Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Revenue	$ 7,061	$ 6,473
Operating Segments
Segment Reporting Information [Line Items]
Revenue	7,076	6,487
Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	4,762	4,273
Operating Segments \| Risk and Insurance Services \| Marsh
Segment Reporting Information [Line Items]
Revenue	3,520	3,081
Operating Segments \| Risk and Insurance Services \| Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	1,242	1,192
Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue	2,314	2,214
Operating Segments \| Consulting \| Mercer
Segment Reporting Information [Line Items]
Revenue	1,496	1,425
Operating Segments \| Consulting \| Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	818	789
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (15)	$ (14)

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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