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Fair Value Measurements - Fair Value Measurements for Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands | Sep. 30, 2023 | Sep. 30, 2022
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | $ 292,735 |
Held-to-maturity debt securities |  | $ 365,033
Total financial assets | 293,082 | 404,295
Money market instruments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Money market instruments | 347 | 39,262
U.S. government bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 31,553 |
Held-to-maturity debt securities |  | 1,973
Municipal securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 7,093 |
Held-to-maturity debt securities |  | 0
Commercial notes |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 22,205 |
Held-to-maturity debt securities |  | 41,727
Corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 231,884 |
Held-to-maturity debt securities |  | 271,333
Certificate of deposits |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Held-to-maturity debt securities |  | 50,000
Level 1 |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 31,553 |
Held-to-maturity debt securities |  | 51,973
Total financial assets | 31,900 | 91,235
Level 1 | Money market instruments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Money market instruments | 347 | 39,262
Level 1 | U.S. government bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 31,553 |
Held-to-maturity debt securities |  | 1,973
Level 1 | Municipal securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 0 |
Held-to-maturity debt securities |  | 0
Level 1 | Commercial notes |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 0 |
Held-to-maturity debt securities |  | 0
Level 1 | Corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 0 |
Held-to-maturity debt securities |  | 0
Level 1 | Certificate of deposits |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Held-to-maturity debt securities |  | 50,000
Level 2 |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 261,182 |
Held-to-maturity debt securities |  | 313,060
Total financial assets | 261,182 | 313,060
Level 2 | Money market instruments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Money market instruments | 0 | 0
Level 2 | U.S. government bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 0 |
Held-to-maturity debt securities |  | 0
Level 2 | Municipal securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 7,093 |
Held-to-maturity debt securities |  | 0
Level 2 | Commercial notes |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 22,205 |
Held-to-maturity debt securities |  | 41,727
Level 2 | Corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 231,884 |
Held-to-maturity debt securities |  | 271,333
Level 2 | Certificate of deposits |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Held-to-maturity debt securities |  | 0
Level 3 |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 0 |
Held-to-maturity debt securities |  | 0
Total financial assets | 0 | 0
Level 3 | Money market instruments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Money market instruments | 0 | 0
Level 3 | U.S. government bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 0 |
Held-to-maturity debt securities |  | 0
Level 3 | Municipal securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 0 |
Held-to-maturity debt securities |  | 0
Level 3 | Commercial notes |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | 0 |
Held-to-maturity debt securities |  | 0
Level 3 | Corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities | $ 0 |
Held-to-maturity debt securities |  | 0
Level 3 | Certificate of deposits |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Held-to-maturity debt securities |  | $ 0

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (aa) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481830/320-10-45-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479130/326-30-45-1 + Details
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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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