Note 3 - Stock-based Compensation (Details Textual) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
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

Percentage of maximum target award under share-based compensation arrangement.

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- Definition

The number of shares to be issued at maximum performance target under share-based compensation arrangement.

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No definition available.

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- Definition

The aggregate tax benefit realized from the exercise of equity instruments during the annual period..

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Number of shares deferred under share-based payment arrangement.

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The weighted-average price of vested compensation awards withheld to satisfy tax withholding obligations..

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- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (i)
-Publisher FASB
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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.

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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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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

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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).

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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

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

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-Topic 718
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- Definition

Number of shares authorized for issuance under share-based payment arrangement.

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- Definition

Number of shares used to settle grantee's tax withholding obligation for award under share-based payment arrangement.

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