Significant Accounting Policies - Additional Information (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2022 USD ($) Portfolio	Sep. 30, 2021 USD ($) Portfolio	Sep. 30, 2020 USD ($)
Significant Accounting Policies [Line Items]
Provision for taxes	$ 800	$ 600	$ 1,200
Provision for taxes on net realized gain on investments	6,200	0	0
Provision for taxes on net unrealized gain on investments	900	0	0
Provision for taxes on net realized and unrealized gains on investments	7,100	0	0
Net realized gain (loss) on investments	43,861	30,021	(11,577)
Prepaid Expenses and Other Assets
Significant Accounting Policies [Line Items]
Offsetting losses expected to refund	4,000
Federal Taxes
Significant Accounting Policies [Line Items]
Net realized gain (loss) on investments	4,000
State and Local Tax | Accrued Other Expenses
Significant Accounting Policies [Line Items]
State and local tax liability	$ 800	$ 800	$ 800
Past Due 30 Days or More
Significant Accounting Policies [Line Items]
Financing receivable, nonaccrual, percent past due on cost basis	1.00%
Financing receivable, nonaccrual, percent past due on fair value basis	0.00%
Number of portfolio on nonaccrual | Portfolio	1	0