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RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($) | 12 Months Ended | 36 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023 | Mar. 31, 2023 | Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016 | Mar. 31, 2023
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Base management fee | 1.75% | 1.75% | 1.75% |  |  |  |  |  |  |  |  |
Base management fee, percent fee | 100.00% |  |  |  |  |  |  |  |  |  |  |
Management and service fee, net of credit | 0.50% | 0.50% |  |  |  |  |  |  |  |  |  |
Management and service fees, loans | 1.50% |  |  |  |  |  |  |  |  |  |  |
Ratio of net investment income (loss) to average net assets – annualized | 5.93% | 7.90% | 8.49% |  | 6.61% | 6.40% | 6.11% | 6.74% | 6.41% | 6.13% | 6.90% |
Incentive fee base on minimum rate of return required on investment | $ 0 |  |  |  |  |  |  |  |  |  |  |
Percentage of pre-incentive fee net investment income is less than the hurdle rate | 100.00% |  |  |  |  |  |  |  |  |  |  |
Investment company, pre-incentive fee net investment income, percent threshold of net assets | 2.1875% |  |  | 2.50% |  |  |  |  |  |  |  |
Pre-incentive fee net investment income exceeds the hurdle rate | 20.00% |  |  |  |  |  |  |  |  |  |  |
Investment company, capital gains-based incentive fees | 20.00% |  |  |  |  |  |  |  |  |  |  |
Capital gains-based incentive fee | $ 0 |  |  |  |  |  |  |  |  |  |  |
Investment company, net investment income to common stockholder distributions ratio, threshold | 100.00% |  | 100.00% |  |  |  |  |  |  |  |  |
Incentive fee credit | $ 2,300,000 | $ 200,000 | $ 0 |  |  |  |  |  |  |  |  |
Investment company, other operating expenses, due to advisor | $ 100,000 | 52,000 |  |  |  |  |  |  |  |  |  |
Gladstone Securities |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Selling commissions, percentage | 7.00% |  |  |  |  |  |  |  |  |  |  |
Dealer manager fees percentage | 3.00% |  |  |  |  |  |  |  |  |  |  |
Payments for selling commissions and dealer-manager fees | $ 1,300,000 | 900,000 | 0 |  |  |  |  |  |  |  |  |
Related Party |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Fee income | 1,100,000 | 400,000 | $ 800,000 |  |  |  |  |  |  |  |  |
Fees due to related party | 3,531,000 | 4,458,000 |  |  |  |  |  |  |  |  |  |
Related Party | Gladstone Investment Corporation |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Fees due to related party | $ 52,000 | $ 75,000 |  |  |  |  |  |  |  |  |  |
Adviser | Related Party | Chief Executive Officer |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest, ownership percentage by parent | 100.00% |  |  |  |  |  |  |  |  |  |  |
Gladstone Securities, LLC | Related Party | Chief Executive Officer |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest, ownership percentage by parent | 100.00% |  |  |  |  |  |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Base management fee | 1.75% |  |  |  |  |  |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ratio of net investment income (loss) to average net assets – annualized | 1.75% |  |  |  |  |  |  |  |  |  |  | 2.00%

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