Annual report [Section 13 and 15(d), not S-K Item 405]

RELATED PARTY TRANSACTIONS - Narrative (Details)

v3.25.3
RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($)
 12 Months Ended 36 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Mar. 31, 2023
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Mar. 31, 2023
Related Party Transaction [Line Items]                        
Base management fee 1.75% 1.75% 1.75%                  
Base management fee, percent fee 100.00%                      
Management and service fee, net of credit 0.50% 0.50%                    
Management and service fees, loans 1.50%                      
Ratio of net investment income (loss) to average net assets – annualized 5.93% 7.90% 8.49%   6.61% 6.40% 6.11% 6.74% 6.41% 6.13% 6.90%  
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%     2.50%                
Pre-incentive fee net investment income exceeds the hurdle rate 20.00%                      
Investment company, capital gains-based incentive fees 20.00%                      
Capital gains-based incentive fee $ 0                      
Investment company, net investment income to common stockholder distributions ratio, threshold 100.00%   100.00%                  
Incentive fee credit $ 2,300,000 $ 200,000 $ 0                  
Investment company, other operating expenses, due to advisor $ 100,000 52,000                    
Gladstone Securities                        
Related Party Transaction [Line Items]                        
Selling commissions, percentage 7.00%                      
Dealer manager fees percentage 3.00%                      
Payments for selling commissions and dealer-manager fees $ 1,300,000 900,000 0                  
Related Party                        
Related Party Transaction [Line Items]                        
Fee income 1,100,000 400,000 $ 800,000                  
Fees due to related party 3,531,000 4,458,000                    
Related Party | Gladstone Investment Corporation                        
Related Party Transaction [Line Items]                        
Fees due to related party $ 52,000 $ 75,000                    
Adviser | Related Party | Chief Executive Officer                        
Related Party Transaction [Line Items]                        
Noncontrolling interest, ownership percentage by parent 100.00%                      
Gladstone Securities, LLC | Related Party | Chief Executive Officer                        
Related Party Transaction [Line Items]                        
Noncontrolling interest, ownership percentage by parent 100.00%                      
Maximum                        
Related Party Transaction [Line Items]                        
Base management fee 1.75%                      
Minimum                        
Related Party Transaction [Line Items]                        
Ratio of net investment income (loss) to average net assets – annualized 1.75%                     2.00%