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BORROWINGS - Narrative (Details) |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |  |  |  |  |  |  |  |  |  |
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May 11, 2022 USD ($) | Sep. 24, 2021 USD ($) | Aug. 09, 2021 USD ($) lender | Aug. 08, 2021 USD ($) lender | Apr. 20, 2021 USD ($) | Sep. 30, 2021 USD ($) | Dec. 31, 2020 USD ($) | Sep. 30, 2022 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2022 USD ($) | Sep. 30, 2021 USD ($) | May 10, 2022 USD ($) | Mar. 31, 2022 USD ($) | Jan. 28, 2022 USD ($) | Nov. 30, 2021 USD ($) | Aug. 31, 2021 USD ($) | May 25, 2021 USD ($) | Feb. 28, 2021 USD ($) | Aug. 31, 2020 USD ($) | Oct. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Apr. 25, 2019
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Asset coverage ratio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 166.00%
Current asset coverage ratio |  |  |  |  |  |  |  | 205.00% |  | 205.00% |  |  |  |  |  |  |  |  |  |  |  |
Recorded Value |  |  |  |  |  |  |  | $ 603,329,000 |  | $ 603,329,000 |  |  | $ 528,588,000 |  |  |  |  |  |  |  |  |
Realized loss on extinguishment of debt |  |  |  |  |  |  |  | 0 | $ (17,087,000) | 0 | $ (17,087,000) |  |  |  |  |  |  |  |  |  |  |
Outstanding Balance |  |  |  |  |  |  |  | 610,000,000 |  | 610,000,000 |  |  | 535,000,000 |  |  |  |  |  |  |  |  |
January 2026 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  | 140,000,000 |  | 140,000,000 |  |  | 140,000,000 |  |  |  |  |  |  |  |  |
October 2026 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  | 150,000,000 |  | 150,000,000 |  |  | 150,000,000 |  |  |  |  |  |  |  |  |
SBA Debentures |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  | 80,000,000 |  | 80,000,000 |  |  | 40,000,000 |  |  |  |  |  |  |  |  |
Notes | January 2026 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recorded Value |  |  |  |  |  |  |  | 138,883,000 |  | 138,883,000 |  |  | 138,714,000 |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  | 1,700,000 | 1,700,000 | 3,300,000 | 3,400,000 |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  | $ 75,000,000 |  |  |  |  |  |  |  |  |  |  | $ 65,000,000 |  |  |  |
Fixed Interest Rate |  |  |  |  |  |  | 4.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Average borrowings |  |  |  |  |  |  |  | $ 140,000,000 | 140,000,000 | $ 140,000,000 | 140,000,000 |  |  |  |  |  |  |  |  |  |  |
Weighted average interest rate |  |  |  |  |  |  |  | 4.46% |  | 4.46% |  |  |  |  |  |  |  |  |  |  |  |
Issuance price percent received |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 102.11% |  |  |  |
Yield-to-maturity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.00% |  |  |  |
Redemption price, percentage |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding Balance |  |  |  |  |  |  |  | $ 140,000,000 |  | $ 140,000,000 |  |  | 140,000,000 |  |  |  |  |  |  |  |  |
Notes | January 2026 Notes | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  |  |  |  |  |  | 120,600,000 |  | 120,600,000 |  |  |  |  |  |  |  |  |  |  |  |
Notes | October 2026 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recorded Value |  |  |  |  |  |  |  | 146,893,000 |  | 146,893,000 |  |  | 146,522,000 |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  | 1,400,000 | 400,000 | 2,900,000 | 400,000 |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000 | $ 100,000,000 |  |  |  |  |  |
Fixed Interest Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.375% | 3.375% |  |  |  |  |  |
Average borrowings |  |  |  |  |  | $ 100,000,000 |  | $ 150,000,000 |  | $ 150,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Issuance price percent received |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 99.993% | 99.418% |  |  |  |  |  |
Yield-to-maturity |  |  |  |  |  |  |  | 3.50% |  | 3.50% |  |  |  |  | 3.375% | 3.50% |  |  |  |  |  |
Redemption price, percentage |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding Balance |  |  |  |  |  |  |  | $ 150,000,000 |  | $ 150,000,000 |  |  | 150,000,000 |  |  |  |  |  |  |  |  |
Notes | October 2026 Notes | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  |  |  |  |  |  | 129,700,000 |  | 129,700,000 |  |  |  |  |  |  |  |  |  |  |  |
Notes | October 2024 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  |  | 1,700,000 |  | 3,600,000 |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000 | $ 10,000,000 | $ 65,000,000 |
