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Acquisition - Narrative (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	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	230,000	$ 118,000
Goodwill impairment	0
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
Core deposits | Bank of Napa, N.A. (Napa)
Business Acquisition [Line Items]
Core deposit intangible asset			4,400,000
Amortization of core deposit intangible	$ 127,000		$ 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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Amount due from customers or clients for goods or services, including trade receivables, that have been delivered or sold in the normal course of business, and amounts due from others, including related parties 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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The amount of financial liabilities assumed (as defined) which have been recognized as of 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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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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