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Summary of Significant Accounting Policies - Narrative (Details)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
payment
branch
Dec. 31, 2017
USD ($)
Jan. 01, 2019
USD ($)
Entity Location [Line Items]        
Number of consecutive payments considered sustained repayment performance | payment   6    
Threshold period a past due loan is charged off   90 days    
Threshold period a past due overdraft account is charged off   60 days    
Advertising costs | $   $ 666 $ 567  
Amount reclassified from AOCI to retained earnings representing stranded tax income effects | $ $ 638      
Marin County        
Entity Location [Line Items]        
Number of branches   10    
Napa County        
Entity Location [Line Items]        
Number of branches   3    
San Francisco        
Entity Location [Line Items]        
Number of branches   1    
Sonoma County        
Entity Location [Line Items]        
Number of branches   6    
Alameda County        
Entity Location [Line Items]        
Number of branches   3    
Accounting Standards Update 2016-02        
Entity Location [Line Items]        
Effect of new accounting guidance, as a percent of consolidated assets   0.50%    
Effect of new accounting guidance, risk-based capital ratio, reduction   10.00%    
Accounting Standards Update 2016-02 | Subsequent event        
Entity Location [Line Items]        
Operating lease, right-of-use asset | $       $ 13,400
Operating lease, liability | $       $ 15,400
Minimum        
Entity Location [Line Items]        
Ownership percentage in limited partnerships related to Low Income Housing Tax Credits   1.00%    
Maximum        
Entity Location [Line Items]        
Ownership percentage in limited partnerships related to Low Income Housing Tax Credits   3.50%    
Core deposit intangible        
Entity Location [Line Items]        
Useful life of core deposit intangible asset   10 years