Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Sep. 30, 2023
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments net assets	$ 357,101	$ 385,664
Debt and equity securities	107,774	104,602
Total assets	464,875	490,266
Financial instruments net liability	5,840	15,408
Registered investment companies
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	28,076	26,685
Bond mutual funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	38,844	37,573
Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Debt securities	37,582	35,507
Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	3,272	4,837
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	66,920	64,258
Total assets	66,920	64,258
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	28,076	26,685
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	38,844	37,573
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	357,101	385,664
Debt and equity securities	40,854	40,344
Total assets	397,955	426,008
Financial instruments gross liability	5,840	15,408
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	37,582	35,507
Significant Other Observable Inputs (Level 2) | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	3,272	4,837
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