Fiduciary Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Fiduciary Assets and Liabilities [Line Items]
Net uncollected premiums and claims receivable and payable	$ 15,200		$ 13,800
Total Segments \| Risk and Insurance Services
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	$ 122	$ 91

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