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INVESTMENTS - Narrative (Details) $ in Thousands | 1 Months Ended | 3 Months Ended | 6 Months Ended |
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Sep. 30, 2023 USD ($) portfolioCompany industry state | Aug. 31, 2023 USD ($) | Jun. 30, 2023 USD ($) | May 31, 2023 USD ($) | Sep. 30, 2023 USD ($) portfolioCompany industry state | Sep. 30, 2022 USD ($) | Sep. 30, 2023 USD ($) portfolioCompany industry state | Sep. 30, 2022 USD ($) | Mar. 31, 2023 USD ($)
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  | $ 114,788 | $ 102,046 |  |
Dividend income |  |  |  |  | $ 0 | $ 4,825 | 1,907 | $ 6,381 |  |
Cost | $ 834,798 |  |  |  | $ 834,798 |  | $ 834,798 |  | $ 720,630 | [1]
Number of investment portfolio company | portfolioCompany | 26 |  |  |  | 26 |  | 26 |  |  |
Number of states which have invested in the company location | state | 19 |  |  |  | 19 |  | 19 |  |  |
Number of industries that have made investments | industry | 16 |  |  |  | 16 |  | 16 |  |  |
Fair Value | $ 915,636 |  |  |  | $ 915,636 |  | $ 915,636 |  | 753,543 | [1]
Allowance for uncollectible receivables | 1,500 |  |  |  | 1,500 |  | $ 1,500 |  | 1,600 |
Uncollectible receivables allowance, minimum required day for uncollectible adjustment |  |  |  |  |  |  | 90 days |  |  |
Gross receivables from portfolio companies | 2,300 |  |  |  | 2,300 |  | $ 2,300 |  | 2,200 |
Five Largest Portfolio Investments | Investment Portfolio Benchmark | Customer Concentration Risk |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Fair Value | 364,900 |  |  |  | 364,900 |  | $ 364,900 |  |  |
Concentration risk, percentage |  |  |  |  |  |  | 39.90% |  |  |
Secured First Lien Debt |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Cost | 541,539 |  |  |  | 541,539 |  | $ 541,539 |  | 471,439 |
Fair Value | 508,504 |  |  |  | 508,504 |  | 508,504 |  | 437,517 |
Preferred Equity |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Cost | 163,787 |  |  |  | 163,787 |  | 163,787 |  | 149,099 |
Fair Value | 267,596 |  |  |  | 267,596 |  | 267,596 |  | 222,585 |
Secured Second Lien Debt |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Cost | 109,158 |  |  |  | 109,158 |  | 109,158 |  | 84,158 |
Fair Value | 102,747 |  |  |  | 102,747 |  | 102,747 |  | 75,734 |
Common Equity/ Equivalents |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Cost | 20,314 |  |  |  | 20,314 |  | 20,314 |  | 15,934 |
Fair Value | 36,789 |  |  |  | $ 36,789 |  | 36,789 |  | $ 17,707 |
Home Concepts Acquisition, Inc |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  | $ 15,300 |  |  |  |  |  |
Home Concepts Acquisition, Inc | Secured First Lien Debt |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  | 12,000 |  |  |  |  |  |
Home Concepts Acquisition, Inc | Preferred Equity |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  | $ 3,300 |  |  |  |  |  |
Old World Christmas, Inc. |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | $ 2,500 |  |  |  |  |  |  |
Proceeds from equity |  |  | 2,200 |  |  |  |  |  |  |
Dividend income |  |  | 1,900 |  |  |  |  |  |  |
Realized gain on preferred equity |  |  | 300 |  |  |  | $ 300 |  |  |
Nth Degree Investment Group, LLC |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | 30,000 |  |  |  |  |  |  |
Nth Degree Investment Group, LLC | Secured Second Lien Debt |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | 25,000 |  |  |  |  |  |  |
Nth Degree Investment Group, LLC | Common Equity/ Equivalents |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | 5,000 |  |  |  |  |  |  |
Specialty Knives & Tools, LLC |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Realized gain on preferred equity |  |  | 900 |  |  |  |  |  |  |
Net proceeds from the sale of investments |  |  | 1,500 |  |  |  |  |  |  |
Cost |  |  | $ 600 |  |  |  |  |  |  |
Nocturne Villa Rentals, Inc. |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments |  | $ 18,700 |  |  |  |  |  |  |  |
The E3 Company, LLC |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments | 46,000 |  |  |  |  |  |  |  |  |
The E3 Company, LLC | Secured First Lien Debt |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments | 34,800 |  |  |  |  |  |  |  |  |
The E3 Company, LLC | Preferred Equity |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |
Purchase of investments | $ 11,200 |  |  |  |  |  |  |  |  |
[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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