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INVESTMENTS - Narrative (Details) $ in Thousands | 1 Months Ended | 12 Months Ended
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Mar. 31, 2024 USD ($) portfolioCompany industry state | Oct. 31, 2023 USD ($) | Sep. 30, 2023 USD ($) | Aug. 31, 2023 USD ($) | Jun. 30, 2023 USD ($) | May 31, 2023 USD ($) | Mar. 31, 2024 USD ($) portfolioCompany industry state | Mar. 31, 2023 USD ($) | Mar. 31, 2022 USD ($)
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  |  |  |  |  |  | $ 183,924 |  | $ 133,756 |  | $ 92,738
Dividend income: |  |  |  |  |  |  |  | 1,907 |  | 10,865 |  | 2,595
Cost | $ 854,290 | [1] |  |  |  |  |  | 854,290 | [1] | 720,630 | [2] |
Success fee income: |  |  |  |  |  |  |  | 3,605 |  | 10,402 |  | 10,308
Principal repayments of investments |  |  |  |  |  |  |  | 28,000 |  | 52,300 |  | 51,398
Net realized gain (loss): |  |  |  |  |  |  |  | $ 30,256 |  | 10,753 |  | $ 12,444
Number of investment portfolio company | portfolioCompany | 24 |  |  |  |  |  |  | 24 |  |  |  |
Number of states which have invested in the company location | state | 18 |  |  |  |  |  |  | 18 |  |  |  |
Number of industries that have made investments | industry | 16 |  |  |  |  |  |  | 16 |  |  |  |
Investments owned, fair value | $ 920,504 | [1] |  |  |  |  |  | $ 920,504 | [1] | 753,543 | [2] |
Uncollectible receivables allowance, minimum required day for uncollectible adjustment |  |  |  |  |  |  |  | 90 days |  |  |  |
Gross receivables from portfolio companies | 2,200 |  |  |  |  |  |  | $ 2,200 |  | 2,200 |  |
Allowance for uncollectible receivables | 1,400 |  |  |  |  |  |  | 1,400 |  | 1,600 |  |
Investment Portfolio Benchmark | Customer Concentration Risk | Five Largest Portfolio Investments |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Investments owned, fair value | 393,500 |  |  |  |  |  |  | $ 393,500 |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  | 42.70% |  |  |  |
Secured First Lien Debt |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cost | 513,425 |  |  |  |  |  |  | $ 513,425 |  | 471,439 |  |
Investments owned, fair value | 474,856 |  |  |  |  |  |  | 474,856 |  | 437,517 |  |
Preferred Equity |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cost | 145,070 |  |  |  |  |  |  | 145,070 |  | 149,099 |  |
Investments owned, fair value | 213,480 |  |  |  |  |  |  | 213,480 |  | 222,585 |  |
Secured Second Lien Debt |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cost | 144,958 |  |  |  |  |  |  | 144,958 |  | 84,158 |  |
Investments owned, fair value | 138,703 |  |  |  |  |  |  | 138,703 |  | 75,734 |  |
Common Equity/ Equivalents |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cost | 50,837 |  |  |  |  |  |  | 50,837 |  | 15,934 |  |
Investments owned, fair value | 93,465 |  |  |  |  |  |  | 93,465 |  | 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 |  | $ 13,400 |  |
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 Luxury Villas, 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 |  |  |  |  |  |  |  |
SFEG Holdings, Inc |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | $ 64,700 |  |  |  |  |  |  |  |  |
Cost |  |  | 4,800 |  |  |  |  |  |  |  |  |
Investments owned, fair value |  |  | 8,600 |  |  |  |  |  |  |  |  |
SFEG Holdings, Inc | Secured Second Lien Debt |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | 39,000 |  |  |  |  |  |  |  |  |
SFEG Holdings, Inc | Common Equity/ Equivalents |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Purchase of investments |  |  | 25,700 |  |  |  |  |  |  |  |  |
Counsel Press, Inc., |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Realized gain on preferred equity |  |  | 43,500 |  |  |  |  |  |  |  |  |
Success fee income: |  |  | 1,400 |  |  |  |  |  |  |  |  |
Principal repayments of investments |  |  | $ 27,500 |  |  |  |  |  |  |  |  |
The Mountain |  |  |  |  |  |  |  |  |  |  |  |
Summary of Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net realized gain (loss): | $ (14,700) |  |  |  |  |  |  |  |  |  |  |
[1] | Cumulative gross unrealized appreciation for federal income tax purposes is $180.5 million; cumulative gross unrealized depreciation for federal income tax purposes is $115.8 million. Cumulative net unrealized appreciation is $64.7 million, based on a tax cost of $855.8 million.
[2] | 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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