Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 20,720	$ 19,820	$ 17,224
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	20,784	19,874	17,313
Pre-tax gain on sale		50
Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	2,537	2,617	2,093
Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Revenue	12,645	12,085	10,337
Fiduciary interest income	120	15	46
Risk and Insurance Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	12,645	12,085	10,337
Fiduciary interest income	120	15	46
Gain (loss) on consolidation of business	(27)
Pre-tax gain on sale		50
Risk and Insurance Services | Marsh & Guy Carpenter
Disaggregation of Revenue [Line Items]
Revenue	12,525	12,070	10,291
Risk and Insurance Services | Marsh
Disaggregation of Revenue [Line Items]
Revenue	10,505	10,203	8,595
Risk and Insurance Services | Marsh | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	10,585	10,214	8,628
Risk and Insurance Services | Guy Carpenter
Disaggregation of Revenue [Line Items]
Revenue	2,020	1,867	1,696
Risk and Insurance Services | Guy Carpenter | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,060	1,871	1,709
Risk and Insurance Services | Total International
Disaggregation of Revenue [Line Items]
Revenue	4,714	4,861	4,058
Risk and Insurance Services | EMEA
Disaggregation of Revenue [Line Items]
Revenue	2,879	2,946	2,575
Risk and Insurance Services | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	1,333	1,462	1,059
Risk and Insurance Services | Latin America
Disaggregation of Revenue [Line Items]
Revenue	502	453	424
Risk and Insurance Services | U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue	5,791	5,342	4,537
Consulting
Disaggregation of Revenue [Line Items]
Revenue	8,139	7,789	6,976
Consulting | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	8,139	7,789	6,976
Consulting | Wealth
Disaggregation of Revenue [Line Items]
Revenue	2,366	2,509	2,348
Consulting | Health
Disaggregation of Revenue [Line Items]
Revenue	2,017	1,855	1,793
Consulting | Career
Disaggregation of Revenue [Line Items]
Revenue	962	890	787
Consulting | Mercer
Disaggregation of Revenue [Line Items]
Revenue	5,345	5,254	4,928
Consulting | Mercer | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	5,345	5,254	4,928
Consulting | Oliver Wyman Group
Disaggregation of Revenue [Line Items]
Revenue	2,794	2,535	2,048
Consulting | Oliver Wyman Group | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,794	2,535	$ 2,048
Oliver Wyman Group | Operating Segments
Disaggregation of Revenue [Line Items]
Gain (loss) on consolidation of business	(12)
Marsh India | Operating Segments
Disaggregation of Revenue [Line Items]
Pre-tax gain on sale		$ 267
Mercer U.S. Affinity Business | Operating Segments
Disaggregation of Revenue [Line Items]
Pre-tax gain on sale	$ 112