Annual report pursuant to Section 13 and 15(d)

RELATED PARTY TRANSACTIONS - Narrative (Details)

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RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($)
 12 Months Ended
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.