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FAIR VALUE HIERARCHY FOR INVESTMENTS—PORTFOLIO COMPOSITION - Narrative (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
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Mar. 31, 2025 USD ($) investmentManager company entity | Mar. 31, 2024 | Mar. 31, 2025 USD ($) investmentManager company entity | Dec. 31, 2024 USD ($) company entity investmentManager
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Annualized total return on investments | 0.106 |  | 0.118 |  |  |  |
Achieved total return on investments |  |  |  |  |  | 0.124 |
Fair value | $ 1,260,853 | [1] |  | $ 1,260,853 | [1] | $ 1,177,507 | [2]
Cost | $ 1,203,330 | [3] |  | $ 1,203,330 | [3] | $ 1,138,745 | [4]
Concentration risk, percentage (as a percent) | 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 (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, percentage (as a percent) | 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 | 84 |  |  | 84 |  | 84 |
Fair value | $ 767,800 |  |  | $ 767,800 |  | $ 677,900 |
Cost | $ 790,000 |  |  | $ 790,000 |  | $ 697,500 |
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 (as a percent) | 26.00% |  | 21.00% | 88.00% |  |  |
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 (as a percent) | 93.50% |  |  |  |  | 93.90% |
Private Loan portfolio investments | Minimum |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Annual revenues | $ 25,000 |  |  |  |  |  |
Investments | $ 1,000 |  |  | $ 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] |  |  |  |  |  |  |
Annual revenues | $ 500,000 |  |  |  |  |  |
Investments | $ 30,000 |  |  | $ 30,000 |  |  |
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 | 57 |  |  | 57 |  | 57 |
Fair value | $ 439,700 |  |  | $ 439,700 |  | $ 436,100 |
Cost | $ 356,300 |  |  | $ 356,300 |  | $ 357,100 |
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 (as a percent) | 26.00% |  | 26.00% | 95.00% |  |  |
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 (as a percent) | 67.70% |  |  |  |  | 67.80% |
LMM portfolio investments | Minimum |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Annual revenues | $ 10,000 |  |  |  |  |  |
Investments | $ 1,000 |  |  | $ 1,000 |  |  |
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 | $ 30,000 |  |  | $ 30,000 |  |  |
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 | 8 |  |  | 8 |  | 10 |
Fair value | $ 30,800 |  |  | $ 30,800 |  | $ 39,400 |
Cost | $ 40,600 |  |  | 40,600 |  | $ 66,300 |
Middle Market 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 (as a percent) | 2.40% |  |  |  |  | 3.30% |
Middle Market 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 (as a percent) | 3.40% |  |  |  |  | 5.80% |
Middle Market portfolio investments | Minimum |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Investments | $ 1,000 |  |  | 1,000 |  |  |
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 | $ 20,000 |  |  | $ 20,000 |  |  |
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 investments in entities | entity | 6 |  |  | 6 |  | 6 |
Number of portfolio investments with investments managers | investmentManager | 4 |  |  | 4 |  | 4 |
Fair value | $ 22,600 |  |  | $ 22,600 |  | $ 24,100 |
Cost | $ 16,500 |  |  | $ 16,500 |  | $ 17,900 |
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 (as a percent) | 1.80% |  |  |  |  | 2.00% |
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 (as a percent) | 1.40% |  |  |  |  | 1.60% |
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. Negative cost is the result of the capitalized discount being greater than the principal amount outstanding on the loan.

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