SIGNIFICANT SUBSIDIARIES - Narrative (Details)			1 Months Ended							3 Months Ended								6 Months Ended		12 Months Ended	56 Months Ended
	Mar. 10, 2021	Apr. 30, 2020 USD ($)	Oct. 31, 2022 USD ($)	Mar. 31, 2021	Mar. 31, 2021	Apr. 30, 2020 USD ($)	Nov. 30, 2019	Nov. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2022 USD ($)		Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)		Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2022 USD ($)		Mar. 31, 2017 USD ($) lender	Apr. 30, 2020 USD ($)	Mar. 25, 2021 USD ($)
Investment Company, Nonconsolidated Subsidiary [Line Items]
Total assets										$ 1,113,171,000			$ 973,957,000					$ 1,113,171,000
Total Investments										1,056,931,000	[1],[2],[3]		936,614,000	[4],[5],[6]				1,056,931,000	[1],[2],[3]
Dividends										14,300,000		$ 16,600,000	11,800,000
Drawn										603,329,000			528,588,000					603,329,000
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										110,000,000								110,000,000
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										176,386,000			189,144,000					176,386,000
Total Investments										168,599,000			176,704,000					168,599,000
Dividends										2,600,000
Drawn										$ 110,000,000			$ 114,500,000					$ 110,000,000
I-45 SLF LLC | Subsequent Event | Capital Southwest Corp
Investment Company, Nonconsolidated Subsidiary [Line Items]
Dividends paid			$ 2,000,000

[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 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.