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INVESTMENTS - Narrative (Details) $ in Thousands | 1 Months Ended | 2 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2023 USD ($) state investment portfolioCompany industry | Jul. 31, 2023 USD ($) | Jun. 30, 2023 USD ($) | May 31, 2023 USD ($) | Apr. 30, 2023 USD ($) | Mar. 31, 2023 USD ($) | Jan. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Oct. 31, 2022 USD ($) | Nov. 30, 2022 USD ($) | Mar. 31, 2023 USD ($) | Sep. 30, 2023 USD ($) state investment portfolioCompany industry | Sep. 30, 2022 USD ($) investment | Sep. 30, 2021 USD ($)
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment | 47 |  |  |  |  |  |  |  |  |  |  |  | 47 |  | 46 |  |
Investments, at fair value | $ 704,815 | [1] |  |  |  |  |  |  |  |  |  |  | $ 704,815 | [1] | $ 649,615 | [2] |
Purchase of investments |  |  |  |  |  |  |  |  |  |  |  |  | 175,477 |  | 274,898 |  | $ 181,841
Line of credit commitment |  |  |  |  |  |  |  |  |  |  |  |  | 45,231 |  | 74,846 |  |
Cost | $ 722,269 | [1] |  |  |  |  |  |  |  |  |  |  | $ 722,269 | [1] | 656,053 | [2] |
Number of investment portfolio company | portfolioCompany | 51 |  |  |  |  |  |  |  |  |  |  |  | 51 |  |  |  |
Number of states which have invested in the company location | state | 23 |  |  |  |  |  |  |  |  |  |  |  | 23 |  |  |  |
Number of industries that have made investments | industry | 12 |  |  |  |  |  |  |  |  |  |  |  | 12 |  |  |  |
Weighted average investment owned, at cost, amount | $ 14,200 |  |  |  |  |  |  |  |  |  |  |  | $ 14,200 |  | 12,600 |  |
Uncollectible receivables allowance, minimum required day for uncollectible adjustment |  |  |  |  |  |  |  |  |  |  |  |  | 90 days |  |  |  |
Gross receivables from portfolio companies | 800 |  |  |  |  |  |  |  |  |  |  |  | $ 800 |  | 500 |  |
Allowance for uncollectible receivables | 9 |  |  |  |  |  |  |  |  |  |  |  | 9 |  | 0 |  |
Leeds Novamark Capital I, L.P |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, at fair value | 200 |  |  |  |  |  |  |  |  |  |  |  | 200 |  |  |  |
Proceeds from equity |  |  |  |  |  |  |  |  |  |  | $ 6,000 |  |  |  |  |  |
Realized investment gains |  |  |  |  |  |  |  |  |  |  | $ 4,400 |  |  |  |  |  |
R2i Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  | $ 19,200 |  |  |  |  |  |  |  |
NeoGraf Solutions, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  | $ 29,000 |  |  |  |  |  |  |  |  |
Line of credit commitment |  |  |  |  |  |  |  | $ 4,500 |  |  |  |  |  |  |  |  |
Salt & Straw, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  |  |  |  | $ 6,300 |  |  |  |  |
Leadpoint Business Services, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  | $ 13,500 |  |  |  |  |  |  |  |  |  |
Line of credit commitment |  |  |  |  |  |  | $ 5,500 |  |  |  |  |  |  |  |  |  |
Funded line of credit |  |  |  |  | $ 5,500 |  |  |  |  |  |  |  |  |  |  |  |
Technical Resource Management, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  | $ 25,000 |  |  |  |  |  |  |  |  |  |  |
Line of credit commitment |  |  |  |  |  | 3,000 |  |  |  |  |  |  |  |  |  |  |
Investment company, delayed draw term loan commitment |  |  |  |  |  | 2,500 |  |  |  |  |  |  |  |  |  |  |
HH-Inspire Acquisition, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  | $ 3,400 |  | 16,800 |  |  |  |  |  |  |  |  |  |  |
Cost |  |  |  |  |  | 35,500 |  |  |  |  |  |  |  |  |  |  |
Investment company, prepayment fee |  |  |  |  |  | 200 |  |  |  |  |  |  |  |  |  |  |
OCI, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  | $ 22,000 |  |  |  |  |  |  |  |  |  |  |  |
PIC 360, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, at fair value |  |  |  | 300 |  |  |  |  |  |  |  |  |  |  |  |  |
Realized investment gains |  |  |  | 3,700 |  |  |  |  |  |  |  |  |  |  |  |  |
Trowbridge Chicago, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  | 6,500 |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit commitment |  |  |  | 2,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Investment company, delayed draw term loan commitment |  |  |  | 5,300 |  |  |  |  |  |  |  |  |  |  |  |  |
Gray Matter Systems, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | $ 5,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sea Link International IRB, Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments | 4,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Antenna Research Associates, Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments | 8,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Encore Dredging Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, at fair value | 4,300 |  |  |  |  |  |  |  |  |  |  |  | 4,300 |  |  |  |
Cost | 3,800 |  |  |  |  |  |  |  |  |  |  |  | 3,800 |  |  |  |
Prepayment fee and early termination fees | 400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Targus Cayman HoldCo Ltd. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Realized investment gains |  |  |  |  |  |  |  |  |  | $ 5,900 |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  |  | 8,000 |  |  |  |  |  |  |
B. Riley Financial, Inc. – Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  | $ 2,400 |  |  |  |  |  |  |  |  |  |  |  |  |
Cost |  |  |  |  |  |  |  |  |  | $ 2,400 |  |  |  |  |  |  |
Investment, interest rate, paid in cash |  |  |  | 6.75% |  |  |  |  |  | 6.75% |  |  |  |  |  |  |
CPM Holdings, Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments | 800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Portfolio Benchmark | Customer Concentration Risk | Five Largest Portfolio Investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, at fair value | 176,900 |  |  |  |  |  |  |  |  |  |  |  | $ 176,900 |  | $ 174,500 |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  |  |  |  | 25.10% |  | 26.90% |  |
Principal or Proprietary Transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, at fair value | $ 695,100 |  |  |  |  |  |  |  |  |  |  |  | $ 695,100 |  | $ 630,800 |  |
Principal or Proprietary Transactions | Investment Portfolio Benchmark | Investment Type Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  |  |  |  | 98.60% |  | 97.10% |  |
Uncalled capital commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit commitment |  |  |  |  |  |  |  |  |  |  |  |  | $ 843 |  | $ 843 |  |
Uncalled capital commitment | HH-Inspire Acquisition, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit commitment |  |  |  |  |  | $ 1,800 |  |  |  |  |  |  |  |  |  |  |
Syndicated Investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment | 4 |  |  |  |  |  |  |  |  |  |  |  | 4 |  | 6 |  |
Investments, at fair value | $ 9,700 |  |  |  |  |  |  |  |  |  |  |  | $ 9,700 |  | $ 18,800 |  |
Syndicated Investments | Investment Portfolio Benchmark | Investment Type Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  |  |  |  | 1.40% |  | 2.90% |  |
[1] | Cumulative gross unrealized depreciation for federal income tax purposes is $56.9 million; cumulative gross unrealized appreciation for federal income tax purposes is $33.7 million. Cumulative net unrealized depreciation is $23.2 million, based on a tax cost of $728.0 million.
[2] | Cumulative gross unrealized depreciation for federal income tax purposes is $52.3 million; cumulative gross unrealized appreciation for federal income tax purposes is $35.2 million. Cumulative net unrealized depreciation is $17.1 million, based on a tax cost of $666.7 million.

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