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Note 6 - Share-based Payments (Details Textual) - USD ($) |  |  |  |  |  | 6 Months Ended | 12 Months Ended | 24 Months Ended | 30 Months Ended | 36 Months Ended | 60 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Apr. 08, 2023 | Dec. 17, 2019 | Apr. 08, 2019 | Feb. 13, 2019 | Feb. 06, 2019 | Dec. 31, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jun. 30, 2019 | Jun. 30, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2018
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total |  |  |  |  |  | $ 3,500,000 | $ 3,500,000 |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition |  |  |  |  |  |  | 1 year 10 months 24 days |  |  |  |  |  |  |  |
Restricted Stock [Member] | Director [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  | $ 776,000 | $ 661,000 | $ 658,000 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  | 15,700 |  |  |  | 15,700 | 18,200 | 17,600 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value |  |  | $ 49.73 |  |  |  | $ 49.73 | $ 43.40 | $ 37.75 |  |  |  |  |  |
Restricted Stock [Member] | Director [Member] | Share-based Payment Arrangement, Tranche One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage |  |  | 33.30% |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock [Member] | Director [Member] | Share-based Payment Arrangement, Tranche Two [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage |  |  | 33.30% |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock [Member] | Director [Member] | Share-based Payment Arrangement, Tranche Three [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage |  |  | 33.30% |  |  |  |  |  |  |  |  |  |  |  |
Restricted Stock [Member] | Employees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  | $ 0 | $ 16,000 | $ 70,000 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  |  |  |  |  | 0 | 0 | 0 |  |  |  |  |  |
Restricted Stock Units (RSUs) [Member] | Employees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  | $ 427,000 | $ 351,000 | $ 331,000 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  |  |  |  |  | 13,270 | 14,780 | 10,995 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value |  |  |  |  |  |  | $ 44.45 | $ 38.99 | $ 33.28 |  |  |  |  |  |
Stock Performance Awards [Member] | Executive Officer [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  | $ 3,597,000 | $ 2,645,000 | $ 2,007,000 |  |  |  |  |  |
Stock Performance Awards [Member] | Executive Officer [Member] | Performance Period 2017 To 2019 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adjusted Return, Targeted Percentage of Equity |  |  |  |  |  |  | 9.50% |  |  |  |  |  |  |  |
Payout as a Percent of Target |  |  |  |  |  |  | 120.19% |  |  |  |  |  |  |  |
Stock Performance Awards [Member] | Executive Officer [Member] | Performance Period 2016 To 2018 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adjusted Return, Targeted Percentage of Equity |  |  |  |  |  |  |  |  |  |  |  |  | 10.00% |  |
Payout as a Percent of Target |  |  |  |  |  |  |  |  |  |  |  |  | 145.17% |  |
Stock Performance Awards [Member] | Executive Officer [Member] | Performance Period 2015 To 2017 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adjusted Return, Targeted Percentage of Equity |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.00% |
Payout as a Percent of Target |  |  |  |  |  |  |  |  |  |  |  |  |  | 136.00% |
The 1999 Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date |  |  |  |  |  |  |  |  |  | 15.00% |  |  |  |  |
Share-based Compensation Arrangements by Share-based Payment Award, Percentage of Market Price at Eligible Employees Granted Options to Purchase Shares |  |  |  |  |  | 85.00% | 100.00% |  |  |  | 100.00% | 15.00% |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  | $ 103,000 |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized |  |  |  |  |  | 1,400,000 | 1,400,000 |  |  |  |  |  |  |  |
The 1999 Stock Incentive Plan [Member] | Restricted Stock [Member] | Employees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0
The 2014 Stock Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized |  |  |  |  |  | 1,900,000 | 1,900,000 |  |  |  |  |  |  |  |
The 2014 Stock Incentive Plan [Member] | Restricted Stock [Member] | Employees [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  |  |  |  |  | 13,270 |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value |  |  |  |  |  |  | $ 44.45 |  |  |  |  |  |  |  |
The 2014 Stock Incentive Plan [Member] | Restricted Stock [Member] | Employees [Member] | Share-based Payment Arrangement, Tranche One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
The 2014 Stock Incentive Plan [Member] | Restricted Stock [Member] | Executive Officer [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period |  |  |  | 15,600 |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value |  |  |  | $ 49.6225 |  |  |  |  |  |  |  |  |  |  |
The 2014 Stock Incentive Plan [Member] | Restricted Stock [Member] | Executive Officer [Member] | Share-based Payment Arrangement, Tranche One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |  |
The 2014 Stock Incentive Plan [Member] | Restricted Stock [Member] | Key Employee [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period |  | 600 |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value |  | $ 52.165 |  |  |  |  |  |  |  |  |  |  |  |  |
The 2014 Stock Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period |  |  |  |  |  |  | 4 years |  |  |  |  |  |  |  |
The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period Specified for Average Adjusted Return |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized |  |  |  | 55,600 |  |  |  |  |  |  |  |  |  |  |
Number of Trading Days |  |  |  | 20 days |  |  |  |  |  |  |  |  |  |  |
Number of Shares Authorized for Actual Payment |  |  |  | 83,400 |  |  |  |  |  |  |  |  |  |  |
The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Target Amount as Actual Payment |  |  |  | 0.00% |  |  |  |  |  |  |  |  |  |  |
The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Target Amount as Actual Payment |  |  |  | 150.00% |  |  |  |  |  |  |  |  |  |  |
The 2014 Stock Incentive Plan [Member] | Stock Performance Awards [Member] | Executive Officer [Member] | Share-based Payment Arrangement, Tranche Two [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized |  |  |  | 27,800 |  |  |  |  |  |  |  |  |  |  |

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- Definition 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) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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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. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Discount rate from fair value on purchase date that participants pay for shares. + 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 -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition 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). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition 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). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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