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, 2023
USD ($)
company
Mar. 31, 2022
USD ($)
investment
Dec. 31, 2022
USD ($)
investment
company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Expenses allocated $ 4,998 $ 2,817  
Annualized total return on investments 0.134 0.118  
Achieved total return on investments     0.111
Fair value $ 4,160,917 $ 4,160,917 [1] $ 4,102,177 [2]
Cost $ 3,797,714 3,797,714 [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, 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%
Affiliated Entity | External Investment Manager      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Expenses allocated $ 5,000 $ 2,800  
Short-term portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of investments | investment   1 1
Short-term portfolio investments | Maximum | Debt      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term 1 year    
LMM portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of portfolio companies | company 79   78
Fair value $ 2,112,600   $ 2,060,500
Cost $ 1,727,900   $ 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, percentage 30.00% 30.00%  
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, percentage 72.90%   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      
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]      
Annual revenues $ 150,000    
Investments $ 75,000    
LMM portfolio investments | Maximum | Debt      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term 7 years    
Private Loan portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of portfolio companies | company 86   85
Fair value $ 1,491,400   $ 1,471,500
Cost $ 1,527,600   $ 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, percentage 25.00% 26.00%  
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, percentage 97.10%   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      
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 $ 75,000    
Private Loan portfolio investments | Maximum | Debt      
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 30   31
Fair value $ 306,200   $ 329,100
Cost $ 391,000   $ 401,700
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, percentage 93.60%   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      
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]      
Annual revenues $ 1,500,000    
Investments $ 25,000    
Middle Market portfolio investments | Maximum | Debt      
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 | company 14   14
Fair value $ 116,100   $ 116,300
Cost $ 119,700   $ 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, percentage 2.80%   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, percentage 3.20%   3.20%
Other Portfolio investments | Minimum | Debt      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term 5 years    
Other Portfolio investments | Maximum | Debt      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment term 10 years    
External Investment Manager investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 132,700   $ 122,900
Cost $ 29,500   $ 29,500
External Investment Manager 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 3.20%   3.00%
External Investment Manager 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 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.