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, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Revenue	$ 6,221	$ 5,876	$ 12,694	$ 11,800
Total Segments
Segment Reporting Information [Line Items]
Revenue	6,238	5,894	12,725	11,831
Total Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	4,022	3,722	8,295	7,628
Total Segments \| Risk and Insurance Services \| Marsh
Segment Reporting Information [Line Items]
Revenue	3,342	3,103	6,423	5,903
Total Segments \| Risk and Insurance Services \| Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	680	619	1,872	1,725
Total Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue	2,216	2,172	4,430	4,203
Total Segments \| Consulting \| Mercer
Segment Reporting Information [Line Items]
Revenue	1,379	1,374	2,804	2,718
Total Segments \| Consulting \| Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	837	798	1,626	1,485
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (17)	$ (18)	$ (31)	$ (31)