| Document and Entity Information | May 28, 2024 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0000711772 |
| Document Type | 8-K |
| Document Period End Date | May 28, 2024 |
| Entity Registrant Name | CAMBRIDGE BANCORP |
| Entity Incorporation State Country Code | MA |
| Entity File Number | 001-38184 |
| Entity Tax Identification Number | 04-2777442 |
| Entity Address, Address Line One | 1336 Massachusetts Avenue |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02138 |
| City Area Code | (617) |
| Local Phone Number | 876-5500 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock |
| Trading Symbol | CATC |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
