v3.25.4

Segment Information (Geographic Area) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2023	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		$ 26,981	$ 24,458	$ 22,736
Fixed Assets, Net		829	859	882
Risk and Insurance Services | Guy Carpenter Reinsurance Group
Segment Reporting, Asset Reconciling Item [Line Items]
Gain from the settlement	$ 58			58
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		27,059	24,528	22,798
Operating Segments | Risk and Insurance Services
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		17,265	15,395	14,089
Operating Segments | Risk and Insurance Services | Guy Carpenter Reinsurance Group
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		2,635	2,544	2,432
Operating Segments | Consulting
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		9,794	9,133	8,709
Operating Segments | Consulting | Marsh Management Consulting
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		3,604	3,390	3,122
Pre-tax gain (loss) on sale			20
Corporate/Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		(78)	(70)	(62)
United States
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		13,344	11,671	10,924
Fixed Assets, Net		469	494	468
United States | Operating Segments | Technology Consulting And Administrative Solutions
Segment Reporting, Asset Reconciling Item [Line Items]
Pre-tax gain (loss) on sale		15
United States | Operating Segments | Mercer U.S. Affinity Business | Consulting
Segment Reporting, Asset Reconciling Item [Line Items]
Pre-tax gain (loss) on sale			(35)
United Kingdom
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		3,816	3,595	3,555
Fixed Assets, Net		136	150	168
United Kingdom | Operating Segments | Consulting | Mercer U K Pension Administration Businesses
Segment Reporting, Asset Reconciling Item [Line Items]
Pre-tax gain (loss) on sale			70
Other
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue		9,899	9,262	8,319
Fixed Assets, Net		224	$ 215	$ 246
Equity Method Investment, Realized Gain (Loss) on Disposal		$ 13

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