Document and Entity Information
Jul. 30, 2019
Cover page.  
Document Type 8-K
Document Period End Date Jul. 30, 2019
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
Entity Central Index Key 0000062709
Amendment Flag false