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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 |  |

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