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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 | 9 Months Ended |  |  |  |
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Jun. 30, 2023 | Dec. 31, 2023 | Dec. 31, 2022 | Sep. 30, 2022 | Sep. 30, 2022 | Dec. 31, 2023 | Dec. 31, 2022 | Sep. 30, 2023 | Mar. 31, 2023 | Aug. 31, 2022 | Mar. 31, 2022
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |  |  |  |
Cost |  | $ 868,504 |  |  |  | $ 868,504 |  |  | $ 720,630 | [1] |  |
Fair Value |  | 902,808 |  |  |  | 902,808 |  |  | 753,543 | [1] |  |
Old World Christmas, Inc. |  |  |  |  |  |  |  |  |  |  |  |
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |  |  |  |
Realized gain (loss) on preferred equity | $ 300 |  |  | $ 13,400 | $ 13,400 | 300 |  |  |  |  |  |
Horizon Facilities Services, Inc. |  |  |  |  |  |  |  |  |  |  |  |
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |  |  |  |
Realized gain (loss) on preferred equity |  |  |  | (12,300) | (12,300) |  |  |  |  |  |  |
Ginsey Home Solutions, Inc. |  |  |  |  |  |  |  |  |  |  |  |
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |  |  |  |
Cost |  |  |  |  |  |  |  |  |  |  | $ 12,200 |
Fair Value |  |  |  |  |  |  |  |  |  |  | $ 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 |  | 902,786 | $ 760,432 | 737,877 | 737,877 | 902,786 | $ 760,432 | $ 915,614 | 753,516 |  |  | $ 714,322
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |  |  |  |
Issuances / originations |  | 68,200 | 31,463 |  |  | 182,988 | 133,518 |  |  |  |  |
Settlements / repayments |  | (27,500) | (2,300) |  |  | (27,500) | (55,396) |  |  |  |  |
Sales |  | (50,453) | (13,372) |  |  | (52,228) | (35,298) |  |  |  |  |
Transfers |  | 0 | 0 |  |  | 0 | 0 |  |  |  |  |
Ending balance |  | 902,786 | 760,432 | 737,877 | 737,877 | 902,786 | 760,432 |  |  |  |  |
Total | Debt and Equity Securities, Realized Gain (Loss) |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |
Total gain (loss) |  | 43,459 | 3,372 |  |  | 44,614 | 10,318 |  |  |  |  |
Total | Debt and Equity Securities, Unrealized Gain (Loss) |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |
Total gain (loss) |  | (2,968) | (6,609) |  |  | 45,055 | (4,783) |  |  |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization |  | (43,566) | 10,001 |  |  | (43,659) | (2,249) |  |  |  |  |
Secured First Lien Debt |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  | 476,126 | 447,491 | 420,907 | 420,907 | 476,126 | 447,491 | 508,504 | 437,517 |  |  | 425,087
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |  |  |  |
Issuances / originations |  | 3,500 | 29,900 |  |  | 73,600 | 106,950 |  |  |  |  |
Settlements / repayments |  | (27,500) | (800) |  |  | (27,500) | (48,800) |  |  |  |  |
Sales |  | 0 | 0 |  |  | 0 | 0 |  |  |  |  |
Transfers |  | 0 | 0 |  |  | 0 | (14,418) |  |  |  |  |
Ending balance |  | 476,126 | 447,491 | 420,907 | 420,907 | 476,126 | 447,491 |  |  |  |  |
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) |  | (7,040) | (2,516) |  |  | (6,153) | (21,328) |  |  |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization |  | (1,338) | 0 |  |  | (1,338) | 0 |  |  |  |  |
Secured Second Lien Debt |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  | 137,480 | 76,169 | 76,751 | 76,751 | 137,480 | 76,169 | 102,747 | 75,734 |  |  | 67,958
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |  |  |  |
Issuances / originations |  | 39,000 | 1,183 |  |  | 64,000 | 5,188 |  |  |  |  |
Settlements / repayments |  | 0 | (1,500) |  |  | 0 | (6,596) |  |  |  |  |
Sales |  | 0 | 0 |  |  | 0 | 0 |  |  |  |  |
Transfers |  | 0 | 0 |  |  | 0 | 14,418 |  |  |  |  |
Ending balance |  | 137,480 | 76,169 | 76,751 | 76,751 | 137,480 | 76,169 |  |  |  |  |
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 | (10,000) |  |  | 0 | (10,000) |  |  |  |  |
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) |  | (4,267) | (266) |  |  | (2,254) | (4,800) |  |  |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization |  | 0 | 10,001 |  |  | 0 | 10,001 |  |  |  |  |
Preferred Equity |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  | 214,664 | 221,774 | 229,430 | 229,430 | 214,664 | 221,774 | 267,596 | 222,585 |  |  | 217,599
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |  |  |  |
Issuances / originations |  | 0 | 0 |  |  | 14,688 | 21,000 |  |  |  |  |
Settlements / repayments |  | 0 | 0 |  |  | 0 | 0 |  |  |  |  |
Sales |  | (50,453) | (13,372) |  |  | (50,726) | (35,298) |  |  |  |  |
Transfers |  | (8,621) | 0 |  |  | (8,621) | 0 |  |  |  |  |
Ending balance |  | 214,664 | 221,774 | 229,430 | 229,430 | 214,664 | 221,774 |  |  |  |  |
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) |  | 43,459 | 13,372 |  |  | 43,732 | 20,318 |  |  |  |  |
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) |  | 4,911 | (7,656) |  |  | 35,234 | 10,405 |  |  |  |  |
Reversal of previously recorded (appreciation) depreciation upon realization |  | (42,228) | 0 |  |  | (42,228) | (12,250) |  |  |  |  |
Common Equity/ Equivalents |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  | 74,516 | 14,998 | 10,789 | 10,789 | 74,516 | 14,998 | $ 36,767 | $ 17,680 |  |  | $ 3,678
New investments, repayments and settlements: |  |  |  |  |  |  |  |  |  |  |  |
Issuances / originations |  | 25,700 | 380 |  |  | 30,700 | 380 |  |  |  |  |
Settlements / repayments |  | 0 | 0 |  |  | 0 | 0 |  |  |  |  |
Sales |  | 0 | 0 |  |  | (1,502) | 0 |  |  |  |  |
Transfers |  | 8,621 | 0 |  |  | 8,621 | 0 |  |  |  |  |
Ending balance |  | 74,516 | 14,998 | $ 10,789 | $ 10,789 | 74,516 | 14,998 |  |  |  |  |
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) |  | 3,428 | 3,829 |  |  | 18,228 | 10,940 |  |  |  |  |
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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