Fair Value of Financial Instruments - Summary of Investments, Cash and Cash Equivalents, Credit Facility, SBA Debentures and Notes (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Sep. 30, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	$ 1,132,511	$ 1,226,301
Cash and cash equivalents	63,135	52,666
Total investments and cash and cash equivalents	1,195,646	1,278,967
Total debt	671,286	704,513
SBA Debentures
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt		19,686
2026 Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	147,218	146,767
2026 Notes-2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	161,800	161,373
Truist Credit Facility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Credit Facility, Fair Value	362,268	376,687
Debt Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	907,544	902,165
Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	224,967	324,136
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		304
Cash and cash equivalents	63,135	52,666
Total investments and cash and cash equivalents	63,135	52,970
Level 1 | Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		304
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	309,018	308,140
Level 2 | 2026 Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	147,218	146,767
Level 2 | 2026 Notes-2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	161,800	161,373
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	1,067,147	1,159,327
Total investments and cash and cash equivalents	1,067,147	1,159,327
Total debt	362,268	396,373
Level 3 | SBA Debentures
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt		19,686
Level 3 | Truist Credit Facility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Credit Facility, Fair Value	362,268	376,687
Level 3 | Debt Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	907,544	902,165
Level 3 | Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	159,603	257,162
Measured at Net Asset Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	65,364	66,670
Total investments and cash and cash equivalents	65,364	66,670
Measured at Net Asset Value | Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	$ 65,364	$ 66,670