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 ($)
company
entity
Mar. 31, 2023
USD ($)
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]        
Expenses allocated $ 5,559 $ 4,998    
Annualized total return on investments (as a percent) 0.167 0.134    
Achieved total return on investments (as a percent)       0.163
Fair value $ 4,548,027 [1]   $ 4,548,027 [1] $ 4,286,271 [2]
Cost $ 3,947,530 [3]   $ 3,947,530 [3] $ 3,725,960 [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%
Affiliated Entity | External Investment Manager        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Expenses allocated $ 5,600 $ 5,000    
Short-term portfolio investments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Number of investments | entity       0
Number of portfolio companies | entity 11   11  
Fair value $ 103,400   $ 103,400  
Cost $ 103,300   $ 103,300  
Short-term 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) 2.30%      
Short-term 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) 2.60%      
Short-term portfolio investments | Maximum | Debt        
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 81   81 80
Fair value $ 2,361,500   $ 2,361,500 $ 2,273,000
Cost $ 1,850,300   $ 1,850,300 $ 1,782,900
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) 24.00% 30.00% 95.00%  
LMM portfolio investments | Debt | 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) 72.50%     72.00%
LMM portfolio investments | Minimum        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Annual revenues $ 10,000      
Investments $ 5,000   $ 5,000  
LMM portfolio investments | Minimum | Debt        
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      
Investments $ 100,000   $ 100,000  
LMM portfolio investments | Maximum | Debt        
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 88   88 87
Fair value $ 1,519,500   $ 1,519,500 $ 1,453,500
Cost $ 1,538,300   $ 1,538,300 $ 1,470,100
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) 27.00% 25.00% 79.00%  
Private Loan portfolio investments | Debt | 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%     94.70%
Private Loan portfolio investments | Minimum        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investments $ 10,000   $ 10,000  
Private Loan portfolio investments | Minimum | Debt        
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   $ 75,000  
Private Loan portfolio investments | Maximum | Debt        
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 22   22 23
Fair value $ 238,600   $ 238,600 $ 243,700
Cost $ 284,800   284,800 $ 294,400
Middle Market portfolio investments | Debt | 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) 91.00%     91.40%
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   3,000  
Middle Market portfolio investments | Minimum | Debt        
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   $ 25,000  
Middle Market portfolio investments | Maximum | Debt        
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 14   14 15
Fair value $ 139,800   $ 139,800 $ 142,000
Cost $ 141,200   141,200 $ 149,100
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) 3.10%     3.30%
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 (as a percent) 3.60%     4.00%
Other Portfolio investments | Minimum | Debt        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investment term (in years) 5 years      
Other Portfolio investments | Maximum | Debt        
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 $ 185,300   185,300 $ 174,100
Cost $ 29,500   $ 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%     4.10%
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) 0.70%     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.