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INVESTMENTS - Narrative (Details) | 1 Months Ended | 2 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
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Jun. 30, 2023 USD ($) state industry investment portfolioCompany | May 31, 2023 USD ($) | 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 ($) | Jun. 30, 2023 USD ($) state industry investment portfolioCompany | Jun. 30, 2022 USD ($) | Sep. 30, 2022 USD ($) investment
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment | 47 |  |  |  |  |  |  |  |  | 47 |  | 46
Investments owned, fair value | $ 715,324,000 |  |  |  |  |  |  |  |  | $ 715,324,000 |  | $ 649,615,000
Purchase of investments |  |  |  |  |  |  |  |  |  | 148,084,000 | $ 188,851,000 |
Line of credit commitment |  |  |  |  |  |  |  |  |  | 63,908,000 |  | 74,846,000
Cost | $ 735,182,000 |  |  |  |  |  |  |  |  | $ 735,182,000 |  | 656,053,000
Number of investment portfolio company | portfolioCompany | 52 |  |  |  |  |  |  |  |  | 52 |  |
Number of states which have invested in the company location | state | 24 |  |  |  |  |  |  |  |  | 24 |  |
Number of industries that have made investments | industry | 12 |  |  |  |  |  |  |  |  | 12 |  |
Weighted average investment owned, at cost, amount | $ 14,100,000 |  |  |  |  |  |  |  |  | $ 14,100,000 |  | 12,600,000
Uncollectible receivables allowance, minimum required day for uncollectible adjustment |  |  |  |  |  |  |  |  |  | 90 days |  |
Gross receivables from portfolio companies | 900,000 |  |  |  |  |  |  |  |  | $ 900,000 |  | 500,000
Allowance for uncollectible receivables | 0 |  |  |  |  |  |  |  |  | 0 |  | 0
Leeds Novamark Capital I, L.P |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Investments owned, fair value | 200,000 |  |  |  |  |  |  |  |  | 200,000 |  |
Proceeds from equity |  |  |  |  |  |  |  | $ 6,000,000 |  |  |  |
Realized investment gains |  |  |  |  |  |  |  | $ 4,400,000 |  |  |  |
R2i Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  | $ 19,200,000 |  |  |  |  |  |
NeoGraf Solutions, LLC |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  | $ 29,000,000 |  |  |  |  |  |  |
Line of credit commitment |  |  |  |  | $ 4,500,000 |  |  |  |  |  |  |
Salt & Straw, LLC |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  | $ 6,300,000 |  |  |
Leadpoint Business Services, LLC |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  | $ 13,500,000 |  |  |  |  |  |  |  |
Line of credit commitment |  |  |  | $ 5,500,000 |  |  |  |  |  |  |  |
Funded line of credit |  | $ 5,500,000 |  |  |  |  |  |  |  |  |  |
Technical Resource Management, LLC |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | $ 25,000,000 |  |  |  |  |  |  |  |  |
Line of credit commitment |  |  | 3,000,000 |  |  |  |  |  |  |  |  |
Investment company, delayed draw term loan commitment |  |  | 2,500,000 |  |  |  |  |  |  |  |  |
HH-Inspire Acquisition, Inc. |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments | 3,400,000 |  | 16,800,000 |  |  |  |  |  |  |  |  |
Cost |  |  | 35,500,000 |  |  |  |  |  |  |  |  |
Investment company, prepayment fee |  |  | 200,000 |  |  |  |  |  |  |  |  |
OCI, LLC |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  | $ 22,000,000 |  |  |  |  |  |  |  |  |  |
PIC 360, LLC |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Investments owned, fair value | 300,000 |  |  |  |  |  |  |  |  | $ 300,000 |  |
Realized investment gains | 3,700,000 |  |  |  |  |  |  |  |  |  |  |
Trowbridge Chicago, LLC |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments | 6,500,000 |  |  |  |  |  |  |  |  |  |  |
Line of credit commitment | 2,000,000 |  |  |  |  |  |  |  |  |  |  |
Investment company, delayed draw term loan commitment | 5,300,000 |  |  |  |  |  |  |  |  |  |  |
Targus Cayman HoldCo Ltd. |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Realized investment gains |  |  |  |  |  |  | $ 5,900,000 |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  | 8,000,000 |  |  |  |  |
B. Riley Financial, Inc. – Term Debt |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments | $ 2,400,000 |  |  |  |  |  |  |  |  |  |  |
Cost |  |  |  |  |  |  | $ 2,400,000 |  |  |  |  |
Investment, interest rate, paid in cash | 6.75% |  |  |  |  |  | 6.75% |  |  | 6.75% |  |
Principal or Proprietary Transactions |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Investments owned, fair value | $ 704,900,000 |  |  |  |  |  |  |  |  | $ 704,900,000 |  | 630,800,000
Uncalled capital commitment |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Line of credit commitment |  |  |  |  |  |  |  |  |  | $ 843,000 |  | $ 843,000
Uncalled capital commitment | HH-Inspire Acquisition, Inc. |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Line of credit commitment |  |  | $ 1,800,000 |  |  |  |  |  |  |  |  |
Syndicated Investments |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment | 5 |  |  |  |  |  |  |  |  | 5 |  | 6
Investments owned, fair value | $ 10,400,000 |  |  |  |  |  |  |  |  | $ 10,400,000 |  | $ 18,800,000
Investment Portfolio Benchmark | Investment Type Concentration Risk | Principal or Proprietary Transactions |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  | 98.50% |  | 97.10%
Investment Portfolio Benchmark | Investment Type Concentration Risk | Syndicated Investments |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  | 1.50% |  | 2.90%
Investment Portfolio Benchmark | Customer Concentration Risk | Five Largest Portfolio Investments |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Investments owned, fair value | $ 172,000,000 |  |  |  |  |  |  |  |  | $ 172,000,000 |  | $ 174,500,000
Concentration risk, percentage |  |  |  |  |  |  |  |  |  | 24.00% |  | 26.90%

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