Note 1 - Nature of Operations and Summary of Significant Accounting Policies (Details Textual) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Reportable Segments	1
Financing Receivable, Allowance for Credit Loss, Ending Balance	$ 9,836,000	$ 9,309,000	$ 8,856,000
Financing Receivable Individually Evaluated for Impairment Estimated Loss Trigger	100,000
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	0
Assets Under Management Fair Value	$ 1,007,113,000	862,517,000
Trust Revenue Earned and Collected Monthly, Percent	82.00%
Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss, Ending Balance	$ 4,788,000	4,591,000	4,235,000
Commercial Portfolio Segment [Member] \| Maximum [Member]
Financing Receivable Individually Evaluated for Impairment Threshold With Estimated Loss	200,000
Residential Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss, Ending Balance	4,182,000	3,986,000	3,963,000
Residential Portfolio Segment [Member] \| Maximum [Member]
Financing Receivable Individually Evaluated for Impairment Threshold With Estimated Loss	400,000
Consumer Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss, Ending Balance	281,000	233,000	$ 159,000
Consumer Portfolio Segment [Member] \| Maximum [Member]
Financing Receivable Individually Evaluated for Impairment Threshold With Estimated Loss	$ 400,000
Unfunded Loan Commitment [Member]
Financing Receivable, Allowance for Credit Loss, Ending Balance		$ 0

The fair value of assets under management.

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- Definition

The amount of estimated loss on a financing receivable with relationships less than $200,000 in the aggregate which will trigger that financing receivable to be individually evaluated for impairment.

+ References

No definition available.

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- Definition

Threshold for determining if a financing receivable with an estimated loss of $100,000 or more, will be individually evaluated for impairment.

+ References

No definition available.

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- Definition

The percent of trust revenue earned and collected monthly.

+ References

No definition available.

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- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

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- Definition

Amount of interest expense accrued for an underpayment of income taxes.

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