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Summary of Significant Accounting Policies - Narrative (Details)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
payment
branch
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
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    
Ownership interest in each limited partnerships formed for affordable housing projects (less than)   2.00%    
Advertising costs | $   $ 567 $ 565 $ 334
Excess tax benefits recorded as a reduction to income tax expense | $   214    
Commitments under operating leases | $   $ 18,772    
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    
Scenario, Forecast [Member]        
Entity Location [Line Items]        
Amount reclassified from AOCI to retained earnings representing stranded tax income effects | $ $ 637