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INVESTMENTS - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |  |
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Jun. 30, 2023 | Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022 | Mar. 31, 2023 | Jun. 30, 2022 | Mar. 31, 2022
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |
Cost |  | $ 834,798 |  | $ 834,798 |  | $ 720,630 | [1] |  |
Fair Value |  | 915,636 |  | 915,636 |  | 753,543 | [1] |  |
Old World Christmas, Inc. |  |  |  |  |  |  |  |  |
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |
Realized gain (loss) on preferred equity | $ 300 |  |  | 300 |  |  |  |  |
Horizon Facilities Services, Inc. |  |  |  |  |  |  |  |  |
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |
Realized gain (loss) on preferred equity |  |  | $ (10,100) |  | $ (10,100) |  |  |  |
Ginsey Home Solutions, Inc. |  |  |  |  |  |  |  |  |
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |
Cost |  |  | 12,200 |  | 12,200 |  |  |  |
Fair Value |  |  | 12,200 |  | 12,200 |  |  |  |
PSI Molded Plastics, Inc |  |  |  |  |  |  |  |  |
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |
Cost |  |  | 26,600 |  | 26,600 |  |  |  |
Fair Value |  |  | 26,600 |  | 26,600 |  |  |  |
Total |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |
Beginning balance | 800,047 | 915,614 | 737,877 | 915,614 | 737,877 | 753,516 |  | $ 689,425 | $ 714,322
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |
Issuances / originations |  | 66,813 | 74,250 | 114,788 | 102,055 |  |  |  |
Settlements / repayments |  | 0 | (5,096) | 0 | (53,096) |  |  |  |
Sales |  | 0 | (12,298) | (1,775) | (21,926) |  |  |  |
Transfers |  | 0 | 0 | 0 | 0 |  |  |  |
Ending balance | 800,047 | 915,614 | 737,877 | 915,614 | 737,877 |  |  |  |
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 | 2,218 | 1,155 | 6,946 |  |  |  |
Total | Debt and Equity Securities, Unrealized Gain (Loss) |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |
Total gain (loss) |  | 48,754 | (10,622) | 48,023 | 1,826 |  |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization |  | 0 | 0 | (93) | (12,250) |  |  |  |
Secured First Lien Debt |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |
Beginning balance | 452,215 | 508,504 | 420,907 | 508,504 | 420,907 | 437,517 |  | 376,752 | 425,087
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |
Issuances / originations |  | 55,400 | 70,250 | 70,100 | 77,050 |  |  |  |
Settlements / repayments |  | 0 | 0 | 0 | (48,000) |  |  |  |
Sales |  | 0 | 0 | 0 | 0 |  |  |  |
Transfers |  | 0 | (14,418) | 0 | (14,418) |  |  |  |
Ending balance | 452,215 | 508,504 | 420,907 | 508,504 | 420,907 |  |  |  |
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 | 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) |  | 889 | (11,677) | 887 | (18,812) |  |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization |  | 0 | 0 | 0 | 0 |  |  |  |
Secured Second Lien Debt |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |
Beginning balance | 104,794 | 102,747 | 76,751 | 102,747 | 76,751 | 75,734 |  | 67,936 | 67,958
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |
Issuances / originations |  | 0 | 4,000 | 25,000 | 4,005 |  |  |  |
Settlements / repayments |  | 0 | (5,096) | 0 | (5,096) |  |  |  |
Sales |  | 0 | 0 | 0 | 0 |  |  |  |
Transfers |  | 0 | 14,418 | 0 | 14,418 |  |  |  |
Ending balance | 104,794 | 102,747 | 76,751 | 102,747 | 76,751 |  |  |  |
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 | 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) |  | (2,047) | (4,507) | 2,013 | (4,534) |  |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization |  | 0 | 0 | 0 | 0 |  |  |  |
Preferred Equity |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |
Beginning balance | 214,258 | 267,596 | 229,430 | 267,596 | 229,430 | 222,585 |  | 238,665 | 217,599
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |
Issuances / originations |  | 11,413 | 0 | 14,688 | 21,000 |  |  |  |
Settlements / repayments |  | 0 | 0 | 0 | 0 |  |  |  |
Sales |  | 0 | (12,298) | (273) | (21,926) |  |  |  |
Transfers |  | 0 | 0 | 0 | 0 |  |  |  |
Ending balance | 214,258 | 267,596 | 229,430 | 267,596 | 229,430 |  |  |  |
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 | 2,218 | 273 | 6,946 |  |  |  |
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) |  | 41,925 | 845 | 30,323 | 18,061 |  |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization |  | 0 | 0 | 0 | (12,250) |  |  |  |
Common Equity/ Equivalents |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |
Beginning balance | 28,780 | 36,767 | 10,789 | 36,767 | 10,789 | $ 17,680 |  | $ 6,072 | $ 3,678
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |
Issuances / originations |  | 0 | 0 | 5,000 | 0 |  |  |  |
Settlements / repayments |  | 0 | 0 | 0 | 0 |  |  |  |
Sales |  | 0 | 0 | (1,502) | 0 |  |  |  |
Transfers |  | 0 | 0 | 0 | 0 |  |  |  |
Ending balance | $ 28,780 | 36,767 | 10,789 | 36,767 | 10,789 |  |  |  |
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 | 882 | 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) |  | 7,987 | 4,717 | 14,800 | 7,111 |  |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization |  | $ 0 | $ 0 | $ (93) | $ 0 |  |  |  |
[1] | Cumulative gross unrealized appreciation for federal income tax purposes is $150.4 million; cumulative gross unrealized depreciation for federal income tax purposes is $119.3 million. Cumulative net unrealized appreciation is $31.1 million, based on a tax cost of $722.4 million.

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