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

INVESTMENTS - Investment Activity Narrative (Details)

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INVESTMENTS - Investment Activity Narrative (Details)
$ in Thousands
 1 Months Ended 12 Months Ended
Aug. 04, 2025
USD ($)
Sep. 30, 2025
USD ($)
investment
Aug. 31, 2025
USD ($)
Jul. 31, 2025
USD ($)
Jun. 30, 2025
USD ($)
May 31, 2025
USD ($)
Apr. 30, 2025
USD ($)
Mar. 31, 2025
USD ($)
Feb. 28, 2025
USD ($)
Jan. 31, 2025
USD ($)
Dec. 31, 2024
USD ($)
Nov. 30, 2024
USD ($)
Oct. 31, 2024
USD ($)
Sep. 30, 2025
USD ($)
investment
Sep. 30, 2024
USD ($)
investment
Sep. 30, 2023
USD ($)
Summary of Investment Holdings [Line Items]                                
Number of proprietary investments | investment   54                       54 47  
Fair Value   $ 859,124 [1]                       $ 859,124 [1] $ 796,260 [2]  
Purchase of investments                           396,796 177,649 $ 175,477
Total   66,396                       66,396 57,594  
Principal repayments on investments                           262,753 133,471 $ 107,505
Cost   876,614 [1]                       876,614 [1] 771,011 [2]  
Unused line of credit commitments                                
Summary of Investment Holdings [Line Items]                                
Total   54,553                       54,553 42,601  
Delayed Draw Term Loan (DDTL)                                
Summary of Investment Holdings [Line Items]                                
Total   11,000                       11,000 14,150  
Perimeter Solutions Group                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments                         $ 15,000      
Investment company, prepayment fee                         500      
Antenna Research Associates, Inc                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments                         31,300      
Realized investment gains (losses)                         $ 59,300      
Giving Home Healthcare, LLC                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments         $ 7,000             $ 28,900        
Wings ‘N More Restaurants, LLC                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments                       10,500        
Wings ‘N More Restaurants, LLC | Unused line of credit commitments                                
Summary of Investment Holdings [Line Items]                                
Total                       1,500        
Wings ‘N More Restaurants, LLC | Delayed Draw Term Loan (DDTL)                                
Summary of Investment Holdings [Line Items]                                
Total                       5,000        
ENET Holdings, LLC                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments                       $ 22,300        
Vet’s Choice Radiology, LLC                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments                     $ 42,800          
Pan-Am Dental, LLC                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments                     28,900          
Freedom Dental Management, Inc.                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments                     15,000          
Tube Bending Technology, LLC                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments                     5,000          
Gladstone Alternative Income Fund                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments                     5,000          
Salt and Straw, LLC                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments                     10,900          
Investment company, prepayment fee                     100          
Realized investment gains (losses)                     2,500          
DKI Ventures, LLC                                
Summary of Investment Holdings [Line Items]                                
Realized investment gains (losses)                     $ (4,100)          
Fix-It Group, LLC                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments                   $ 20,600            
Investment company, prepayment fee                   100            
Sokol & Company Holdings, LLC                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments                 $ 1,100 $ 5,400            
Realized investment gains (losses)                 4,700              
Dutch Gold Honey, Inc.                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments                 18,900              
Viron International, LLC                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments                 $ 19,400              
Viva Railings, LLC                                
Summary of Investment Holdings [Line Items]                                
Principal repayments on investments               $ 6,000                
MCG Energy Solutions, LLC                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments               20,400                
Realized investment gains (losses)               $ 3,000                
SpaceCo Holdings, LLC                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments             $ 42,200                  
Eegee Acquisition Corporation                                
Summary of Investment Holdings [Line Items]                                
Realized investment gains (losses)             4,400                  
Altior Healthcare, LLC                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments           $ 46,000                    
Cafe Zupas                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments         36,100                      
Investment company, prepayment fee         400                      
RF Technologies, LLC                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments         12,600                      
Total         $ 3,500                      
MASSiv Brands, LLC                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments       $ 25,000                        
Alsay, Inc.                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments       15,000                        
Snif-Snax, LLC                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments       23,700                        
Zero Case Holding Inc.                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments       13,000                        
Total       4,000                        
OCI, LLC                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments $ 16,600                              
Foodservices Brand Group (“FBG”)                                
Summary of Investment Holdings [Line Items]                                
Purchase of investments     $ 30,000                          
Total     $ 10,000                          
FES Resources Holdings, LLC                                
Summary of Investment Holdings [Line Items]                                
Realized investment gains (losses)   (6,400)                            
Ohio Armor Holdings, LLC                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments   15,400                            
8th Avenue Food and Provisions, Inc.                                
Summary of Investment Holdings [Line Items]                                
Net proceeds from the sale of investments       $ 3,700                        
Principal or Proprietary Transactions                                
Summary of Investment Holdings [Line Items]                                
Fair Value   859,100                       $ 859,100 $ 792,900  
Principal or Proprietary Transactions | Investment Portfolio Benchmark | Investment Type Concentration Risk                                
Summary of Investment Holdings [Line Items]                                
Concentration risk, percentage                           100.00% 99.60%  
Secured first lien debt                                
Summary of Investment Holdings [Line Items]                                
Fair Value   622,371                       $ 622,371 $ 554,937  
Cost   646,131                       646,131 580,736  
Secured first lien debt | Eegee Acquisition Corporation                                
Summary of Investment Holdings [Line Items]                                
Cost             12,800                  
Common equity/equivalents                                
Summary of Investment Holdings [Line Items]                                
Fair Value   54,664                       54,664 96,229  
Cost   $ 42,562                       $ 42,562 $ 31,369  
Common equity/equivalents | Eegee Acquisition Corporation                                
Summary of Investment Holdings [Line Items]                                
Cost             $ 8,500                  
Syndicated Investments                                
Summary of Investment Holdings [Line Items]                                
Number of proprietary investments | investment   1                       1 2  
Fair Value   $ 100                       $ 100 $ 3,300  
Syndicated Investments | Investment Portfolio Benchmark | Investment Type Concentration Risk                                
Summary of Investment Holdings [Line Items]                                
Concentration risk, percentage                           0.10% 0.40%  
[1] Cumulative gross unrealized depreciation for federal income tax purposes is $59.7 million; cumulative gross unrealized appreciation for federal income tax purposes is $34.1 million. Cumulative net unrealized depreciation is $25.5 million, based on a tax cost of $884.7 million.
[2] Cumulative gross unrealized depreciation for federal income tax purposes is $66.1 million; cumulative gross unrealized appreciation for federal income tax purposes is $85.8 million. Cumulative net unrealized appreciation is $19.7 million, based on a tax cost of $776.5 million