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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 |  |  |

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