Quarterly report [Sections 13 or 15(d)]

RELATED PARTY TRANSACTIONS - Narrative (Details)

v3.26.3
RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Mar. 31, 2026
Related Party Transaction [Line Items]          
Base management fee (as a percent) 2.00% 2.00% 2.00%    
Base management fee, percent fee     100.00%    
Investment company, excess expense reimbursable $ 100,000 $ 100,000 $ 200,000 $ 200,000  
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     3.22% 1.88%  
Investment company, capital gains-based incentive fees     $ 0   $ 0
Investment company, capital gains-based incentive fees (reversal) accrual 2,400,000 4,900,000 (3,100,000) $ 4,700,000  
Accrued capital gains-based incentive fee 74,200,000   74,200,000   77,300,000
Administration fee [1] 479,000 520,000 971,000 953,000  
Success fee income 0 0 5,591,000 754,000  
Related Party          
Related Party Transaction [Line Items]          
Success fee income 2,100,000 $ 700,000 2,100,000 $ 1,300,000  
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.