v3.26.1
Document and Entity Information
Jul. 09, 2025
Entity Information [Line Items]  
Document Type 8-K
Document Period End Date Jun. 02, 2026
Entity Registrant Name Marsh & McLennan Companies, Inc.
Entity Incorporation, State or Country Code DE
Entity File Number 1-5998
Entity Tax Identification Number 36-2668272
Entity Address, Address Line One 1166 Avenue of the Americas,
Entity Address, City or Town New York,
Entity Address, State or Province NY
Entity Address, Postal Zip Code 10036
City Area Code (212)
Local Phone Number 345-5000
Written Communications false
Soliciting Material false
Pre-commencement Tender Offer false
Pre-commencement Issuer Tender Offer false
Entity Emerging Growth Company false
Title of 12(b) Security Common Stock, par value $1.00 per share
Trading Symbol MRSH
Entity Central Index Key 0000062709
Amendment Flag false
New York Stock Exchange  
Entity Information [Line Items]  
Security Exchange Name NYSE
NYSE Texas  
Entity Information [Line Items]  
Security Exchange Name CHX