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BORROWINGS - Narrative (Details) |  |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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Mar. 20, 2024 USD ($) | Mar. 01, 2024 | Aug. 02, 2023 USD ($) | Apr. 20, 2021 USD ($) | Aug. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Sep. 30, 2024 USD ($) facility | Sep. 30, 2023 USD ($) | Sep. 30, 2024 USD ($) facility | Sep. 30, 2023 USD ($) | Sep. 12, 2024 USD ($) lender | Sep. 11, 2024 USD ($) | Jun. 20, 2024 USD ($) | Jun. 19, 2024 USD ($) | Dec. 20, 2023 USD ($) | Dec. 07, 2023 USD ($) lender | Dec. 06, 2023 USD ($) | Aug. 01, 2023 USD ($) | Jun. 30, 2023 USD ($) | Nov. 22, 2022 USD ($) | Jan. 28, 2022 USD ($) | Nov. 30, 2021 USD ($) | May 25, 2021 USD ($) | Feb. 28, 2021 USD ($) | Apr. 25, 2019
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Asset coverage ratio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 166.00%
Current asset coverage ratio |  |  |  |  |  |  | 224.00% |  | 224.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
January 2026 Notes | Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average effective interest rate |  |  |  |  |  |  | 4.46% | 4.46% | 4.46% | 4.46% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  | $ 75,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 65,000,000 |
Fixed interest rate |  |  |  |  |  | 4.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance price percent received |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 102.11% |
Yield-to-maturity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.00% |
Redemption price, percentage |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
October 2026 Notes | Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average effective interest rate |  |  |  |  |  |  | 3.50% | 3.50% | 3.50% | 3.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000 |  |  |
Fixed interest rate |  |  |  |  | 3.375% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.375% |  |  |
Issuance price percent received |  |  |  |  | 99.418% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 99.993% |  |  |
Yield-to-maturity |  |  |  |  | 3.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.375% |  |  |
Redemption price, percentage |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
August 2028 Notes | Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average effective interest rate |  |  |  |  |  |  | 7.75% | 7.75% | 7.75% | 7.75% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 71,900,000 |  |  |  |  |  |
Fixed interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.75% |  |  |  |  |  |
August 2028 Notes Overallotment | Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 9,400,000 |  |  |  |  |  |
SBA Debentures | Unsecured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average effective interest rate |  |  |  |  |  |  | 4.43% | 4.23% | 4.41% | 4.09% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SBA Debentures | Unsecured Debt | SBIC I |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory maximum borrowing capacity |  |  |  | $ 175,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory capital |  |  |  | $ 87,500,000 |  |  | $ 87,500,000 |  | $ 87,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Leverage commitment amount |  |  |  |  |  |  | 175,000,000 |  | 175,000,000 |  |  |  |  |  | $ 45,000,000 |  |  |  |  | $ 50,000,000 | $ 40,000,000 |  | $ 40,000,000 |  |
Available leverage commitment |  |  |  |  |  |  | 87,500,000 |  | 87,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unused commitments |  |  |  |  |  |  | $ 22,000,000 |  | $ 22,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate Credit Facility | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average effective interest rate |  |  |  |  |  |  | 7.77% |  | 7.78% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Average borrowings |  |  |  |  |  |  | $ 237,500,000 |  | $ 237,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate Credit Facility | Corporate Credit Facility | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of revolving credit facility | facility |  |  |  |  |  |  | 1 |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average effective interest rate |  |  |  |  |  |  | 7.74% | 7.70% | 7.75% | 7.52% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Average borrowings |  |  |  |  |  |  | $ 166,685,000 | $ 185,054,000 | $ 178,579,000 | $ 106,120,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  | 2.15% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  | $ 435,000,000 |  |  |  |  |  |  |  | $ 485,000,000 | $ 460,000,000 |  |  |  | $ 460,000,000 | $ 435,000,000 | $ 400,000,000 |  |  |  |  |  |  |
Accordion feature increase limit |  |  | $ 750,000,000 |  |  |  |  |  |  |  | $ 25,000,000 |  |  |  |  | $ 25,000,000 |  |  |  |  |  |  |  |  |
Number of lenders | lender |  |  |  |  |  |  |  |  |  |  | 11 |  |  |  |  | 10 |  |  |  |  |  |  |  |  |
Minimum senior coverage ratio |  | 2.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum asset coverage ratio |  | 1.50 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest coverage ratio |  | 2.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate Credit Facility | Corporate Credit Facility | Line of Credit | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment fee percentage |  | 0.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate Credit Facility | Corporate Credit Facility | Line of Credit | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment fee percentage |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt | SPV Credit Facility | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of revolving credit facility | facility |  |  |  |  |  |  | 1 |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average effective interest rate |  |  |  |  |  |  | 7.85% | 0.00% | 7.87% | 0.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Average borrowings |  |  |  |  |  |  | $ 70,826,000 | $ 0 | $ 58,617,000 | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity | $ 150,000,000 |  |  |  |  |  |  |  |  |  |  |  |  | $ 150,000,000 |  |  |  |  |  |  |  |  |  |  |
Accordion feature increase limit |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Accordion feature that allows increases (up to) | $ 400,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt | SPV Credit Facility | Line of Credit | Before March 20 2027 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate | 2.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt | SPV Credit Facility | Line of Credit | After March 20 2027 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate | 2.85% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt | SPV Credit Facility | Line of Credit | Before April 20 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment fee percentage | 0.10% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt | SPV Credit Facility | Line of Credit | After April 20 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment fee percentage | 0.35% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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