Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Revenue [Line Items]
Revenue	$ 7,404	$ 6,974	$ 15,001	$ 14,035
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	7,425	6,996	15,034	14,072
Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(21)	(22)	(33)	(37)
Risk and Insurance Services \| Operating Segments
Disaggregation of Revenue [Line Items]
Fiduciary interest income	88	99	173	202
Revenue	4,823	4,625	9,874	9,387
Risk and Insurance Services \| Operating Segments \| Subtotal
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	4,735	4,526	9,701	9,185
Risk and Insurance Services \| Operating Segments \| Marsh Risk
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	4,071	3,849	7,797	7,302
Revenue	4,131	3,915	7,913	7,435
Risk and Insurance Services \| Operating Segments \| Guy Carpenter
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	664	677	1,904	1,883
Revenue	692	710	1,961	1,952
Risk and Insurance Services \| Operating Segments \| Total International
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,655	1,547	3,368	3,065
Risk and Insurance Services \| Operating Segments \| EMEA
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	1,063	1,006	2,271	2,065
Risk and Insurance Services \| Operating Segments \| Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	439	409	808	744
Risk and Insurance Services \| Operating Segments \| Latin America
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	153	132	289	256
Risk and Insurance Services \| Operating Segments \| U.S./Canada
Disaggregation of Revenue [Line Items]
Revenue, excluding fiduciary interest income	2,416	2,302	4,429	4,237
Consulting \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,602	2,371	5,160	4,685
Consulting \| Operating Segments \| Wealth
Disaggregation of Revenue [Line Items]
Revenue	740	685	1,492	1,355
Consulting \| Operating Segments \| Health
Disaggregation of Revenue [Line Items]
Revenue	611	594	1,272	1,202
Consulting \| Operating Segments \| Career
Disaggregation of Revenue [Line Items]
Revenue	247	219	495	437
Consulting \| Operating Segments \| Mercer
Disaggregation of Revenue [Line Items]
Revenue	1,598	1,498	3,259	2,994
Consulting \| Operating Segments \| Marsh Management Consulting
Disaggregation of Revenue [Line Items]
Revenue	$ 1,004	$ 873	$ 1,901	$ 1,691