Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 6,221	$ 5,876	$ 12,694	$ 11,800
Total Segments
Disaggregation of Revenue [Line Items]
Revenue	6,238	5,894	12,725	11,831
Risk and Insurance Services | Total Segments
Disaggregation of Revenue [Line Items]
Fiduciary interest income	125	108	247	199
Revenue	4,022	3,722	8,295	7,628
Risk and Insurance Services | Subtotal | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	3,897	3,614	8,048	7,429
Risk and Insurance Services | Total Marsh | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	3,265	3,038	6,268	5,782
Risk and Insurance Services | Guy Carpenter | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	632	576	1,780	1,647
Risk and Insurance Services | Total International | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,440	1,352	2,926	2,711
Risk and Insurance Services | EMEA | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	912	858	1,937	1,790
Risk and Insurance Services | Asia Pacific | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	391	357	727	669
Risk and Insurance Services | Latin America | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	137	137	262	252
Risk and Insurance Services | U.S./Canada | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,825	1,686	3,342	3,071
Consulting | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	2,216	2,172	4,430	4,203
Consulting | Mercer | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	1,379	1,374	2,804	2,718
Consulting | Mercer | Wealth | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	612	637	1,284	1,218
Consulting | Mercer | Health | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	547	518	1,085	1,063
Consulting | Mercer | Career | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	220	219	435	437
Consulting | Oliver Wyman Group | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	$ 837	$ 798	$ 1,626	$ 1,485