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BORROWINGS - Narrative (Details) |  |  |  | 1 Months Ended | 3 Months Ended | 4 Months Ended | 6 Months Ended |  |  |  |  |  |  |  |  |
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Aug. 02, 2023 USD ($) | May 11, 2022 | Apr. 20, 2021 USD ($) | Aug. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Sep. 30, 2023 USD ($) | Sep. 30, 2022 USD ($) | Sep. 30, 2023 USD ($) | Sep. 30, 2023 USD ($) | Sep. 30, 2022 USD ($) | Aug. 01, 2023 USD ($) | Jun. 30, 2023 USD ($) | Mar. 31, 2023 USD ($) | 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 |  |  |  |  |  | 229.00% |  | 229.00% | 229.00% |  |  |  |  |  |  |  |  |  |
Recorded value |  |  |  |  |  | $ 732,667,000 |  | $ 732,667,000 | $ 732,667,000 |  |  |  | $ 637,644,000 |  |  |  |  |  |
Outstanding balance |  |  |  |  |  | 741,875,000 |  | 741,875,000 | 741,875,000 |  |  |  | 645,000,000 |  |  |  |  |  |
January 2026 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  | 140,000,000 |  | 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 |  |  |  | 150,000,000 |  |  |  |  |  |
August 2028 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  | 71,875,000 |  | 71,875,000 | 71,875,000 |  |  |  | 0 |  |  |  |  |  |
SBA Debentures |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  | 130,000,000 |  | 130,000,000 | 130,000,000 |  |  |  | 120,000,000 |  |  |  |  |  |
Notes | January 2026 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recorded value |  |  |  |  |  | 139,220,000 |  | 139,220,000 | 139,220,000 |  |  |  | 139,051,000 |  |  |  |  |  |
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% |
Outstanding balance |  |  |  |  |  | $ 140,000,000 |  | $ 140,000,000 | $ 140,000,000 |  |  |  | 140,000,000 |  |  |  |  |  |
Weighted average interest rate |  |  |  |  |  | 4.46% |  | 4.46% | 4.46% |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  | $ 1,700,000 | $ 3,300,000 |  | $ 1,700,000 | $ 3,300,000 |  |  |  |  |  |  |  |  |
Average borrowings |  |  |  |  |  | 140,000,000 | 140,000,000 |  | 140,000,000 | 140,000,000 |  |  |  |  |  |  |  |  |
Redemption price, percentage |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notes | January 2026 Notes | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  |  |  |  | 117,300,000 |  | $ 117,300,000 | 117,300,000 |  |  |  |  |  |  |  |  |  |
Notes | October 2026 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recorded value |  |  |  |  |  | $ 147,633,000 |  | $ 147,633,000 | $ 147,633,000 |  |  |  | 147,263,000 |  |  |  |  |  |
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% |  | 3.50% |  | 3.50% | 3.50% |  |  |  |  |  |  | 3.375% |  |  |
Outstanding balance |  |  |  |  |  | $ 150,000,000 |  | $ 150,000,000 | $ 150,000,000 |  |  |  | 150,000,000 |  |  |  |  |  |
Interest expense |  |  |  |  |  | 1,500,000 | 1,400,000 |  | 2,900,000 | 2,900,000 |  |  |  |  |  |  |  |  |
Average borrowings |  |  |  |  |  | 150,000,000 | $ 150,000,000 |  | 150,000,000 | $ 150,000,000 |  |  |  |  |  |  |  |  |
Redemption price, percentage |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notes | October 2026 Notes | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  |  |  |  | 126,100,000 |  | 126,100,000 | 126,100,000 |  |  |  |  |  |  |  |  |  |
Notes | August 2028 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recorded value |  |  |  |  |  | 69,438,000 |  | 69,438,000 | 69,438,000 |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  | 71,900,000 |  | 71,900,000 | 71,900,000 |  |  | $ 71,900,000 |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  |  |  |  |  |  |  |  | 7.75% |  |  |  |  |  |  |
Outstanding balance |  |  |  |  |  | $ 71,875,000 |  | $ 71,875,000 | $ 71,875,000 |  |  |  |  |  |  |  |  |  |
Weighted average interest rate |  |  |  |  |  | 7.75% |  | 7.75% | 7.75% |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  | $ 1,500,000 |  |  | $ 1,800,000 |  |  |  |  |  |  |  |  |  |
Average borrowings |  |  |  |  |  |  |  | $ 71,900,000 |  |  |  |  |  |  |  |  |  |  |
Notes | August 2028 Notes | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  |  |  |  | 73,600,000 |  | 73,600,000 | 73,600,000 |  |  |  |  |  |  |  |  |  |
Notes | August 2028 Notes Overallotment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  | $ 9,400,000 |  |  |  |  |  |  |
Unsecured Debt | SBA Debentures |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recorded value |  |  |  |  |  | $ 126,376,000 |  | 126,376,000 | $ 126,376,000 |  |  |  | 116,330,000 |  |  |  |  |  |
Weighted-average interest rate |  |  |  |  |  | 4.23% | 2.58% |  | 4.09% | 2.55% |  |  |  |  |  |  |  |  |
Outstanding balance |  |  |  |  |  | $ 130,000,000 |  | 130,000,000 | $ 130,000,000 |  |  |  | $ 120,000,000 |  |  |  |  |  |
Interest expense |  |  |  |  |  | 1,500,000 | $ 600,000 |  | 2,900,000 | $ 1,000,000 |  |  |  |  |  |  |  |  |
Average borrowings |  |  |  |  |  | 128,200,000 | 80,000,000 |  | 126,000,000 | 65,900,000 |  |  |  |  |  |  |  |  |
Unsecured Debt | SBA Debentures | SBIC I |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument term |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory maximum borrowing capacity |  |  | $ 175,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory capital |  |  | $ 87,500,000 |  |  | 87,500,000 |  | 87,500,000 | 87,500,000 |  |  |  |  |  |  |  |  |  |
Leverage commitment amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000 | $ 40,000,000 |  | $ 40,000,000 |  |
Available leverage commitment |  |  |  |  |  | 79,000,000 |  | 79,000,000 | 79,000,000 |  |  |  |  |  |  |  |  |  |
Unsecured Debt | SBA Debentures | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  |  |  |  | 116,200,000 |  | 116,200,000 | 116,200,000 |  |  |  |  |  |  |  |  |  |
Credit Facility | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity | $ 435,000,000 |  |  |  |  |  |  |  |  |  | $ 400,000,000 |  |  |  |  |  |  |  |
Accordion feature increase limit | $ 750,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum senior coverage ratio |  | 2.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum asset coverage ratio |  | 1.50 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest coverage ratio |  | 2.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recorded value |  |  |  |  |  | 250,000,000 |  | 250,000,000 | 250,000,000 |  |  |  |  |  |  |  |  |  |
Amortization of deferred loan costs |  |  |  |  |  | $ 4,300,000 | $ 2,900,000 |  | $ 9,300,000 | $ 4,800,000 |  |  |  |  |  |  |  |  |
Weighted-average interest rate |  |  |  |  |  | 7.70% | 4.46% |  | 7.52% | 3.86% |  |  |  |  |  |  |  |  |
Average outstanding amount |  |  |  |  |  | $ 185,100,000 | $ 214,600,000 |  | $ 106,100,000 | $ 198,800,000 |  |  |  |  |  |  |  |  |
Outstanding balance |  |  |  |  |  | $ 250,000,000 |  | $ 250,000,000 | $ 250,000,000 |  |  |  |  |  |  |  |  |  |
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 | SOFR |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate | 2.15% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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