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 ($)
company
investment
entity
Dec. 31, 2022
USD ($)
entity
company
investment
Dec. 31, 2021
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Expenses allocated $ 22,050 $ 12,965 $ 10,277
Achieved total return on investments (as a percent) 0.163 0.111  
Fair value $ 4,286,271 [1] $ 4,102,177 [2]  
Cost $ 3,725,960 [3] $ 3,773,752 [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 (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 (as a percent) 100.00% 100.00%  
External Investment Manager | Related Party      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Expenses allocated $ 22,100 $ 13,000 $ 10,300
Short-term portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of investments | investment 0 1  
Short-term portfolio investments | Maximum | Debt investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term (in years) 1 year    
LMM portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of portfolio companies | company 80 78  
Fair value $ 2,273,000 $ 2,060,500  
Cost $ 1,782,900 $ 1,719,900  
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 (as a percent) 95.00% 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 (as a percent) 72.00% 73.70%  
LMM portfolio investments | Minimum      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Annual revenues $ 10,000    
Investments $ 5,000    
LMM portfolio investments | Minimum | Debt investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term (in years) 5 years    
LMM portfolio investments | Maximum      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Annual revenues $ 150,000    
LMM portfolio investments | Maximum | Debt investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term (in years) 7 years    
Private Loan portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of portfolio companies | company 87 85  
Fair value $ 1,453,500 $ 1,471,500  
Cost $ 1,470,100 $ 1,500,300  
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 (as a percent) 82.00% 76.00%  
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 (as a percent) 94.70% 97.10%  
Private Loan portfolio investments | Minimum      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments $ 10,000    
Private Loan portfolio investments | Minimum | Debt investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term (in years) 3 years    
Private Loan portfolio investments | Maximum      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments $ 75,000    
Private Loan portfolio investments | Maximum | Debt investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term (in years) 7 years    
Middle Market portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of portfolio companies | company 23 31  
Fair value $ 243,700 $ 329,100  
Cost $ 294,400 $ 401,700  
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 (as a percent) 91.40% 93.80%  
Middle Market portfolio investments | Minimum      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Annual revenues $ 150,000    
Investments $ 3,000    
Middle Market portfolio investments | Minimum | Debt investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term (in years) 3 years    
Middle Market portfolio investments | Maximum      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Annual revenues $ 1,500,000    
Investments $ 25,000    
Middle Market portfolio investments | Maximum | Debt investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term (in years) 7 years    
Other Portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of portfolio companies | entity 15 14  
Fair value $ 142,000 $ 116,300  
Cost $ 149,100 $ 120,400  
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 (as a percent) 3.30% 2.80%  
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 (as a percent) 4.00% 3.20%  
Other Portfolio investments | Minimum | Debt investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term (in years) 5 years    
Other Portfolio investments | Maximum | Debt investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term (in years) 10 years    
External Investment Manager      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 174,100 $ 122,900  
Cost $ 29,500 $ 29,500  
External Investment Manager | Investment Owned, At Fair Value | Investment Type Concentration Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Concentration risk (as a percent) 4.10% 3.00%  
External Investment Manager | Investment Owned, At Cost | Investment Type Concentration Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Concentration risk (as a percent) 0.80% 0.80%  
[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.