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INVESTMENTS - Investment Activity Narrative (Details) $ in Thousands |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Aug. 04, 2025 USD ($) | Sep. 30, 2025 USD ($) investment | Aug. 31, 2025 USD ($) | Jul. 31, 2025 USD ($) | Jun. 30, 2025 USD ($) | May 31, 2025 USD ($) | Apr. 30, 2025 USD ($) | Mar. 31, 2025 USD ($) | Feb. 28, 2025 USD ($) | Jan. 31, 2025 USD ($) | Dec. 31, 2024 USD ($) | Nov. 30, 2024 USD ($) | Oct. 31, 2024 USD ($) | Sep. 30, 2025 USD ($) investment | Sep. 30, 2024 USD ($) investment | Sep. 30, 2023 USD ($)
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment |  | 54 |  |  |  |  |  |  |  |  |  |  |  |  | 54 |  | 47 |  |
Fair Value |  | $ 859,124 | [1] |  |  |  |  |  |  |  |  |  |  |  | $ 859,124 | [1] | $ 796,260 | [2] |
Purchase of investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 396,796 |  | 177,649 |  | $ 175,477
Total |  | 66,396 |  |  |  |  |  |  |  |  |  |  |  |  | 66,396 |  | 57,594 |  |
Principal repayments on investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 262,753 |  | 133,471 |  | $ 107,505
Cost |  | 876,614 | [1] |  |  |  |  |  |  |  |  |  |  |  | 876,614 | [1] | 771,011 | [2] |
Unused line of credit commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total |  | 54,553 |  |  |  |  |  |  |  |  |  |  |  |  | 54,553 |  | 42,601 |  |
Delayed Draw Term Loan (DDTL) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total |  | 11,000 |  |  |  |  |  |  |  |  |  |  |  |  | 11,000 |  | 14,150 |  |
Perimeter Solutions Group |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,000 |  |  |  |  |
Investment company, prepayment fee |  |  |  |  |  |  |  |  |  |  |  |  |  | 500 |  |  |  |  |
Antenna Research Associates, Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  |  |  |  |  |  | 31,300 |  |  |  |  |
Realized investment gains (losses) |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 59,300 |  |  |  |  |
Giving Home Healthcare, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  | $ 7,000 |  |  |  |  |  |  | $ 28,900 |  |  |  |  |  |
Wings ‘N More Restaurants, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  |  |  |  |  | 10,500 |  |  |  |  |  |
Wings ‘N More Restaurants, LLC | Unused line of credit commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total |  |  |  |  |  |  |  |  |  |  |  |  | 1,500 |  |  |  |  |  |
Wings ‘N More Restaurants, LLC | Delayed Draw Term Loan (DDTL) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total |  |  |  |  |  |  |  |  |  |  |  |  | 5,000 |  |  |  |  |  |
ENET Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  |  |  |  |  | $ 22,300 |  |  |  |  |  |
Vet’s Choice Radiology, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  |  |  |  | $ 42,800 |  |  |  |  |  |  |
Pan-Am Dental, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  |  |  |  | 28,900 |  |  |  |  |  |  |
Freedom Dental Management, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  |  |  |  | 15,000 |  |  |  |  |  |  |
Tube Bending Technology, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  |  |  |  | 5,000 |  |  |  |  |  |  |
Gladstone Alternative Income Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  |  |  |  | 5,000 |  |  |  |  |  |  |
Salt and Straw, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  |  |  |  | 10,900 |  |  |  |  |  |  |
Investment company, prepayment fee |  |  |  |  |  |  |  |  |  |  |  | 100 |  |  |  |  |  |  |
Realized investment gains (losses) |  |  |  |  |  |  |  |  |  |  |  | 2,500 |  |  |  |  |  |  |
DKI Ventures, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Realized investment gains (losses) |  |  |  |  |  |  |  |  |  |  |  | $ (4,100) |  |  |  |  |  |  |
Fix-It Group, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  |  |  | $ 20,600 |  |  |  |  |  |  |  |
Investment company, prepayment fee |  |  |  |  |  |  |  |  |  |  | 100 |  |  |  |  |  |  |  |
Sokol & Company Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  |  | $ 1,100 | $ 5,400 |  |  |  |  |  |  |  |
Realized investment gains (losses) |  |  |  |  |  |  |  |  |  | 4,700 |  |  |  |  |  |  |  |  |
Dutch Gold Honey, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  |  | 18,900 |  |  |  |  |  |  |  |  |
Viron International, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  |  |  | $ 19,400 |  |  |  |  |  |  |  |  |
