| Document and Entity Information | Oct. 29, 2019 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Oct. 29, 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 |
