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Note 6 - Share-based Payments - Status of Dividends Equivalent Restricted Stock Unit Awards Granted to Executive and Key Employees (Details) - Executive Officers and Key Employees [Member] - Dividends Equivalent Restricted Stock Unit [Member] - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Nonvested, Beginning of Year (in shares)	45,300	47,750	41,825
Nonvested, Beginning of Year, weighted average grant-date fair value (in dollars per share)	$ 35.70	$ 32.71	$ 30.23
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	16,200	15,200	15,900
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 49.72	$ 41.325	$ 37.65
Vested (in shares)	15,850	17,650	9,975
Vested, weighted average grant-date fair value (in dollars per share)	$ 34.46	$ 32.462	$ 30.16
Forfeited (in shares)	0	0	0
Nonvested, End of Year (in shares)	45,650	45,300	47,750
Nonvested, End of Year, weighted average grant-date fair value (in dollars per share)	$ 41.10	$ 35.70	$ 32.71
Share-based Payment Arrangement, Expense	$ 1,055,000	$ 769,000	$ 576,000
Fair Value of Awards Vested	$ 546,000	$ 573,000	$ 301,000

X

- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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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