Segment Information (Operating Segment Revenue) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	$ 24,458	$ 22,736	$ 20,720
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	24,528	22,798	20,784
Operating Segments \| Risk and Insurance Services
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	15,395	14,089	12,645
Operating Segments \| Risk and Insurance Services \| Marsh Insurance Group
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	12,851	11,657	10,585
Operating Segments \| Risk and Insurance Services \| Guy Carpenter Reinsurance Group
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	2,544	2,432	2,060
Operating Segments \| Consulting Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	9,133	8,709	8,139
Operating Segments \| Consulting Segment \| Mercer Consulting Group
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	5,743	5,587	5,345
Operating Segments \| Consulting Segment \| Oliver Wyman Group
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	3,390	3,122	2,794
Corporate/Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	$ (70)	$ (62)	$ (64)

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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