| Document and Entity Information | Aug. 17, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001752036 |
| Document Type | 8-K |
| Document Period End Date | Aug. 17, 2021 |
| Entity Registrant Name | CALIFORNIA BANCORP |
| Entity Incorporation State Country Code | CA |
| Entity File Number | 001-39242 |
| Entity Tax Identification Number | 82-1751097 |
| Entity Address, Address Line One | 1300 Clay Street |
| Entity Address, Address Line Two | Suite 500 |
| Entity Address, City or Town | Oakland |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94612 |
| City Area Code | (510) |
| Local Phone Number | 457-3737 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock |
| Trading Symbol | CALB |
| Entity Address, Country | CA |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
