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Note 7 - Share-based Compensation (Details Textual)	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 $ / shares shares
Number Of Active Stock Incentive Plans	1
Number Of Inactive Stock Option Plans	1
Restricted Stock Units and Performance Share Award Target Level, Percentage	100.00%
Director [Member]
Share-based Goods and Nonemployee Services Transaction, Quantity of Securities Issued	11,172	14,944
Share-based Compensation Arrangement by Stock-based Payment Award, Grant Date Fair Value | $ / shares	$ 21.48	$ 14.72
Minimum [Member]
Performance Awards Issued Multiplier	0.00%
Maximum [Member]
Performance Awards Issued Multiplier	200.00%
Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Number At Target Level Of Performance	40,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options | $	$ 0
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years

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- Definition

The number of active stock incentive plans.

+ References

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- Definition

The number of inactive stock options plans.

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- Definition

The percentage of the performance awards issued multiplier.

+ References

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- Definition

Represents the target level percentage for restricted stock unit and performance share awards.

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- Definition

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

+ References

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- Definition

Represents the stock-based payment award grant date fair value under a share-based compensation arrangement.

+ References

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- Definition

Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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The number of shares or units of entity securities issued in the transaction in which equity securities were issued to pay for goods or nonemployee services.

+ References

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-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 2
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-Name Accounting Standards Codification
-Topic 505
-SubTopic 50
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=68057994&loc=d3e25284-112666

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