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INVESTMENTS - Narrative (Details) $ in Thousands | 1 Months Ended | 9 Months Ended | 12 Months Ended
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Jun. 30, 2025 USD ($) investment industry portfolioCompany state | May 31, 2025 USD ($) | 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 ($) | Jun. 30, 2025 USD ($) investment industry portfolioCompany state | Jun. 30, 2024 USD ($) | Sep. 30, 2024 USD ($) investment
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of proprietary investments | investment | 49 |  |  |  |  |  |  |  |  | 49 |  | 47 |
Fair Value | $ 751,260 |  |  |  |  |  |  |  |  | $ 751,260 |  | $ 796,260 | [1]
Purchase of investments |  |  |  |  |  |  |  |  |  | 270,163 | $ 149,122 |  |
Financial support to investee, not provided | 48,669 |  |  |  |  |  |  |  |  | 48,669 |  | 57,594 |
Principal repayments on investments |  |  |  |  |  |  |  |  |  | 239,258 | $ 120,946 |  |
Cost | $ 777,851 |  |  |  |  |  |  |  |  | $ 777,851 |  | 771,011 | [1]
Number of investment portfolio company | portfolioCompany | 51 |  |  |  |  |  |  |  |  | 51 |  |  |
Number of states which have invested in the company location | state | 22 |  |  |  |  |  |  |  |  | 22 |  |  |
Number of industries that have made investments | industry | 14 |  |  |  |  |  |  |  |  | 14 |  |  |
Weighted average investment owned, at cost, amount | $ 15,300 |  |  |  |  |  |  |  |  | $ 15,300 |  | 15,700 |
Uncollectible receivables allowance, minimum required day for uncollectible adjustment |  |  |  |  |  |  |  |  |  | 90 days |  |  |
Gross receivables from portfolio companies | 2,400 |  |  |  |  |  |  |  |  | $ 2,400 |  | 1,700 |
Allowance for uncollectible receivables | 100 |  |  |  |  |  |  |  |  | 100 |  | 75 |
Unused line of credit commitments |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided | 40,326 |  |  |  |  |  |  |  |  | 40,326 |  | 42,601 |
Delayed draw term loans |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided | 7,500 |  |  |  |  |  |  |  |  | 7,500 |  | 14,150 |
Perimeter Solutions Group |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  |  |  |  |  |  |  | $ 15,000 |  |  |  |
Prepayment fee income |  |  |  |  |  |  |  |  | 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] |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided |  |  |  |  |  |  |  | 1,500 |  |  |  |  |
Wings ‘N More Restaurants, LLC | Delayed draw term loans |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided |  |  |  |  |  |  |  | 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 |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |
Prepayment fee income |  |  |  |  |  |  | 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 |  |  |  |  |  |  |
Prepayment fee income |  |  |  |  |  | 100 |  |  |  |  |  |  |
Sokol |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |
Prepayment fee income | 400 |  |  |  |  |  |  |  |  |  |  |  |
RF Technologies, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments | 12,600 |  |  |  |  |  |  |  |  |  |  |  |
Financial support to investee, not provided | 3,500 |  |  |  |  |  |  |  |  | 3,500 |  |  |
Investment Portfolio Benchmark | Customer Concentration Risk | Five Largest Portfolio Investments |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value | 189,700 |  |  |  |  |  |  |  |  | $ 189,700 |  | $ 232,700 |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  | 25.30% |  | 29.20% |
Principal or Proprietary Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value | 747,500 |  |  |  |  |  |  |  |  | $ 747,500 |  | $ 792,900 |
Principal or Proprietary Transactions | Investment Portfolio Benchmark | Investment Type Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  | 99.50% |  | 99.60% |
Secured First Lien Debt |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value | 529,149 |  |  |  |  |  |  |  |  | $ 529,149 |  | $ 554,937 |
Cost | 555,347 |  |  |  |  |  |  |  |  | 555,347 |  | 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 | 51,741 |  |  |  |  |  |  |  |  | 51,741 |  | 96,229 |
Cost | $ 40,451 |  |  |  |  |  |  |  |  | $ 40,451 |  | $ 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 | 2 |  |  |  |  |  |  |  |  | 2 |  | 2 |
Fair Value | $ 3,800 |  |  |  |  |  |  |  |  | $ 3,800 |  | $ 3,300 |
Syndicated Investments | Investment Portfolio Benchmark | Investment Type Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  |  | 0.50% |  | 0.40% |
[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.

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