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Note 2 - Shareholders' Equity (Details Textual) - USD ($) |  | 1 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Apr. 25, 2017 | Feb. 29, 2020 | Jun. 30, 2021 | Dec. 31, 2020 | Jun. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2021 | Sep. 30, 2020 | Nov. 13, 2014
Stock Repurchase Program, Remaining Authorized Repurchase Amount |  |  |  |  |  |  | $ 4,980,671 |  | $ 8,000,000
Stock Repurchase Program, Increase in Authorized Amount | $ 4,000,000 |  |  |  |  |  |  |  |
Stock Repurchase Program, Authorized Amount | $ 12,000,000 |  |  |  |  |  |  |  |
Common Stock, Shares Authorized (in shares) |  |  |  |  |  |  | 50,000,000 | 50,000,000 |
Common Stock, Par or Stated Value Per Share (in dollars per share) |  |  |  |  |  |  | $ 0.01 | $ 0.01 |
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  |  | 500,000 | 500,000 |
Preferred Stock, Shares Issued, Total (in shares) |  |  |  |  |  |  | 0 | 0 |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares) |  |  |  |  |  |  | 667,465 |  |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total |  |  |  |  |  |  | $ 2,529,274 |  |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) |  |  |  |  |  |  | 3 years 1 month 6 days |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  | $ 1,280,135 | $ 773,555 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares) |  |  |  |  |  |  | 105,089 | 121,350 |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year) |  |  |  |  |  |  | 5 years | 5 years 8 months 15 days |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  | 3 years | 4 years 8 months 15 days |
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price (in dollars per share) |  |  |  |  |  |  | $ 23.74 | $ 12.43 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number (in shares) |  |  |  |  |  |  | 79,833 | 44,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (in shares) |  |  |  |  |  |  | 101,966 | 26,750 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value |  |  |  |  |  |  | $ 1,314,900 | $ 332,468 |
Stock Repurchased During Period, Shares (in shares) |  |  |  |  |  |  | 11,754 | 10,038 |
Treasury Stock Acquired, Average Cost Per Share (in dollars per share) |  |  |  |  |  |  | $ 39.32 | $ 17.57 |
Payment, Tax Withholding, Share-based Payment Arrangement |  |  |  |  |  |  | $ 462,115 | $ 176,416 |
Preferred Stock, Shares Outstanding, Ending Balance (in shares) |  |  |  |  |  |  | 0 | 0 |
Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares) |  |  |  |  |  |  | 194,873 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Market Price Percentage, Offering Date |  |  |  |  |  |  | 85.00% |  |
Stock Issued During Period, Shares, Employee Stock Purchase Plans (in shares) |  |  | 9,739 | 15,011 | 15,116 | 15,107 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price (in dollars per share) |  |  | $ 21.01 | $ 11.93 | $ 11.85 | $ 11.23 |  |  |
Payment, Tax Withholding, Share-based Payment Arrangement |  |  |  |  |  |  | $ 143,768 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized (in shares) |  | 200,000 |  |  |  |  |  |  |
Minimum [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  |  | 3 years |
Maximum [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  |  | 5 years |
Share-based Payment Arrangement, Option [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  | 3 years |  |
Share-based Payment Arrangement, Option [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year) |  |  |  |  |  |  | 6 years |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  | 5 years |  |
Cashless Method Stock Option Exercised [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (in shares) |  |  |  |  |  |  | 101,966 | 14,688 |
Restricted Stock [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value |  |  |  |  |  |  | $ 1,364,007 | $ 496,829 |
Restricted Stock [Member] | Minimum [Member] | Employees [Member] | Stock Compensation Plan 2007 [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  | 1 year |  |
Restricted Stock [Member] | Maximum [Member] | Employees [Member] | Stock Compensation Plan 2007 [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  |  |  | 10 years |  |
Selling, General and Administrative Expenses [Member] |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  | $ 1,222,756 | 752,011 |
Cost of Sales [Member] |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  | $ 57,379 | $ 21,544 |
Undesignated Shares [Member] |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  |  | 5,000,000 |  |
Series B Preferred Stock [Member] |  |  |  |  |  |  |  |  |
Preferred Stock, Shares Authorized (in shares) |  |  |  |  |  |  | 500,000 |  |
Preferred Stock, Shares Issued, Total (in shares) |  |  |  |  |  |  | 0 |  |
Preferred Stock, Shares Outstanding, Ending Balance (in shares) |  |  |  |  |  |  | 0 |  |

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- Definition The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123467817&loc=SL123496158-112644 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + 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 (d)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Number of additional shares authorized for issuance under share-based payment arrangement. + References No definition available. + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + 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=120381028&loc=d3e5070-113901 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into 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 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Gross number of share options (or share units) granted during the period. + References No definition available. + Details
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- Definition Number of fully vested and expected to vest exercisable options that may be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur. + 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 (e)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Period from grant date that an equity-based award expires, 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 -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Total cost of shares repurchased divided by the total number of shares repurchased. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655 + Details
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