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RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022 | Mar. 31, 2023
Related Party Transaction [Line Items] |  |  |  |  |  |
Base management fee |  | 2.00% | 2.00% | 2.00% | 2.00% |
Investment company, excess expense reimbursable |  | $ 83,000 | $ 70,000 | $ 158,000 | $ 106,000 |
Ratio of net investment income (loss) to average net assets – annualized |  | (5.39%) | 7.01% | (0.50%) | 5.69% |
Investment company, capital gains-based incentive fees |  |  |  | $ 0 |  | $ 0
Investment company, capital gains-based incentive fees payable (reversal) |  | $ 9,800,000 | $ (1,700,000) | 9,900,000 | $ (700,000) |
Administration fee | [1] | 334,000 | 562,000 | 856,000 | 942,000 |
Success fee income |  | 0 | 1,733,000 | 0 | 6,733,000 |
Related Party |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Success fee income |  |  | $ 1,000,000 | 300,000 | $ 1,300,000 |
Due from related parties |  | $ 19,000 |  | $ 19,000 |  | $ 0
Affiliated Entity | Advisory Agreement |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Base management fee |  |  |  | 2.00% |  |
Base management fee, percent fee |  |  |  | 100.00% |  |
Maximum base management fee paid cannot exceed a percentage of total assets |  |  |  | 2.00% |  |
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% |  |
Pre-incentive fee net investment income exceeds the hurdle rate |  |  |  | 20.00% |  |
Investment company, capital gains-based incentive fees |  | 20.00% |  | 20.00% |  |
Affiliated Entity | Advisory Agreement | Minimum |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Ratio of net investment income (loss) to average net assets – annualized |  |  |  | 1.75% |  |
Adviser | Chief Executive Officer | Related Party |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Noncontrolling interest, ownership percentage by parent |  | 100.00% |  | 100.00% |  |
Administrator | Chief Executive Officer | Related Party |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Noncontrolling interest, ownership percentage by parent |  | 100.00% |  | 100.00% |  |
Gladstone Securities, LLC | Chief Executive Officer | Related Party |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Noncontrolling interest, ownership percentage by parent |  | 100.00% |  | 100.00% |  |
[1] | Refer to Note 4 — Related Party Transactions in the accompanying Notes to Consolidated Financial Statements for additional information.

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