Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 5,697	$ 5,382	$ 18,391	$ 17,182
Total Segments
Disaggregation of Revenue [Line Items]
Revenue	5,715	5,396	18,440	17,227
Risk and Insurance Services | Total Segments
Disaggregation of Revenue [Line Items]
Fiduciary interest income	138	131	385	330
Revenue	3,453	3,190	11,748	10,818
Risk and Insurance Services | Subtotal | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	3,315	3,059	11,363	10,488
Risk and Insurance Services | Total Marsh | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	2,934	2,700	9,202	8,482
Risk and Insurance Services | Guy Carpenter | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	381	359	2,161	2,006
Risk and Insurance Services | Total International | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,223	1,137	4,149	3,848
Risk and Insurance Services | EMEA | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	747	692	2,684	2,482
Risk and Insurance Services | Asia Pacific | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	342	311	1,069	980
Risk and Insurance Services | Latin America | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	134	134	396	386
Risk and Insurance Services | U.S./Canada | Total Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,711	1,563	5,053	4,634
Consulting | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	2,262	2,206	6,692	6,409
Consulting | Mercer | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	1,452	1,425	4,256	4,143
Consulting | Mercer | Wealth | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	625	635	1,909	1,853
Consulting | Mercer | Health | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	520	496	1,605	1,559
Consulting | Mercer | Career | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	307	294	742	731
Consulting | Oliver Wyman Group | Total Segments
Disaggregation of Revenue [Line Items]
Revenue	$ 810	$ 781	$ 2,436	$ 2,266