Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2021	Feb. 10, 2022	Jun. 30, 2021
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	1-5998
Entity Registrant Name	Marsh & McLennan Companies, Inc.
Entity Incorporation, State or Country Code	DE
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
Title of 12(b) Security	Common Stock, par value $1.00 per share
Trading Symbol	MMC
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
ICFR Auditor Attestation Flag	true
Entity Public Float			$ 53,818,358,381
Entity Common Stock, Shares Outstanding		502,765,629
Documents Incorporated by Reference	Portions of Marsh & McLennan Companies, Inc.’s Notice of Annual Meeting and Proxy Statement for the 2022 Annual Meeting of Stockholders (the "2022 Proxy Statement") are incorporated by reference in Part III of this Form 10-K.
Entity Central Index Key	0000062709
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Amendment Flag	false
New York Stock Exchange
Entity Information [Line Items]
Security Exchange Name	NYSE
Chicago Stock Exchange
Entity Information [Line Items]
Security Exchange Name	CHX