Note 3 - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Outstanding-Number of options (in shares)	95,075	85,525
Granted-Number of options (in shares)	24,700	25,000
Exercised-Number of options (in shares)	(8,775)	(10,000)
Forfeited-Number of options (in shares)	(1,500)	(1,600)
Outstanding ending-Number of options (in shares)	109,500	98,925
Exercisable-Number of options (in shares)	36,350	26,525
Outstanding-Weighted average exercise price (in dollars per share)	$ 35.11	$ 30.94
Granted-Weighted average exercise price (in dollars per share)	38.15	43.60
Exercised-Weighted average exercise price (in dollars per share)	25.64	24.86
Forfeited-Weighted average exercise price (in dollars per share)	37.20	32.52
Outstanding ending-Weighted average exercise price (in dollars per share)	36.53	34.73
Exercisable-Weighted average exercise price (in dollars per share)	$ 29.95	$ 27.46
Outstanding ending-Weighted average remaining contractual term	2 years 4 months 2 days	2 years 8 months 26 days
Exercisable-Weighted average remaining contractual term	8 months 19 days	10 months 24 days
Outstanding ending-Aggregate intrinsic value	$ 357	$ 1,105
Exercisable-Aggregate intrinsic value	$ 318	$ 489

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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