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Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2022 | Sep. 30, 2021
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Financial instruments net assets | $ 488,157 | $ 230,686
Debt and equity securities | 98,356 | 108,071
Total assets | 586,513 | 338,757
Financial instruments net liability | 198 | 5,269
Registered investment companies |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 28,370 | 35,175
Bond mutual funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 32,667 | 34,298
Bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Debt securities | 33,729 | 35,655
Money market funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 3,590 | 2,943
Quoted Prices in Active Markets (Level 1) |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Financial instruments gross assets | 0 | 0
Debt and equity securities | 61,037 | 69,473
Total assets | 61,037 | 69,473
Financial instruments gross liability | 0 | 0
Quoted Prices in Active Markets (Level 1) | Registered investment companies |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 28,370 | 35,175
Quoted Prices in Active Markets (Level 1) | Bond mutual funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 32,667 | 34,298
Quoted Prices in Active Markets (Level 1) | Bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 0
Quoted Prices in Active Markets (Level 1) | Money market funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Significant Other Observable Inputs (Level 2) |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Financial instruments gross assets | 488,157 | 230,686
Debt and equity securities | 37,319 | 38,598
Total assets | 525,476 | 269,284
Financial instruments gross liability | 198 | 5,269
Significant Other Observable Inputs (Level 2) | Registered investment companies |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Significant Other Observable Inputs (Level 2) | Bond mutual funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Significant Other Observable Inputs (Level 2) | Bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Debt securities | 33,729 | 35,655
Significant Other Observable Inputs (Level 2) | Money market funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 3,590 | 2,943
Significant Other Unobservable Inputs (Level 3) |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Financial instruments gross assets | 0 | 0
Debt and equity securities | 0 | 0
Total assets | 0 | 0
Financial instruments gross liability | 0 | 0
Significant Other Unobservable Inputs (Level 3) | Registered investment companies |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Significant Other Unobservable Inputs (Level 3) | Bond mutual funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Significant Other Unobservable Inputs (Level 3) | Bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 0
Significant Other Unobservable Inputs (Level 3) | Money market funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | $ 0 | $ 0

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