Cover Page	3 Months Ended
	Mar. 31, 2020 shares
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Period End Date	Mar. 31, 2020
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Document Quarterly Report	true
Entity Registrant Name	California BanCorp
Entity Central Index Key	0001752036
Entity Filer Category	Non-accelerated Filer
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Common Stock, Shares Outstanding	8,121,848
Title of 12(b) Security	Common Stock
Trading Symbol	CALB
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	CA
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Address, Country	CA