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.