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SIGNIFICANT SUBSIDIARIES - Narrative (Details) |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 56 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 10, 2021 | Apr. 30, 2020 USD ($) | Jan. 31, 2023 USD ($) | Mar. 31, 2021 | Mar. 31, 2021 | Nov. 30, 2019 | Nov. 30, 2015 USD ($) | Sep. 30, 2015 USD ($) | Dec. 31, 2022 USD ($) | Sep. 30, 2022 USD ($) | Jun. 30, 2022 USD ($) | Mar. 31, 2022 USD ($) | Dec. 31, 2021 | Sep. 30, 2021 | Jun. 30, 2021 | Dec. 31, 2022 USD ($) | Mar. 31, 2017 USD ($) lender | Apr. 30, 2020 USD ($) | Mar. 25, 2021 USD ($)
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total assets |  |  |  |  |  |  |  |  | $ 1,202,157,000 |  |  |  | $ 973,957,000 |  |  |  |  | $ 1,202,157,000 |  |  |  |
Total Investments |  |  |  |  |  |  |  |  | 1,150,046,000 | [1],[2],[3] |  |  | 936,614,000 | [4],[5],[6] |  |  |  | 1,150,046,000 | [1],[2],[3] |  |  |
Dividends |  |  |  |  |  |  |  |  | 19,300,000 |  | $ 14,300,000 | $ 16,600,000 | 11,800,000 |  |  |  |  |  |  |  |  |
Drawn |  |  |  |  |  |  |  |  | 611,627,000 |  |  |  | 528,588,000 |  |  |  |  | 611,627,000 |  |  |  |
Secured Debt | I-45 Credit Facility | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  |  | $ 75,000,000 |  |  |  |  |  |  |  |  |  |  |  |  | $ 165,000,000 |  |
Debt instrument term |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt-to-equity leverage ratio |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accordion feature increase limit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 90,000,000 |  |
Number of additional lenders | lender |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 |  |
Debt prepayment |  | $ 15,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Drawn |  |  |  |  |  |  |  |  | 104,000,000 |  |  |  |  |  |  |  |  | 104,000,000 |  |  |  |
Secured Debt | I-45 Credit Facility | Line of Credit | LIBOR |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  | 2.15% |  | 2.25% | 2.50% |  |  |  |  |  |  |  |  |  |  |  |  | 2.40% |  |
I-45 SLF LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Initial equity capital commitment |  |  |  |  |  |  |  | $ 85,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Main Street Capital Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Initial equity capital commitment |  | 3,200,000 |  |  |  |  |  | 17,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contributed capital |  |  |  |  |  |  |  |  | 19,000,000 |  |  |  |  |  |  |  |  | $ 19,000,000 |  |  |  |
Corporate Joint Venture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Initial equity capital commitment |  | $ 12,800,000 |  |  |  |  |  | $ 68,000,000 |  |  |  |  |  |  |  |  |  |  |  |  | $ 80,800,000 |
Allocated current profits interest | 75.60% |  |  |  | 76.2625% |  |  |  |  |  |  |  |  |  | 78.25% | 77.5875% | 76.925% | 78.25% |  |  |  |
Contributed capital |  |  |  |  |  |  |  |  | $ 76,000,000 |  |  |  |  |  |  |  |  | $ 76,000,000 |  |  |  |
Corporate Joint Venture | I-45 SLF LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percent |  |  |  |  |  |  |  |  | 80.00% |  |  |  |  |  |  |  |  | 80.00% |  |  |  |
Corporate Joint Venture | Main Street Capital Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Initial equity capital commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,200,000 |
Allocated current profits interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21.75% |  |  |  |
Corporate Joint Venture | Main Street Capital Corporation | I-45 SLF LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest |  |  |  |  |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  | 20.00% |  |  |  |
I-45 SLF LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital dividend |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 10,000,000
Total funded equity capital |  |  |  |  |  |  |  |  | $ 95,000,000 |  |  |  |  |  |  |  |  | $ 95,000,000 |  |  |  |
Total assets |  |  |  |  |  |  |  |  | 166,235,000 |  |  |  | 189,144,000 |  |  |  |  | 166,235,000 |  |  |  |
Total Investments |  |  |  |  |  |  |  |  | 160,998,000 |  |  |  | $ 176,704,000 |  |  |  |  | $ 160,998,000 |  |  |  |
Dividends |  |  |  |  |  |  |  |  | $ 2,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |
I-45 SLF LLC | Capital Southwest Corp | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends paid |  |  | $ 1,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | All debt investments are income-producing, unless otherwise noted. Equity investments are non-income producing, unless otherwise noted.
[2] | Equity ownership may be held in shares or units of a company that is either wholly owned by the portfolio company or under common control by the same parent company to the portfolio company.
[3] | The Company's investment portfolio is comprised entirely of debt and equity securities of privately held companies for which quoted prices falling within the categories of Level 1 and Level 2 inputs are not readily available. Therefore, the Company values all of its portfolio investments at fair value, as determined in good faith by the Board of Directors, using significant unobservable Level 3 inputs. Refer to Note 4 - Fair Value Measurements for further discussion.
[4] | All debt investments are income-producing, unless otherwise noted. Equity investments and warrants are non-income producing, unless otherwise noted.
[5] | Equity ownership may be held in shares or units of a company that is either wholly owned by the portfolio company or under common control by the same parent company to the portfolio company.
[6] | The Company's investment portfolio is comprised entirely of debt and equity securities of privately held companies for which quoted prices falling within the categories of Level 1 and Level 2 inputs are not readily available. Therefore, the Company values all of its portfolio investments at fair value, as determined in good faith by the Board of Directors, using significant unobservable Level 3 inputs. Refer to Note 4 for further discussion.

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