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 ($)
company
Mar. 31, 2022
Dec. 31, 2022
USD ($)
company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Achieved total return on investments 0.114 0.103 0.091
Fair value $ 1,067,094 [1]   $ 1,068,143 [2]
Cost $ 1,062,976 [3]   $ 1,059,886 [4]
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%
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%
Private Loan portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of portfolio companies | company 71   70
Fair value $ 567,700   $ 559,800
Cost $ 571,900   $ 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 24.00% 26.00%  
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 96.30%   96.20%
Private Loan portfolio investments | Minimum      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, per company $ 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, per company $ 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 49   48
Fair value $ 362,200   $ 352,700
Cost $ 316,600   $ 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 39.00% 40.00%  
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 71.60%   73.20%
LMM portfolio investments | Minimum      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, per company $ 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, per company $ 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 20   21
Fair value $ 109,500   $ 126,700
Cost $ 150,700   $ 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 94.70%   95.00%
Middle Market portfolio investments | Minimum      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, per company $ 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, per company $ 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 | company 4   4
Fair value $ 27,700   $ 29,000
Cost $ 23,700   $ 24,700
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 2.20%   2.30%
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 2.60%   2.70%
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.