Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 19,820	$ 17,224	$ 16,652
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	19,874	17,313	16,742
Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	2,617	2,093	2,001
Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue	12,085	10,337	9,599
Fiduciary interest income	15	46	105
Risk and insurance services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	12,085	10,337	9,599
Fiduciary interest income	15	46	105
Risk and insurance services | Total International
Disaggregation of Revenue [Line Items]
Revenue	4,861	4,058	3,895
Risk and insurance services | EMEA
Disaggregation of Revenue [Line Items]
Revenue	2,946	2,575	2,482
Risk and insurance services | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	1,462	1,059	953
Risk and insurance services | Latin America
Disaggregation of Revenue [Line Items]
Revenue	453	424	460
Risk and insurance services | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	5,342	4,537	4,119
Consulting
Disaggregation of Revenue [Line Items]
Revenue	7,789	6,976	7,143
Consulting | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	7,789	6,976	7,143
Consulting | Wealth
Disaggregation of Revenue [Line Items]
Revenue	2,509	2,348	2,369
Consulting | Health
Disaggregation of Revenue [Line Items]
Revenue	1,855	1,793	1,796
Consulting | Career
Disaggregation of Revenue [Line Items]
Revenue	890	787	856
Marsh & Guy Carpenter | Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue	12,070	10,291	9,494
Marsh | Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue	10,203	8,595	8,014
Marsh | Risk and insurance services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	10,214	8,628	8,085
Guy Carpenter | Risk and insurance services
Disaggregation of Revenue [Line Items]
Revenue	1,867	1,696	1,480
Guy Carpenter | Risk and insurance services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,871	1,709	1,514
Mercer | Consulting
Disaggregation of Revenue [Line Items]
Revenue	5,254	4,928	5,021
Mercer | Consulting | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	5,254	4,928	5,021
Oliver Wyman Group | Consulting
Disaggregation of Revenue [Line Items]
Revenue	2,535	2,048	2,122
Oliver Wyman Group | Consulting | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	$ 2,535	$ 2,048	$ 2,122