Quarterly report pursuant to Section 13 or 15(d)

INVESTMENTS - Narrative (Details)

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INVESTMENTS - Narrative (Details)
$ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2024
USD ($)
investment
state
industry
portfolioCompany
Nov. 30, 2024
USD ($)
Oct. 31, 2024
USD ($)
Dec. 31, 2024
USD ($)
investment
state
industry
portfolioCompany
Dec. 31, 2023
USD ($)
Sep. 30, 2024
USD ($)
investment
Summary of Investment Holdings [Line Items]            
Number of proprietary investments | investment 49     49   47
Fair Value $ 799,497     $ 799,497   $ 796,260 [1]
Purchase of investments       151,616 $ 57,998  
Financial support to investee, not provided $ 54,089     $ 54,089   57,594
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 14     14    
Weighted average investment owned, at cost, amount $ 16,000     $ 16,000   15,700
Uncollectible receivables allowance, minimum required day for uncollectible adjustment       90 days    
Gross receivables from portfolio companies 2,200     $ 2,200   1,700
Allowance for uncollectible receivables 59     59   75
Delayed draw term loans            
Summary of Investment Holdings [Line Items]            
Financial support to investee, not provided 13,200     13,200   14,150
Unused line of credit commitments            
Summary of Investment Holdings [Line Items]            
Financial support to investee, not provided 40,046     40,046   42,601
Perimeter Solutions Group            
Summary of Investment Holdings [Line Items]            
Net proceeds from the sale of investments     $ 15,000      
Prepayment fee income     500      
Antenna Research Associates, Inc            
Summary of Investment Holdings [Line Items]            
Net proceeds from the sale of investments     31,300      
Realized investment gains     $ 59,300      
Giving Home Healthcare, LLC            
Summary of Investment Holdings [Line Items]            
Purchase of investments   $ 28,900        
Wings ‘N More Restaurants, LLC            
Summary of Investment Holdings [Line Items]            
Purchase of investments   10,500        
Wings ‘N More Restaurants, LLC | Delayed draw term loans            
Summary of Investment Holdings [Line Items]            
Financial support to investee, not provided   5,000        
Wings ‘N More Restaurants, LLC | Unused line of credit commitments            
Summary of Investment Holdings [Line Items]            
Financial support to investee, not provided   1,500        
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            
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          
Prepayment fee income 100          
Realized investment gains 2,500          
DKI Ventures, LLC            
Summary of Investment Holdings [Line Items]            
Realized investment gains 4,100          
Investment Portfolio Benchmark | Customer Concentration Risk | Five Largest Portfolio Investments            
Summary of Investment Holdings [Line Items]            
Fair Value 190,600     $ 190,600   $ 232,700
Concentration risk, percentage       23.80%   29.20%
Principal or Proprietary Transactions            
Summary of Investment Holdings [Line Items]            
Fair Value $ 796,000     $ 796,000   $ 792,900
Principal or Proprietary Transactions | Investment Portfolio Benchmark | Investment Type Concentration Risk            
Summary of Investment Holdings [Line Items]            
Concentration risk, percentage       99.60%   99.60%
Syndicated Investments            
Summary of Investment Holdings [Line Items]            
Number of proprietary investments | investment 2     2   2
Fair Value $ 3,500     $ 3,500   $ 3,300
Syndicated Investments | Investment Portfolio Benchmark | Investment Type Concentration Risk            
Summary of Investment Holdings [Line Items]            
Concentration risk, percentage       0.40%   0.40%
[1] 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.