Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Segment Reporting Information [Line Items]
Revenue	$ 7,597	$ 7,061
Operating Segments
Segment Reporting Information [Line Items]
Revenue	7,609	7,076
Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	5,051	4,762
Operating Segments \| Risk and Insurance Services \| Marsh Risk
Segment Reporting Information [Line Items]
Revenue	3,782	3,520
Operating Segments \| Risk and Insurance Services \| Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	1,269	1,242
Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue	2,558	2,314
Operating Segments \| Consulting \| Mercer
Segment Reporting Information [Line Items]
Revenue	1,661	1,496
Operating Segments \| Consulting \| Marsh Management Consulting
Segment Reporting Information [Line Items]
Revenue	897	818
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (12)	$ (15)

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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