Quarterly report pursuant to Section 13 or 15(d)

INVESTMENTS - Narrative (Details)

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INVESTMENTS - Narrative (Details)
$ in Thousands
 1 Months Ended 2 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2024
USD ($)
investment
state
industry
portfolioCompany
May 31, 2024
USD ($)
Apr. 30, 2024
USD ($)
Mar. 31, 2024
USD ($)
Feb. 29, 2024
USD ($)
Jan. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Nov. 30, 2023
USD ($)
Mar. 31, 2024
USD ($)
Jun. 30, 2024
USD ($)
investment
state
industry
portfolioCompany
Jun. 30, 2023
USD ($)
Sep. 30, 2023
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investment
Summary of Investment Holdings [Line Items]                        
Number of proprietary investments | investment 48                 48   47
Investments, at fair value $ 757,745                 $ 757,745   $ 704,815 [1]
Purchase of investments                   149,122 $ 148,084  
Financial support to investee, not provided 53,344                 53,344   45,231
Investment owned, at cost $ 753,263                 $ 753,263   722,269 [1]
Number of investment portfolio company | portfolioCompany 51                 51    
Number of states which have invested in the company location | state 23                 23    
Number of industries that have made investments | industry 13                 13    
Weighted average investment owned, at cost, amount $ 14,800                 $ 14,800   14,200
Gross receivables from portfolio companies 1,500                 1,500   800
Allowance for uncollectible receivables 58                 58   9
Quality Environmental Midco, Inc                        
Summary of Investment Holdings [Line Items]                        
Purchase of investments         $ 5,000     $ 11,000        
Financial support to investee, not provided               2,000        
Financial support to investee         3,000              
Cafe Zupas                        
Summary of Investment Holdings [Line Items]                        
Financial support to investee, not provided               10,500        
Investment company, delayed draw term loan commitment             $ 1,400          
Net proceeds from the sale of investments               7,300        
PIC 360, LLC                        
Summary of Investment Holdings [Line Items]                        
Realized investment gains               $ 300        
ALS Education, LLC                        
Summary of Investment Holdings [Line Items]                        
Purchase of investments             14,300          
Leadpoint Business Services, LLC                        
Summary of Investment Holdings [Line Items]                        
Purchase of investments             12,000          
Salt & Straw, LLC                        
Summary of Investment Holdings [Line Items]                        
Purchase of investments             7,000          
Financial support to investee             $ 2,900          
SpaceCo Holdings, LLC                        
Summary of Investment Holdings [Line Items]                        
Purchase of investments                 $ 13,500      
Perimeter Solutions Group                        
Summary of Investment Holdings [Line Items]                        
Purchase of investments         $ 15,000              
Trowbridge Chicago, LLC                        
Summary of Investment Holdings [Line Items]                        
Net proceeds from the sale of investments       $ 8,400                
Realized investment gains       200                
Investment company, prepayment fee       200                
Investment owned, at cost       $ 300         $ 300      
Total Access Elevator, LLC                        
Summary of Investment Holdings [Line Items]                        
Purchase of investments     $ 7,300                  
Investment company, line of credit commitment     3,000                  
Investment company, delayed draw term loan commitment     2,500                  
Home Healthcare, LLC ("Giving Home")                        
Summary of Investment Holdings [Line Items]                        
Net proceeds from the sale of investments     29,700                  
Investment company, prepayment fee     900                  
Proceeds from equity     $ 2,500                  
Gray Matter Systems, LLC                        
Summary of Investment Holdings [Line Items]                        
Net proceeds from the sale of investments   $ 14,000                    
Proceeds from disposition of investment, prepayment fees   200                    
Pansophic Learning, Ltd.                        
Summary of Investment Holdings [Line Items]                        
Net proceeds from the sale of investments   33,000                    
RPM Freight Systems, LLC                        
Summary of Investment Holdings [Line Items]                        
Purchase of investments   20,000                    
Financial support to investee, not provided   5,000                    
Funko Acquisition Holdings, LLC                        
Summary of Investment Holdings [Line Items]                        
Investments, at fair value                       22
Realized investment gains   2                    
Investment owned, at cost   $ 21                    
Workforce QA, LLC                        
Summary of Investment Holdings [Line Items]                        
Purchase of investments 7,400                      
CPM Holdings, Inc                        
Summary of Investment Holdings [Line Items]                        
Net proceeds from the sale of investments           $ 3,000            
Investment Portfolio Benchmark | Customer Concentration Risk | Five Largest Portfolio Investments                        
Summary of Investment Holdings [Line Items]                        
Investments, at fair value 207,100                 $ 207,100   $ 176,900
Concentration risk, percentage                   27.30%   25.10%
Principal or Proprietary Transactions                        
Summary of Investment Holdings [Line Items]                        
Investments, at fair value $ 749,300                 $ 749,300   $ 695,100
Principal or Proprietary Transactions | Investment Portfolio Benchmark | Investment Type Concentration Risk                        
Summary of Investment Holdings [Line Items]                        
Concentration risk, percentage                   98.90%   98.60%
Syndicated Investments                        
Summary of Investment Holdings [Line Items]                        
Number of proprietary investments | investment 3                 3   4
Investments, at fair value $ 8,400                 $ 8,400   $ 9,700
Syndicated Investments | Investment Portfolio Benchmark | Investment Type Concentration Risk                        
Summary of Investment Holdings [Line Items]                        
Concentration risk, percentage                   1.10%   1.40%
[1] Cumulative gross unrealized depreciation for federal income tax purposes is $56.9 million; cumulative gross unrealized appreciation for federal income tax purposes is $33.7 million. Cumulative net unrealized depreciation is $23.2 million, based on a tax cost of $728.0 million.