Quarterly report [Sections 13 or 15(d)]

RELATED PARTY TRANSACTIONS - Narrative (Details)

v3.25.3
RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Mar. 31, 2025
Related Party Transaction [Line Items]          
Base management fee (as a percent) 2.00% 2.00% 2.00% 2.00%  
Base management fee, percent fee     100.00%    
Investment company, excess expense reimbursable $ 130,000 $ 77,000 $ 239,000 $ 152,000  
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     1.88% 5.84%  
Investment company, capital gains-based incentive fees     $ 0   $ 4,900,000
Investment company, capital gains-based incentive fees payable (reversal) 4,900,000 1,600,000 4,700,000 $ 2,200,000  
Accrued capital gains-based incentive fee 44,000,000.0   44,000,000.0   39,300,000
Administration fee [1] 520,000 567,000 953,000 1,073,000  
Success fee income 0 150,000 754,000 1,703,000  
Related Party          
Related Party Transaction [Line Items]          
Success fee income 700,000 $ 200,000 1,300,000 $ 200,000  
Due from related parties $ 100,000   100,000   $ 100,000
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%   20.00%    
Affiliated Entity | Minimum | Advisory Agreement          
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.