Fair Value of Financial Instruments - Summary of Investments, Cash and Cash Equivalents, Credit Facility, SBA Debentures and Notes (Details) - USD ($) $ in Thousands		Sep. 30, 2025		Sep. 30, 2024
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		$ 1,287,273		$ 1,328,050
Cash and cash equivalents, Fair Value		51,783		49,861
Total investments and cash and cash equivalents		1,339,056		1,377,911
Credit Facility, Fair Value		425,477		460,361
Total debt		738,883		772,012
2026 Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt		149,473	[1]	148,571	[2]
2026 Notes-2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt		163,933	[1]	163,080	[2]
Debt Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		801,754		916,796
U.S. Government Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		124,788	[3]	99,632	[4]
Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		360,731		311,622
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Fair Value		51,783		49,861
Total investments and cash and cash equivalents		51,783		49,861
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		124,788		99,632
Total investments and cash and cash equivalents		124,788		99,632
Total debt		313,406		311,651
Level 2 | 2026 Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt		149,473	[1]	148,571	[2]
Level 2 | 2026 Notes-2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt		163,933	[1]	163,080	[2]
Level 2 | U.S. Government Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	[4]	124,788	[3]	99,632
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		1,087,964		1,152,369
Total investments and cash and cash equivalents		1,087,964		1,152,369
Credit Facility, Fair Value		425,477		460,361
Total debt		425,477		460,361
Level 3 | 2026 Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	[1]	0
Level 3 | 2026 Notes-2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt	[1]	0
Level 3 | Debt Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		801,754		916,796
Level 3 | U.S. Government Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments	[3]	0
Level 3 | Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		286,210		235,573
Measured at Net Asset Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		74,521	[5]	76,049	[6]
Total investments and cash and cash equivalents		74,521	[5]	76,049	[6]
Measured at Net Asset Value | Equity Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total investments		$ 74,521	[5]	$ 76,049	[6]

[1] We elected not to apply ASC 825-10 to the 2026 Notes, and the 2026 Notes-2 and thus the balance reported in the Consolidated Statement of Assets and Liabilities represents the carrying value.

[2] We elected not to apply ASC 825-10 to the 2026 Notes, and the 2026 Notes-2 and thus the balance reported in the Consolidated Statement of Assets and Liabilities represents the carrying value.

[3] Our U.S. Treasury Bills are classified as Level 2, as they were valued by the pricing service who utilize broker-supplied prices.

[4] Our U.S. Treasury Bills are classified as Level 2, as they were valued by the pricing service who utilize broker-supplied prices.

[5] In accordance with ASC Subtopic 820-10, Fair Value Measurements and Disclosures, or ASC 820-10, our equity investment in PSLF and PTSF II is measured using the net asset value per share (or its equivalent) as a practical expedient for fair value in accordance with the specialized accounting guidance for investment companies, and thus has not been classified in the fair value hierarchy.

[6] In accordance with ASC Subtopic 820-10, Fair Value Measurements and Disclosures, or ASC 820-10, our equity investment in PSLF and PTSF II is measured using the net asset value per share (or its equivalent) as a practical expedient for fair value in accordance with the specialized accounting guidance for investment companies, and thus has not been classified in the fair value hierarchy.