Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUE HIERARCHY FOR INVESTMENTS???PORTFOLIO COMPOSITION - Schedule of Investments (Details)

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FAIR VALUE HIERARCHY FOR INVESTMENTS—PORTFOLIO COMPOSITION - Schedule of Investments (Details)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2024
USD ($)
company
Dec. 31, 2023
USD ($)
company
Summary of Investment Holdings [Line Items]    
Fair value $ 4,548,027 [1] $ 4,286,271 [2]
Cost $ 3,947,530 [3] $ 3,725,960 [4]
Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 100.00% 100.00%
Non-accrual Status | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 2.00% 2.30%
Equity | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 16.50% 16.80%
First lien debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 83.00% 82.70%
LMM portfolio investments    
Summary of Investment Holdings [Line Items]    
Number of portfolio companies | company 81 80
Fair value $ 2,361,500 $ 2,273,000
Cost $ 1,850,300 $ 1,782,900
Weighted-average annual effective yield (as a percent) 12.80% 13.00%
Average EBITDA $ 8,700 $ 8,200
Diluted equity ownership (as a percent) 40.00% 40.00%
Investment company, number of companies within portfolio excluded from EBITDA calculation | company 4 2
LMM portfolio investments | Non-accrual Status    
Summary of Investment Holdings [Line Items]    
Weighted-average annual effective yield (as a percent) 12.70% 12.90%
LMM portfolio investments | Debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 72.50% 72.00%
LMM portfolio investments | Equity | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 27.50% 28.00%
LMM portfolio investments | First lien debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 99.20% 99.20%
Private Loan portfolio investments    
Summary of Investment Holdings [Line Items]    
Number of portfolio companies | company 88 87
Fair value $ 1,519,500 $ 1,453,500
Cost $ 1,538,300 $ 1,470,100
Weighted-average annual effective yield (as a percent) 12.80% 12.90%
Average EBITDA $ 32,800 $ 27,200
Investment company, number of companies within portfolio excluded from EBITDA calculation | company 2 2
Private Loan portfolio investments | Non-accrual Status    
Summary of Investment Holdings [Line Items]    
Weighted-average annual effective yield (as a percent) 12.40% 12.50%
Private Loan portfolio investments | Debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 94.70% 94.70%
Private Loan portfolio investments | Equity | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 5.30% 5.30%
Private Loan portfolio investments | First lien debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 99.90% 100.00%
Middle Market portfolio investments    
Summary of Investment Holdings [Line Items]    
Number of portfolio companies | company 22 23
Fair value $ 238,600 $ 243,700
Cost $ 284,800 $ 294,400
Weighted-average annual effective yield (as a percent) 12.90% 12.50%
Average EBITDA $ 61,100 $ 64,200
Investment company, number of companies within portfolio excluded from EBITDA calculation | company 1  
Middle Market portfolio investments | Non-accrual Status    
Summary of Investment Holdings [Line Items]    
Weighted-average annual effective yield (as a percent) 11.40% 10.80%
Middle Market portfolio investments | Debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 91.00% 91.40%
Middle Market portfolio investments | Equity | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 9.00% 8.60%
Middle Market portfolio investments | First lien debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk (as a percent) 99.00% 99.10%
[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.