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INVESTMENTS - Schedule of Changes in Level 3 Fair Value Measurements of Investments (Details) - USD ($) $ in Thousands | 3 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Mar. 31, 2025 | Mar. 31, 2024
New investments, repayments and settlements: |  |  |  |  |
Cost | $ 997,538 |  | $ 939,066 | [1] |
PSI Molded |  |  |  |  |
New investments, repayments and settlements: |  |  |  |  |
Cost | 10,600 |  |  |  |
Total |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance | 1,031,735 | $ 899,138 | 974,345 |  | $ 920,486
New investments, repayments and settlements: |  |  |  |  |
Issuances / originations | 62,842 | 598 |  |  |
Settlements / repayments | (4,370) | (3,000) |  |  |
Sales | 0 | 0 |  |  |
Transfers | 0 | 0 |  |  |
Ending balance | 1,031,735 | 899,138 |  |  |
Total | Debt and Equity Securities, Realized Gain (Loss) |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Total gain (loss) | 0 | 0 |  |  |
Total | Debt and Equity Securities, Unrealized Gain (Loss) |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Total gain (loss) | (1,082) | (18,946) |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization | 0 | 0 |  |  |
Secured First Lien Debt |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance | 557,057 | 463,219 | 514,334 |  | 474,856
New investments, repayments and settlements: |  |  |  |  |
Issuances / originations | 45,941 | 598 |  |  |
Settlements / repayments | (4,370) | (3,000) |  |  |
Sales | 0 | 0 |  |  |
Transfers | 0 | 0 |  |  |
Ending balance | 557,057 | 463,219 |  |  |
Secured First Lien Debt | Debt and Equity Securities, Realized Gain (Loss) |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Total gain (loss) | 0 | 0 |  |  |
Secured First Lien Debt | Debt and Equity Securities, Unrealized Gain (Loss) |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Total gain (loss) | 1,152 | (9,235) |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization | 0 | 0 |  |  |
Secured Second Lien Debt |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance | 93,340 | 137,827 | 103,580 |  | 138,703
New investments, repayments and settlements: |  |  |  |  |
Issuances / originations | 0 | 0 |  |  |
Settlements / repayments | 0 | 0 |  |  |
Sales | 0 | 0 |  |  |
Transfers | (10,616) | 0 |  |  |
Ending balance | 93,340 | 137,827 |  |  |
Secured Second Lien Debt | PSI Molded |  |  |  |  |
New investments, repayments and settlements: |  |  |  |  |
Cost | 10,600 |  |  |  |
Secured Second Lien Debt | Debt and Equity Securities, Realized Gain (Loss) |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Total gain (loss) | 0 | 0 |  |  |
Secured Second Lien Debt | Debt and Equity Securities, Unrealized Gain (Loss) |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Total gain (loss) | 376 | (876) |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization | 0 | 0 |  |  |
Preferred Equity |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance | 325,997 | 212,591 | 302,163 |  | 213,480
New investments, repayments and settlements: |  |  |  |  |
Issuances / originations | 16,901 | 0 |  |  |
Settlements / repayments | 0 | 0 |  |  |
Sales | 0 | 0 |  |  |
Transfers | 10,616 | 0 |  |  |
Ending balance | 325,997 | 212,591 |  |  |
Preferred Equity | Debt and Equity Securities, Realized Gain (Loss) |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Total gain (loss) | 0 | 0 |  |  |
Preferred Equity | Debt and Equity Securities, Unrealized Gain (Loss) |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Total gain (loss) | (3,683) | (889) |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization | 0 | 0 |  |  |
Common Equity/ Equivalents |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance | 55,341 | 85,501 | $ 54,268 |  | $ 93,447
New investments, repayments and settlements: |  |  |  |  |
Issuances / originations | 0 | 0 |  |  |
Settlements / repayments | 0 | 0 |  |  |
Sales | 0 | 0 |  |  |
Transfers | 0 | 0 |  |  |
Ending balance | 55,341 | 85,501 |  |  |
Common Equity/ Equivalents | Debt and Equity Securities, Realized Gain (Loss) |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Total gain (loss) | 0 | 0 |  |  |
Common Equity/ Equivalents | Debt and Equity Securities, Unrealized Gain (Loss) |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Total gain (loss) | 1,073 | (7,946) |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization | $ 0 | $ 0 |  |  |
[1] | Cumulative gross unrealized appreciation for federal income tax purposes is $183.3 million; cumulative gross unrealized depreciation for federal income tax purposes is $144.9 million. Cumulative net unrealized appreciation is $38.5 million, based on a tax cost of $940.9 million.

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