Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTY TRANSACTIONS - Narrative (Details)

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RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2023
Dec. 31, 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 $ 57,000 $ 40,000 $ 215,000 $ 146,000  
Ratio of net investment income (loss) to average net assets – annualized 4.73% 5.15% 1.32% 5.50%  
Investment company, capital gains-based incentive fees     $ 1,100,000   $ 0
Investment company, capital gains-based incentive fees payable (reversal) $ (600,000) $ 1,400,000 9,300,000 $ 700,000  
Administration fee [1] 450,000 410,000 1,306,000 1,352,000  
Success fee income 1,382,000 1,061,000 1,382,000 7,794,000  
Related Party          
Related Party Transaction [Line Items]          
Success fee income   $ 300,000 300,000 $ 1,600,000  
Due from related parties $ 22,000   $ 22,000   $ 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]          
Ownership percentage by parent 100.00%   100.00%    
Administrator | Chief Executive Officer | Related Party          
Related Party Transaction [Line Items]          
Ownership percentage by parent 100.00%   100.00%    
Gladstone Securities, LLC | Chief Executive Officer | Related Party          
Related Party Transaction [Line Items]          
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.