Note 6 - Stock-based Compensation Plan - Summary of Nonvested Restricted Shares (Details) - Restricted Stock [Member] - $ / shares	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018
Nonvested, number of shares (in shares)	24,900
Nonvested, weight average grant date fair value (in dollars per share)	$ 34.10
Granted, number of shares (in shares)	9,750
Granted, weight average grant date fair value (in dollars per share)	$ 52.09
Vested, number of shares (in shares)	6,900	4,200
Vested, weight average grant date fair value (in dollars per share)	$ 31.43
Forfeited, number of shares (in shares)
Forfeited, weight average grant date fair value (in dollars per share)
Nonvested, number of shares (in shares)	27,750
Nonvested, weight average grant date fair value (in dollars per share)	$ 41.08

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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-Name Accounting Standards Codification
-Topic 718
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-Subparagraph (c)(1)(iv)(3)
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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