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, 2022
USD ($)
company
investment
Dec. 31, 2021
USD ($)
company
investment
Dec. 31, 2020
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Expenses allocated $ 12,965 $ 10,277 $ 7,429
Achieved total return on investments 0.111 0.166  
Fair value $ 4,102,177 [1],[2] $ 3,561,831 [3],[4]  
Cost $ 3,773,752 [2],[5] $ 3,259,246 [4],[6]  
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%  
External Investment Manager | Affiliated Entity      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Expenses allocated $ 13,000 $ 10,300 $ 7,400
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 investments      
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 78 73  
Fair value $ 2,060,500 $ 1,716,400  
Cost $ 1,719,900 $ 1,455,700  
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 94.00% 81.00%  
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 73.70% 70.90%  
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 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 investments      
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 85 75  
Fair value $ 1,471,500 $ 1,141,800  
Cost $ 1,500,300 $ 1,157,500  
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 76.00% 60.00%  
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 97.10% 95.70%  
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 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 7 years    
Middle Market portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of portfolio companies | company 31 36  
Fair value $ 329,100 $ 395,200  
Cost $ 401,700 $ 440,900  
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 93.80% 93.30%  
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 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 7 years    
Other portfolio investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of portfolio companies | company 14 13  
Fair value $ 116,300 $ 166,100  
Cost $ 120,400 $ 173,700  
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% 4.70%  
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% 5.30%  
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    
External investment manager      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 122,900 $ 140,400  
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, percentage 3.00% 3.90%  
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, percentage 0.80% 0.90%  
[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] All investments are Lower Middle Market portfolio investments, unless otherwise noted. See Note C — Fair Value Hierarchy for Investments — Portfolio Composition for a description of Lower Middle Market portfolio investments. All of the Company’s investments, unless otherwise noted, are encumbered either as security for the Company’s Corporate Facility or SPV Facility (each as defined in Note B.5. — Deferred Financing Costs, and together the “Credit Facilities”) or in support of the SBA-guaranteed debentures issued by the Funds.
[3] 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.
[4] All investments are Lower Middle Market portfolio investments, unless otherwise noted. See Note C — Fair Value Hierarchy for Investments — Portfolio Composition for a description of Lower Middle Market portfolio investments. All of the Company’s investments, unless otherwise noted, are encumbered either as security for the Company’s Corporate Facility or in support of the SBA-guaranteed debentures issued by the Funds.
[5] 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.
[6] Principal is net of repayments. Cost is net of repayments and accumulated unearned income.