Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUE HIERARCHY FOR INVESTMENTS???PORTFOLIO COMPOSITION - Narrative (Details)

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FAIR VALUE HIERARCHY FOR INVESTMENTS—PORTFOLIO COMPOSITION - Narrative (Details)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2024
USD ($)
company
entity
Mar. 31, 2023
Mar. 31, 2024
USD ($)
company
entity
Dec. 31, 2023
USD ($)
company
entity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Annualized total return on investments 0.118 0.114    
Achieved total return on investments       0.136
Fair value $ 1,115,982 [1]   $ 1,115,982 [1] $ 1,092,895 [2]
Cost $ 1,062,538 [3]   $ 1,062,538 [3] $ 1,038,318 [4]
Investment Owned, At Fair Value | Investment Type Concentration Risk        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Concentration risk, percentage (as a percent) 100.00%     100.00%
Investment Owned, At Cost | Investment Type Concentration Risk        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Concentration risk, percentage (as a percent) 100.00%     100.00%
Private Loan portfolio investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Number of portfolio companies | company 79   79 78
Fair value $ 631,400   $ 631,400 $ 595,300
Cost $ 624,300   $ 624,300 $ 586,400
Private Loan portfolio investments | Investment Owned, At Fair Value | Investment Type Concentration Risk        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Concentration risk, percentage (as a percent) 21.00% 24.00% 78.00%  
Private Loan portfolio investments | Debt investments | Investment Owned, At Cost | Investment Type Concentration Risk        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Concentration risk, percentage (as a percent) 94.60%     94.10%
Private Loan portfolio investments | Minimum        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investments, per company $ 1,000   $ 1,000  
Private Loan portfolio investments | Minimum | Debt investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investment, term 3 years      
Private Loan portfolio investments | Maximum        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investments, per company $ 30,000   $ 30,000  
Private Loan portfolio investments | Maximum | Debt investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investment, term 7 years      
LMM portfolio investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Number of portfolio companies | company 51   51 50
Fair value $ 391,600   $ 391,600 $ 387,000
Cost $ 319,200   $ 319,200 $ 315,700
LMM portfolio investments | Investment Owned, At Fair Value | Investment Type Concentration Risk        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Concentration risk, percentage (as a percent) 26.00% 39.00% 94.00%  
LMM portfolio investments | Debt investments | Investment Owned, At Cost | Investment Type Concentration Risk        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Concentration risk, percentage (as a percent) 70.30%     70.20%
LMM portfolio investments | Minimum        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investments, per company $ 1,000   $ 1,000  
Investments, annual revenues per company $ 10,000      
LMM portfolio investments | Minimum | Debt investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investment, term 5 years      
LMM portfolio investments | Maximum        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investments, per company $ 20,000   $ 20,000  
Investments, annual revenues per company $ 150,000      
LMM portfolio investments | Maximum | Debt investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investment, term 7 years      
Middle Market portfolio investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Number of portfolio companies | company 15   15 16
Fair value $ 68,400   $ 68,400 $ 86,000
Cost $ 97,700   97,700 $ 114,700
Middle Market portfolio investments | Debt investments | Investment Owned, At Cost | Investment Type Concentration Risk        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Concentration risk, percentage (as a percent) 91.70%     93.10%
Middle Market portfolio investments | Minimum        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investments, per company $ 1,000   1,000  
Investments, annual revenues per company $ 150,000      
Middle Market portfolio investments | Minimum | Debt investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investment, term 3 years      
Middle Market portfolio investments | Maximum        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investments, per company $ 20,000   $ 20,000  
Investments, annual revenues per company $ 1,500,000      
Middle Market portfolio investments | Maximum | Debt investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investment, term 7 years      
Other Portfolio investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Number of portfolio companies | entity 4   4 4
Fair value $ 24,600   $ 24,600 $ 24,600
Cost $ 21,400   $ 21,400 $ 21,500
Other Portfolio investments | Investment Owned, At Fair Value | Investment Type Concentration Risk        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Concentration risk, percentage (as a percent) 2.20%     2.30%
Other Portfolio investments | Investment Owned, At Cost | Investment Type Concentration Risk        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Concentration risk, percentage (as a percent) 2.00%     2.10%
Other Portfolio investments | Minimum | Debt investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investment, term 5 years      
Other Portfolio investments | Maximum | Debt investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investment, term 10 years      
[1] Investment fair value was determined using significant unobservable inputs, unless otherwise noted. See Note C—Fair Value Hierarchy for Investments—Portfolio Composition for further discussion. Negative fair value is the result of the capitalized discount on the loan or the unfunded commitment being valued below par.
[2] Investment fair value was determined using significant unobservable inputs, unless otherwise noted. See Note C — Fair Value Hierarchy for Investments—Portfolio Composition for further discussion. Negative fair value is the result of the capitalized discount on the loan or the unfunded commitment being valued below par.
[3] Principal is net of repayments. Cost is net of repayments and accumulated unearned income. Negative cost is the result of the capitalized discount being greater than the principal amount outstanding on the loan.
[4] Principal is net of repayments. Cost is net of repayments and accumulated unearned income. Negative cost is the result of the capitalized discount being greater than the principal amount outstanding on the loan.