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Note 3 - Stock-based Compensation (Details Textual) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Period (Year) | 3 years |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized (in shares) | 1,000,000 |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Shares Deferred (in shares) | 244,618 |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Fair Value Assumptions, Expected Dividend Rate | 0.00% | 0.00% | 0.00%
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount | $ 12.1 |  |
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) | 1 year 8 months 12 days |  |
Performance-contingent Restricted Share Units [Member] |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 116,390 |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 79.88 |  |
Performance-contingent Restricted Share Units [Member] | Executive Officers and Key Employees [Member] |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 116,390 |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 79.88 | $ 67.43 | $ 68.69
Share Based Compensation Arrangement By Share Based Payment Award, Maximum Target Award Percentage | 200.00% |  |
Share Based Compensation Arrangement By Share Based Payment Award, Number of Shares to Be Issued at Maximum Performance Target (in shares) | 696,706 |  |
Time-vested Restricted Stock [Member] |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 68,244 |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 72.6 |  |
Time-vested Restricted Stock [Member] | Executive Officers and Key Employees [Member] |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 55,524 |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 71.91 | 63.57 | 64.29
Time-vested Restricted Stock [Member] | Non Employee Directors [Member] |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 12,720 |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 75.59 | $ 66.31 | $ 62.35
Performance-contingent Restricted Stock Unit and Time-vested Restricted Stocks [Member] |  |  |
Share-Based Payment Arrangement, Shares Withheld for Tax Withholding Obligation (in shares) | 18,428 | 28,055 | 16,787
Share-Based Compensation Arrangement by Share-Based Payment Award, Shares Withheld for Tax Withholding Obligations, Weighted Average Price Per Share (in dollars per share) | $ 72.03 | $ 63.97 | $ 65.99
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value | $ 8.6 | $ 10.1 | $ 6.5
Share Based Compensation Tax Benefit Realized From Exercise Of Equity Instruments Other Than Stock Options | $ 3.2 | $ 3.8 | $ 2.5

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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://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(01) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + 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://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(01) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + Details
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