Note 7 - Share-based Compensation (Details Textual) $ / shares in Units, $ in Millions	9 Months Ended
Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 $ / shares shares
Number Of Active Stock Incentive Plans	1
Restricted Stock Units and Performance Share Award Target Level, Percentage	100.00%	100.00%
Performance Awards Issued Multiplier	0.00%
Director [Member]
Share-based Goods and Nonemployee Services Transaction, Quantity of Securities Issued	11,924	11,172
Share-based Compensation Arrangement by Stock-based Payment Award, Grant Date Fair Value \| $ / shares	$ 25.16	$ 21.48
Minimum [Member]
Performance Awards Issued Multiplier	0.00%
Maximum [Member]
Performance Awards Issued Multiplier	150.00%
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Number At Target Level Of Performance	64,000
Share-based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount \| $	$ 1.5
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	1 year 109 days

The number of active stock incentive plans.

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The percentage of the performance awards issued multiplier.

+ References

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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

No definition available.

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- Definition

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

+ References

No definition available.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ 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)
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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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- Definition

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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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