Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)	Jun. 30, 2021	Sep. 30, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Netting and Cash Collateral	$ 0	$ 0
Financial instruments net assets	238,919,000	80,678,000
Debt and equity securities	105,717,000	103,952,000
Total assets	344,636,000	184,630,000
Netting and Cash Collateral	0	0
Financial instruments net liability	5,402,000	2,015,000
Registered investment companies
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity Securities	35,166,000	37,831,000
Bond mutual funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity Securities	34,270,000	29,166,000
Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Debt securities	32,264,000	32,900,000
Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity Securities	4,017,000	4,055,000
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	69,436,000	66,997,000
Total assets	69,436,000	66,997,000
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	35,166,000	37,831,000
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	34,270,000	29,166,000
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	238,919,000	80,678,000
Debt and equity securities	36,281,000	36,955,000
Total assets	275,200,000	117,633,000
Financial instruments gross liability	5,402,000	2,015,000
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	32,264,000	32,900,000
Significant Other Observable Inputs (Level 2) | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity Securities	4,017,000	4,055,000
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