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INVESTMENTS - Narrative (Details) $ in Thousands | 1 Months Ended | 2 Months Ended | 12 Months Ended
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Sep. 30, 2024 USD ($) portfolioCompany state investment industry | Jul. 31, 2024 USD ($) | Jun. 30, 2024 USD ($) | May 31, 2024 USD ($) | 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 ($) | Sep. 30, 2024 USD ($) portfolioCompany state investment industry | Sep. 30, 2023 USD ($) investment | Sep. 30, 2022 USD ($)
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment | 47 |  |  |  |  |  |  |  |  |  |  |  | 47 |  | 47 |  |
Fair Value | $ 796,260 | [1] |  |  |  |  |  |  |  |  |  |  | $ 796,260 | [1] | $ 704,815 | [2] |
Purchase of investments |  |  |  |  |  |  |  |  |  |  |  |  | 177,649 |  | 175,477 |  | $ 274,898
Financial support to investee, not provided | 57,594 |  |  |  |  |  |  |  |  |  |  |  | 57,594 |  | 45,231 |  |
Investment owned, at cost | $ 771,011 | [1] |  |  |  |  |  |  |  |  |  |  | $ 771,011 | [1] | 722,269 | [2] |
Number of investment portfolio company | portfolioCompany | 49 |  |  |  |  |  |  |  |  |  |  |  | 49 |  |  |  |
Number of states which have invested in the company location | state | 22 |  |  |  |  |  |  |  |  |  |  |  | 22 |  |  |  |
Number of industries that have made investments | industry | 13 |  |  |  |  |  |  |  |  |  |  |  | 13 |  |  |  |
Weighted average investment owned, at cost, amount | $ 15,700 |  |  |  |  |  |  |  |  |  |  |  | $ 15,700 |  | 14,200 |  |
Uncollectible receivables allowance, minimum required day for uncollectible adjustment |  |  |  |  |  |  |  |  |  |  |  |  | 90 days |  |  |  |
Gross receivables from portfolio companies | 1,700 |  |  |  |  |  |  |  |  |  |  |  | $ 1,700 |  | 800 |  |
Allowance for uncollectible receivables | 75 |  |  |  |  |  |  |  |  |  |  |  | 75 |  | 9 |  |
Unused line of credit commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided | 42,601 |  |  |  |  |  |  |  |  |  |  |  | 42,601 |  | 32,349 |  |
Delayed Draw Term Loan (DDTL) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided | 14,150 |  |  |  |  |  |  |  |  |  |  |  | 14,150 |  | 12,039 |  |
Investment Portfolio Benchmark | Customer Concentration Risk | Five Largest Portfolio Investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value | 232,700 |  |  |  |  |  |  |  |  |  |  |  | $ 232,700 |  | $ 176,900 |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  |  |  |  | 29.20% |  | 25.10% |  |
Principal or Proprietary Transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value | $ 792,900 |  |  |  |  |  |  |  |  |  |  |  | $ 792,900 |  | $ 695,100 |  |
Principal or Proprietary Transactions | Investment Portfolio Benchmark | Investment Type Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  |  |  |  | 99.60% |  | 98.60% |  |
Syndicated Investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment | 2 |  |  |  |  |  |  |  |  |  |  |  | 2 |  | 4 |  |
Fair Value | $ 3,300 |  |  |  |  |  |  |  |  |  |  |  | $ 3,300 |  | $ 9,700 |  |
Syndicated Investments | Investment Portfolio Benchmark | Investment Type Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  |  |  |  | 0.40% |  | 1.40% |  |
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 |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  |  |  | 7,300 |  |  |  |  |  |
Cafe Zupas | Delayed Draw Term Loan (DDTL) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee |  |  |  |  |  |  |  |  |  | $ 1,400 |  |  |  |  |  |  |
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 | 300 |  |  |  |  |  | $ 8,400 |  |  |  |  |  |  |  |  |  |
Realized investment gains |  |  |  |  |  |  | 200 |  |  |  |  |  |  |  |  |  |
Investment company, prepayment fee |  |  |  |  |  |  | $ 200 |  |  |  |  |  |  |  |  |  |
Total Access Elevator, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  | $ 7,300 |  |  |  |  |  |  |  |  |  |  |
Total Access Elevator, LLC | Unused line of credit commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided |  |  |  |  |  | 3,000 |  |  |  |  |  |  |  |  |  |  |
Total Access Elevator, LLC | Delayed Draw Term Loan (DDTL) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided |  |  |  |  |  | 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] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |
Turn Key Health Clinics, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | $ 6,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee |  |  | 2,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Arc Drilling Holdings LLC – Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments | 13,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Arc Drilling Holdings LLC – Common Stock | Unused line of credit commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided | 4,000 |  |  |  |  |  |  |  |  |  |  |  | $ 4,000 |  |  |  |
Amount funded | $ 900 |  |  |  |  |  |  |  |  |  |  |  | $ 900 |  |  |  |
CPM Holdings, Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  | $ 3,000 |  |  |  |  |  |  |  |
Tailwind Smith Cooper Immediate Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  | $ 5,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Cumulative gross unrealized depreciation for federal income tax purposes is $66.1 million; cumulative gross unrealized appreciation for federal income tax purposes is $85.8 million. Cumulative net unrealized appreciation is $19.7 million, based on a tax cost of $776.5 million.
[2] | 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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