Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Revenue	$ 5,549	$ 5,083
Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	3,549	3,225
Fiduciary interest income	4	5
Risk and Insurance Services Segment | Total International
Disaggregation of Revenue [Line Items]
Revenue	1,267	1,201
Risk and Insurance Services Segment | EMEA
Disaggregation of Revenue [Line Items]
Revenue	842	837
Risk and Insurance Services Segment | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	321	274
Risk and Insurance Services Segment | Latin America
Disaggregation of Revenue [Line Items]
Revenue	104	90
Risk and Insurance Services Segment | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	1,279	1,124
Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	2,010	1,873
Consulting Segment | Wealth
Disaggregation of Revenue [Line Items]
Revenue	617	623
Consulting Segment | Health
Disaggregation of Revenue [Line Items]
Revenue	524	487
Consulting Segment | Career
Disaggregation of Revenue [Line Items]
Revenue	202	178
Marsh & Guy Carpenter | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	3,545	3,220
Marsh Insurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	2,546	2,325
Guy Carpenter Reinsurance Group | Risk and Insurance Services Segment
Disaggregation of Revenue [Line Items]
Revenue	999	895
Mercer Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	1,343	1,288
Oliver Wyman Group Consulting Group | Consulting Segment
Disaggregation of Revenue [Line Items]
Revenue	$ 667	$ 585