Acquisitions and Dispositions (Acquired Finite-Lived Intangible Assets by Major class) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2025 USD ($)
Finite-Lived Intangible Assets [Line Items]
Amount	$ 100
Client relationships
Finite-Lived Intangible Assets [Line Items]
Amount	$ 96
Weighted Average Amortization Period	11 years 4 months 24 days
Other
Finite-Lived Intangible Assets [Line Items]
Amount	$ 4
Weighted Average Amortization Period	4 years 2 months 12 days

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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MARSH & MCLENNAN COMPANIES, INC. 10-Q