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		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Revenue	$ 6,351	$ 5,697	$ 20,386	$ 18,391
Operating Segments
Segment Reporting Information [Line Items]
Revenue	6,372	5,715	20,444	18,440
Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,907	3,453	13,294	11,748
Operating Segments \| Risk and Insurance Services \| Marsh
Segment Reporting Information [Line Items]
Revenue	3,472	3,023	10,907	9,446
Operating Segments \| Risk and Insurance Services \| Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	435	430	2,387	2,302
Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue	2,465	2,262	7,150	6,692
Operating Segments \| Consulting \| Mercer
Segment Reporting Information [Line Items]
Revenue	1,579	1,452	4,573	4,256
Operating Segments \| Consulting \| Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	886	810	2,577	2,436
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (21)	$ (18)	$ (58)	$ (49)