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Acquisition - Narrative (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Nov. 21, 2017 USD ($) branch
Business Acquisition [Line Items]
Goodwill	$ 30,140,000		$ 30,140,000
Amortization of core deposit intangible	690,000	$ 354,000
Bank of Napa, N.A. (Napa)
Business Acquisition [Line Items]
Number of branch offices | branch				2
Loans acquired				$ 134,700,000
Deposits assumed				249,900,000
Investment securities acquired				$ 75,500,000
Shares received by Napa shareholders in acquisition per acquiree share				0.307
Goodwill				$ 23,700,000
Goodwill impairment	0
Core deposits | Bank of Napa, N.A. (Napa)
Business Acquisition [Line Items]
Core deposit intangible asset				$ 4,400,000
Amortization of core deposit intangible	$ 381,000		$ 56,000
Useful life of core deposit intangible asset	10 years

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Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Entity Shares Issued Per Acquiree Share, Ratio

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Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer, acquired at the acquisition date.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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