Amount, including 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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenue	$ 5,382	$ 4,770	$ 17,182	$ 15,698
Operating Segments
Segment Reporting Information [Line Items]
Revenue	5,396	4,789	17,227	15,745
Operating Segments \| Risk and Insurance Services
Segment Reporting Information [Line Items]
Revenue	3,190	2,838	10,818	9,700
Operating Segments \| Risk and Insurance Services \| Marsh
Segment Reporting Information [Line Items]
Revenue	2,782	2,497	8,685	7,833
Operating Segments \| Risk and Insurance Services \| Guy Carpenter
Segment Reporting Information [Line Items]
Revenue	408	341	2,133	1,867
Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenue	2,206	1,951	6,409	6,045
Operating Segments \| Consulting \| Mercer
Segment Reporting Information [Line Items]
Revenue	1,425	1,284	4,143	4,016
Operating Segments \| Consulting \| Oliver Wyman Group
Segment Reporting Information [Line Items]
Revenue	781	667	2,266	2,029
Corporate/Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (14)	$ (19)	$ (45)	$ (47)