Fixed Interest Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.375% |
Repurchased face amount |  | $ 125,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of principal amount redeemed |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unamortized debt issuance costs |  |  |  |  |  |  |  |  | 1,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Realized loss on extinguishment of debt |  |  |  |  |  |  |  |  | 15,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Average borrowings |  |  |  |  |  |  |  |  | $ 115,500,000 |  | $ 120,200,000 |  |  |  |  |  |  |  |  |  |  |
Weighted average interest rate |  |  |  |  |  | 5.375% |  |  | 5.375% |  | 5.375% |  |  |  |  |  |  |  |  |  |  |
Unsecured Debt | SBA Debentures |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recorded Value |  |  |  |  |  |  |  | 77,553,000 |  | 77,553,000 |  |  | 38,352,000 |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  | $ 600,000 | $ 46,000 | $ 1,000,000 | $ 53,700 |  |  |  |  |  |  |  |  |  |  |
Weighted average interest rate |  |  |  |  |  |  |  | 2.58% | 0.85% | 2.55% | 0.84% |  |  |  |  |  |  |  |  |  |  |
Average borrowings |  |  |  |  |  |  |  | $ 80,000,000 | $ 9,200,000 | $ 65,900,000 | $ 4,600,000 |  |  |  |  |  |  |  |  |  |  |
Debt instrument term |  |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory maximum borrowing capacity |  |  |  |  | $ 175,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory capital |  |  |  |  | $ 87,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Leverage commitment amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 40,000,000 |  |  | $ 40,000,000 |  |  |  |  |
Outstanding Balance |  |  |  |  |  |  |  | 80,000,000 |  | 80,000,000 |  |  | $ 40,000,000 |  |  |  |  |  |  |  |  |
Unsecured Debt | SBA Debentures | SBIC I |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory capital |  |  |  |  |  |  |  | 65,000,000 |  | 65,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Available leverage commitment |  |  |  |  |  |  |  | 50,000,000 |  | 50,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Unsecured Debt | SBA Debentures | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  |  |  |  |  |  | 73,800,000 |  | 73,800,000 |  |  |  |  |  |  |  |  |  |  |  |
Credit Facility | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
LIBOR floor |  |  | 0.00% | 0.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity | $ 380,000,000 |  | $ 335,000,000 | $ 340,000,000 |  |  |  |  |  |  |  | $ 335,000,000 |  |  |  |  |  |  |  |  |  |
Number of lenders | lender |  |  | 10 | 11 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Covenants increase the minimum obligors |  |  | $ 200,000,000 | $ 180,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accordion feature increase limit | $ 400,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum senior coverage ratio | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum asset coverage ratio | 1.50 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest coverage ratio | 2.25 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding advances exceed percentage | 90.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum liquidity | 0.10 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recorded Value |  |  |  |  |  |  |  | 240,000,000 |  | 240,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Amortization of deferred loan costs |  |  |  |  |  |  |  | 2,900,000 |  | 4,800,000 |  |  |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  | $ 1,500,000 |  | $ 2,900,000 |  |  |  |  |  |  |  |  |  |  |  |
Weighted average interest rate |  |  |  |  |  |  |  | 4.46% | 2.58% | 3.86% | 2.64% |  |  |  |  |  |  |  |  |  |  |
Average outstanding amount |  |  |  |  |  |  |  | $ 214,600,000 | $ 159,600,000 | $ 198,800,000 | $ 148,500,000 |  |  |  |  |  |  |  |  |  |  |
Outstanding Balance |  |  |  |  |  |  |  | $ 240,000,000 |  | $ 240,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Credit Facility | Line of Credit | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment fee percentage | 0.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit Facility | Line of Credit | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment fee percentage | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit Facility | Line of Credit | LIBOR |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  | 2.15% | 2.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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