	17	8
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	1,245	1,246	2,512	2,447
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	745	796	1,587	1,633
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	382	347	703	621
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	118	103	222	193
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,533	1,404	2,812	2,528
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	2,084	1,892	4,094	3,765
Consulting Segment | Wealth
Disaggregation of Revenue [Line Items]
Revenue	597	625	1,214	1,248
Consulting Segment | Health
Disaggregation of Revenue [Line Items]
Revenue	587	462	1,111	949
Consulting Segment | Career
Disaggregation of Revenue [Line Items]
Revenue	205	187	407	365
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	3,300	3,138	6,845	6,358
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,778	2,650	5,324	4,975
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	522	488	1,521	1,383
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,389	1,274	2,732	2,562
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 695	$ 618	$ 1,362	$ 1,203