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Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands		3 Months Ended	12 Months Ended
	Jan. 01, 2020	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) payment branch	Dec. 31, 2018 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			$ 775	$ 666
Amount reclassified from AOCI to retained earnings representing stranded tax income effects		$ 638
Operating lease right-of-use assets			11,002
Operating lease liabilities			$ 12,615
Marin County
Entity Location [Line Items]
Number of branches | branch			10
Napa County
Entity Location [Line Items]
Number of branches | branch			3
San Francisco
Entity Location [Line Items]
Number of branches | branch			1
Sonoma County
Entity Location [Line Items]
Number of branches | branch			5
Alameda County
Entity Location [Line Items]
Number of branches | branch			3
Accounting Standards Update 2016-02
Entity Location [Line Items]
Operating lease right-of-use assets					$ 13,400
Operating lease liabilities					$ 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
Forecast | Minimum | Accounting Standards Update 2016-13
Entity Location [Line Items]
Allowance for loan and lease losses, percent increase (decrease)	5.00%
Forecast | Maximum | Accounting Standards Update 2016-13
Entity Location [Line Items]
Allowance for loan and lease losses, percent increase (decrease)	15.00%

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