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 6 Months Ended 12 Months Ended
Jun. 30, 2024
USD ($)
investment
entity
Jun. 30, 2023
USD ($)
Jun. 30, 2024
USD ($)
investment
entity
Jun. 30, 2023
USD ($)
Jun. 30, 2024
USD ($)
investment
entity
Dec. 31, 2023
USD ($)
investment
entity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Expenses allocated $ 5,887 $ 5,688 $ 11,446 $ 10,686    
Annualized total return on investments (as a percent) 0.157 0.167 0.162 0.151    
Achieved total return on investments (as a percent)           0.163
Fair value $ 4,737,511 [1]   $ 4,737,511 [1]   $ 4,737,511 [1] $ 4,286,271 [2]
Cost 4,114,074 [3]   $ 4,114,074 [3]   $ 4,114,074 [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 $ 5,900 $ 5,700 $ 11,400 $ 10,700    
Short-term portfolio investments            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Number of portfolio investments | investment 0   0   0 0
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]            
Fair value $ 2,440,000   $ 2,440,000   $ 2,440,000 $ 2,273,000
Cost $ 1,920,900   $ 1,920,900   $ 1,920,900 $ 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) 21.00% 23.00%     94.00%  
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)     72.60%     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   $ 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   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]            
Fair value 1,747,500   $ 1,747,500   1,747,500 $ 1,453,500
Cost $ 1,768,000   $ 1,768,000   $ 1,768,000 $ 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) 22.00% 24.00%     72.00%  
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)     95.40%     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   $ 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 100,000   $ 100,000   100,000  
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]            
Fair value 184,000   $ 184,000   184,000 $ 243,700
Cost 220,600   $ 220,600   220,600 $ 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)     88.30%     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   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   $ 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 investments in entities | entity 14   14   14 15
Fair value $ 179,500   $ 179,500   $ 179,500 $ 142,000
Cost 175,000   $ 175,000   175,000 $ 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)     3.80%     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)     4.30%     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 186,600   $ 186,600   186,600 $ 174,100
Cost $ 29,500   $ 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)     3.90%     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.