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Summary of Significant Accounting Policies - Nature of Operations and Basis of Presentation (Details)	Dec. 31, 2020 branch loanProductionOffice
Marin County
Product Information [Line Items]
Number of branches and loan production offices	10
Napa County
Product Information [Line Items]
Number of branches and loan production offices	2
San Francisco
Product Information [Line Items]
Number of branches and loan production offices	1
Sonoma County
Product Information [Line Items]
Number of branches and loan production offices	5
Alameda County
Product Information [Line Items]
Number of branches and loan production offices	3
Contra Costa County
Product Information [Line Items]
Number of branches and loan production offices | loanProductionOffice	1
San Mateo County
Product Information [Line Items]
Number of branches and loan production offices | loanProductionOffice	1

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