Annual report pursuant to Section 13 and 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
 12 Months Ended
Dec. 31, 2023
USD ($)
company
entity
Dec. 31, 2022
USD ($)
entity
company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Achieved total return on investments 0.136 0.091
Fair value $ 1,092,895 [1] $ 1,068,143 [2]
Cost $ 1,038,318 [3] $ 1,059,886 [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 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 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 78 70
Fair value $ 595,300 $ 559,800
Cost $ 586,400 $ 563,000
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 79.00% 83.00%
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 94.10% 96.20%
Private Loan portfolio investments | Minimum    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, made $ 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, made $ 20,000  
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 50 48
Fair value $ 387,000 $ 352,700
Cost $ 315,700 $ 312,500
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 95.00% 97.00%
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 70.20% 73.20%
LMM portfolio investments | Minimum    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, made $ 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, made   $ 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 16 21
Fair value $ 86,000 $ 126,700
Cost $ 114,700 $ 159,700
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 93.10% 95.00%
Middle Market portfolio investments | Minimum    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, made $ 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, made $ 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
Fair value $ 24,600 $ 29,000
Cost $ 21,500 $ 24,700
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 2.30% 2.70%
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 2.10% 2.30%
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.