Fair Value of Financial Instruments - Summary of Investments, Cash and Cash Equivalents, Credit Facility, SBA Debentures and Notes (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Sep. 30, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	$ 1,196,035	$ 1,226,301
Cash and cash equivalents	28,556	52,666
Total investments and cash and cash equivalents	1,224,591	1,278,967
Total debt	695,588	704,513
SBA Debentures
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	19,701	19,686
2026 Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	146,993	146,767
2026 Notes-2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	161,586	161,373
Truist Credit Facility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Credit Facility, Fair Value	367,308	376,687
Debt Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	940,621	902,165
Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	255,414	324,136
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	509	304
Cash and cash equivalents	28,556	52,666
Total investments and cash and cash equivalents	29,065	52,970
Level 1 | Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	509	304
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	308,579	308,140
Level 2 | 2026 Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	146,993	146,767
Level 2 | 2026 Notes-2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	161,586	161,373
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	1,126,730	1,159,327
Total investments and cash and cash equivalents	1,126,730	1,159,327
Total debt	387,009	396,373
Level 3 | SBA Debentures
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	19,701	19,686
Level 3 | Truist Credit Facility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Credit Facility, Fair Value	367,308	376,687
Level 3 | Debt Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	940,621	902,165
Level 3 | Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	186,109	257,162
Measured at Net Asset Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	68,796	66,670
Total investments and cash and cash equivalents	68,796	66,670
Measured at Net Asset Value | Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	$ 68,796	$ 66,670