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INVESTMENTS - Narrative (Details) $ in Thousands | 1 Months Ended | 2 Months Ended | 9 Months Ended | 12 Months Ended
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Jun. 30, 2024 USD ($) investment state industry portfolioCompany | May 31, 2024 USD ($) | Apr. 30, 2024 USD ($) | Mar. 31, 2024 USD ($) | Feb. 29, 2024 USD ($) | Jan. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) | Nov. 30, 2023 USD ($) | Mar. 31, 2024 USD ($) | Jun. 30, 2024 USD ($) investment state industry portfolioCompany | Jun. 30, 2023 USD ($) | Sep. 30, 2023 USD ($) investment
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment | 48 |  |  |  |  |  |  |  |  | 48 |  | 47 |
Investments, at fair value | $ 757,745 |  |  |  |  |  |  |  |  | $ 757,745 |  | $ 704,815 | [1]
Purchase of investments |  |  |  |  |  |  |  |  |  | 149,122 | $ 148,084 |  |
Financial support to investee, not provided | 53,344 |  |  |  |  |  |  |  |  | 53,344 |  | 45,231 |
Investment owned, at cost | $ 753,263 |  |  |  |  |  |  |  |  | $ 753,263 |  | 722,269 | [1]
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 | 13 |  |  |  |  |  |  |  |  | 13 |  |  |
Weighted average investment owned, at cost, amount | $ 14,800 |  |  |  |  |  |  |  |  | $ 14,800 |  | 14,200 |
Gross receivables from portfolio companies | 1,500 |  |  |  |  |  |  |  |  | 1,500 |  | 800 |
Allowance for uncollectible receivables | 58 |  |  |  |  |  |  |  |  | 58 |  | 9 |
Quality Environmental Midco, Inc |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  | $ 5,000 |  |  | $ 11,000 |  |  |  |  |
Financial support to investee, not provided |  |  |  |  |  |  |  | 2,000 |  |  |  |  |
Financial support to investee |  |  |  |  | 3,000 |  |  |  |  |  |  |  |
Cafe Zupas |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided |  |  |  |  |  |  |  | 10,500 |  |  |  |  |
Investment company, delayed draw term loan commitment |  |  |  |  |  |  | $ 1,400 |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  | 7,300 |  |  |  |  |
PIC 360, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Realized investment gains |  |  |  |  |  |  |  | $ 300 |  |  |  |  |
ALS Education, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  | 14,300 |  |  |  |  |  |
Leadpoint Business Services, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  | 12,000 |  |  |  |  |  |
Salt & Straw, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  | 7,000 |  |  |  |  |  |
Financial support to investee |  |  |  |  |  |  | $ 2,900 |  |  |  |  |  |
SpaceCo Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  | $ 13,500 |  |  |  |
Perimeter Solutions Group |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  | $ 15,000 |  |  |  |  |  |  |  |
Trowbridge Chicago, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  | $ 8,400 |  |  |  |  |  |  |  |  |
Realized investment gains |  |  |  | 200 |  |  |  |  |  |  |  |  |
Investment company, prepayment fee |  |  |  | 200 |  |  |  |  |  |  |  |  |
Investment owned, at cost |  |  |  | $ 300 |  |  |  |  | $ 300 |  |  |  |
Total Access Elevator, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | $ 7,300 |  |  |  |  |  |  |  |  |  |
Investment company, line of credit commitment |  |  | 3,000 |  |  |  |  |  |  |  |  |  |
Investment company, delayed draw term loan commitment |  |  | 2,500 |  |  |  |  |  |  |  |  |  |
Home Healthcare, LLC ("Giving Home") |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  | 29,700 |  |  |  |  |  |  |  |  |  |
Investment company, prepayment fee |  |  | 900 |  |  |  |  |  |  |  |  |  |
Proceeds from equity |  |  | $ 2,500 |  |  |  |  |  |  |  |  |  |
Gray Matter Systems, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  | $ 14,000 |  |  |  |  |  |  |  |  |  |  |
Proceeds from disposition of investment, prepayment fees |  | 200 |  |  |  |  |  |  |  |  |  |  |
Pansophic Learning, Ltd. |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  | 33,000 |  |  |  |  |  |  |  |  |  |  |
RPM Freight Systems, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  | 20,000 |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided |  | 5,000 |  |  |  |  |  |  |  |  |  |  |
Funko Acquisition Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, at fair value |  |  |  |  |  |  |  |  |  |  |  | 22 |
Realized investment gains |  | 2 |  |  |  |  |  |  |  |  |  |  |
Investment owned, at cost |  | $ 21 |  |  |  |  |  |  |  |  |  |  |
Workforce QA, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments | 7,400 |  |  |  |  |  |  |  |  |  |  |  |
CPM Holdings, Inc |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  | $ 3,000 |  |  |  |  |  |  |
Investment Portfolio Benchmark | Customer Concentration Risk | Five Largest Portfolio Investments |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, at fair value | 207,100 |  |  |  |  |  |  |  |  | $ 207,100 |  | $ 176,900 |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  | 27.30% |  | 25.10% |
Principal or Proprietary Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, at fair value | $ 749,300 |  |  |  |  |  |  |  |  | $ 749,300 |  | $ 695,100 |
Principal or Proprietary Transactions | Investment Portfolio Benchmark | Investment Type Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  | 98.90% |  | 98.60% |
Syndicated Investments |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment | 3 |  |  |  |  |  |  |  |  | 3 |  | 4 |
Investments, at fair value | $ 8,400 |  |  |  |  |  |  |  |  | $ 8,400 |  | $ 9,700 |
Syndicated Investments | Investment Portfolio Benchmark | Investment Type Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  | 1.10% |  | 1.40% |
[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.

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