Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 6,351	$ 5,697	$ 20,386	$ 18,391
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	6,372	5,715	20,444	18,440
Risk and Insurance Services | Operating Segments
Disaggregation of Revenue [Line Items]
Fiduciary interest income	109	138	311	385
Revenue	3,907	3,453	13,294	11,748
Risk and Insurance Services | Subtotal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	3,798	3,315	12,983	11,363
Risk and Insurance Services | Marsh | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	3,400	2,934	10,702	9,202
Revenue	3,472	3,023	10,907	9,446
Risk and Insurance Services | Guy Carpenter | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	398	381	2,281	2,161
Revenue	435	430	2,387	2,302
Risk and Insurance Services | Total International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,311	1,223	4,376	4,149
Risk and Insurance Services | EMEA | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	813	747	2,878	2,684
Risk and Insurance Services | Asia Pacific | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	361	342	1,105	1,069
Risk and Insurance Services | Latin America | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	137	134	393	396
Risk and Insurance Services | U.S./Canada | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	2,089	1,711	6,326	5,053
Consulting | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,465	2,262	7,150	6,692
Consulting | Mercer | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,579	1,452	4,573	4,256
Consulting | Mercer | Wealth | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	705	625	2,060	1,909
Consulting | Mercer | Health | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	555	520	1,757	1,605
Consulting | Mercer | Career | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	319	307	756	742
Consulting | Oliver Wyman Group | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	$ 886	$ 810	$ 2,577	$ 2,436