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RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($) | 12 Months Ended
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Mar. 31, 2024 | Mar. 31, 2023 | Mar. 31, 2022 | Mar. 31, 2021 | Mar. 31, 2020 | Mar. 31, 2019 | Mar. 31, 2018 | Mar. 31, 2017 | Mar. 31, 2016 | Mar. 31, 2015
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Base management fee |  | 2.00% | 2.00% | 2.00% |  |  |  |  |  |  |
Base management fee, percent fee |  | 100.00% |  |  |  |  |  |  |  |  |
Investment company, excess expense reimbursable |  | $ 300,000 | $ 200,000 | $ 300,000 |  |  |  |  |  |  |
Maximum base management fee paid cannot exceed a percentage of total assets |  | 2.00% |  |  |  |  |  |  |  |  |
Ratio of net investment income (loss) to average net assets – annualized |  | 1.53% | 5.66% | 0.31% | 2.16% | 6.20% | (1.22%) | 3.66% | 4.19% | 3.94% | 5.26%
Investment company, capital gains-based incentive fees |  | $ 1,100,000 | $ 0 | $ 5,300,000 |  |  |  |  |  |  |
Investment company, capital gains-based incentive fees payable (reversal) |  | 12,700,000 | (300,000) | 18,300,000 |  |  |  |  |  |  |
Administration fee | [1] | 1,789,000 | 1,811,000 | 1,806,000 |  |  |  |  |  |  |
Success fee income: |  | 3,605,000 | 10,402,000 | 10,308,000 |  |  |  |  |  |  |
Related Party |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Co-investment expense |  | 100,000 | 0 |  |  |  |  |  |  |  |
Affiliated Entity |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Success fee income: |  | 300,000 | $ 1,600,000 | $ 3,200,000 |  |  |  |  |  |  |
Affiliated Entity | Advisory Agreement |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
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% |  |  |  |  |  |  |  |  |
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% |  |  |  |  |  |  |  |  |
Administrator | Chief Executive Officer | Related Party |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest, ownership percentage by parent |  | 100.00% |  |  |  |  |  |  |  |  |
Gladstone Securities, LLC | Chief Executive Officer | Related Party |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest, ownership percentage by parent |  | 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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