Note 6 - Stock-based Compensation - Stock Options (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Balance, Shares (in shares)	45,130	64,630	82,670	82,670
Balance, Weighted-Average Exercise Price (in dollars per share)	$ 23.37	$ 23.37	$ 23.37	$ 23.37
Balance, Weighted-Average Remaining Contractual Life (Year)	146 days	255 days	328 days			1 year 36 days
Balance, Aggregate Intrinsic Value	$ 659	$ 658,000	$ 925,000	$ 659		$ 1,211,000
Exercised, Shares (in shares)	(6,000)	(19,500)	(18,040)	(43,540)	(2,110)
Exercised, Weighted-Average Exercise Price (in dollars per share)	$ 23.37	$ 23.37	$ 23.37
Balance, Shares (in shares)	39,130	45,130	64,630	39,130		82,670
Balance, Weighted-Average Exercise Price (in dollars per share)	$ 23.37	$ 23.37	$ 23.37	$ 23.37		$ 23.37
Exercisable, Shares (in shares)	39,130			39,130
Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 23.37			$ 23.37
Exercisable, Weighted-Average Remaining Contractual Life (Year)	146 days
Exercisable, Aggregate Intrinsic Value	$ 659,000			$ 659,000

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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-Name Accounting Standards Codification
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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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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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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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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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