Net uncollected premiums and claims and the related payables are not assets and liabilities of MMC and are not included in the accompanying consolidated balance sheets.

No definition available.

Name:	mmc_UncollectedPremiumsAndClaimsNetOfRelatedPayables
Namespace Prefix:	mmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fiduciary Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Fiduciary Assets and Liabilities [Line Items]
Net uncollected premiums and claims receivable and payable	$ 15,300		$ 15,300		$ 15,100
Operating Segments \| Risk and Insurance Services
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	$ 109	$ 138	$ 311	$ 385