Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting [Line Items]
Revenue	$ 7,404	$ 6,974	$ 15,001	$ 14,035
Operating Segments
Segment Reporting [Line Items]
Revenue	7,425	6,996	15,034	14,072
Operating Segments \| Risk and Insurance Services
Segment Reporting [Line Items]
Revenue	4,823	4,625	9,874	9,387
Operating Segments \| Risk and Insurance Services \| Marsh Risk
Segment Reporting [Line Items]
Revenue	4,131	3,915	7,913	7,435
Operating Segments \| Risk and Insurance Services \| Guy Carpenter
Segment Reporting [Line Items]
Revenue	692	710	1,961	1,952
Operating Segments \| Consulting
Segment Reporting [Line Items]
Revenue	2,602	2,371	5,160	4,685
Operating Segments \| Consulting \| Mercer
Segment Reporting [Line Items]
Revenue	1,598	1,498	3,259	2,994
Operating Segments \| Consulting \| Marsh Management Consulting
Segment Reporting [Line Items]
Revenue	1,004	873	1,901	1,691
Corporate Eliminations
Segment Reporting [Line Items]
Revenue	$ (21)	$ (22)	$ (33)	$ (37)