| Document and Entity Information | Oct. 20, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 20, 2020 |
| Entity Registrant Name | CAMBRIDGE BANCORP |
| Entity Central Index Key | 0000711772 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-38184 |
| Entity Incorporation, State or Country Code | MA |
| 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 |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | CATC |
| Security Exchange Name | NASDAQ |