Viva Railings, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal repayments on investments |  |  |  |  |  |  |  |  | $ 6,000 |  |  |  |  |  |  |  |  |  |
MCG Energy Solutions, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  | 20,400 |  |  |  |  |  |  |  |  |  |
Realized investment gains (losses) |  |  |  |  |  |  |  |  | $ 3,000 |  |  |  |  |  |  |  |  |  |
SpaceCo Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  | $ 42,200 |  |  |  |  |  |  |  |  |  |  |
Eegee Acquisition Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Realized investment gains (losses) |  |  |  |  |  |  |  | 4,400 |  |  |  |  |  |  |  |  |  |  |
Altior Healthcare, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  | $ 46,000 |  |  |  |  |  |  |  |  |  |  |  |
Cafe Zupas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  | 36,100 |  |  |  |  |  |  |  |  |  |  |  |  |
Investment company, prepayment fee |  |  |  |  |  | 400 |  |  |  |  |  |  |  |  |  |  |  |  |
RF Technologies, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  | 12,600 |  |  |  |  |  |  |  |  |  |  |  |  |
Total |  |  |  |  |  | $ 3,500 |  |  |  |  |  |  |  |  |  |  |  |  |
MASSiv Brands, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  | $ 25,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alsay, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  | 15,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Snif-Snax, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  | 23,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Zero Case Holding Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  | 13,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total |  |  |  |  | 4,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
OCI, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments | $ 16,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Foodservices Brand Group (“FBG”) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  | $ 30,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total |  |  |  | $ 10,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
FES Resources Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Realized investment gains (losses) |  | (6,400) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ohio Armor Holdings, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  | 15,400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
8th Avenue Food and Provisions, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  | $ 3,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal or Proprietary Transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value |  | 859,100 |  |  |  |  |  |  |  |  |  |  |  |  | $ 859,100 |  | $ 792,900 |  |
Principal or Proprietary Transactions | Investment Portfolio Benchmark | Investment Type Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  | 99.60% |  |
Secured first lien debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value |  | 622,371 |  |  |  |  |  |  |  |  |  |  |  |  | $ 622,371 |  | $ 554,937 |  |
Cost |  | 646,131 |  |  |  |  |  |  |  |  |  |  |  |  | 646,131 |  | 580,736 |  |
Secured first lien debt | Eegee Acquisition Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost |  |  |  |  |  |  |  | 12,800 |  |  |  |  |  |  |  |  |  |  |
Common equity/equivalents |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value |  | 54,664 |  |  |  |  |  |  |  |  |  |  |  |  | 54,664 |  | 96,229 |  |
Cost |  | $ 42,562 |  |  |  |  |  |  |  |  |  |  |  |  | $ 42,562 |  | $ 31,369 |  |
Common equity/equivalents | Eegee Acquisition Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost |  |  |  |  |  |  |  | $ 8,500 |  |  |  |  |  |  |  |  |  |  |
Syndicated Investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  | 2 |  |
Fair Value |  | $ 100 |  |  |  |  |  |  |  |  |  |  |  |  | $ 100 |  | $ 3,300 |  |
Syndicated Investments | Investment Portfolio Benchmark | Investment Type Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.10% |  | 0.40% |  |
[1] | Cumulative gross unrealized depreciation for federal income tax purposes is $59.7 million; cumulative gross unrealized appreciation for federal income tax purposes is $34.1 million. Cumulative net unrealized depreciation is $25.5 million, based on a tax cost of $884.7 million.
[2] | 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

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