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Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Sep. 30, 2022
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Financial instruments net assets | $ 387,856 | $ 381,991
Debt and equity securities | 98,256 | 96,012
Total assets | 486,112 | 478,003
Financial instruments net liability | 12,040 | 4,129
Registered investment companies |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 26,984 | 26,367
Bond mutual funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 32,753 | 32,367
Bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Debt securities | 35,696 | 33,433
Money market funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 2,823 | 3,845
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 | 59,737 | 58,734
Total assets | 59,737 | 58,734
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 | 26,984 | 26,367
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 | 32,753 | 32,367
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 | 387,856 | 381,991
Debt and equity securities | 38,519 | 37,278
Total assets | 426,375 | 419,269
Financial instruments gross liability | 12,040 | 4,129
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 | 35,696 | 33,433
Significant Other Observable Inputs (Level 2) | Money market funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 2,823 | 3,845
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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