FAIR VALUE MEASUREMENTS - Schedule of Valuation Techniques and Significant Level 3 Inputs (Details)
$ in Thousands

Mar. 31, 2026

USD ($)

Mar. 31, 2025

USD ($)

Investments at fair value

$ 2,097,446

^{[1],[2],[3],[4],[5]}

$ 1,785,299

^{[6],[7],[8],[9],[10]}

First lien loans

Investments at fair value

1,890,406

1,586,622

Second lien loans

Investments at fair value

24,931

18,066

Subordinated debt

Investments at fair value

1,157

1,218

Preferred equity

Investments at fair value

74,638

102,918

Common equity & warrants

Investments at fair value

102,276

76,475

Investments at fair value

4,038

Significant Unobservable Inputs (Level 3)

Investments at fair value

2,097,446

1,785,299

Significant Unobservable Inputs (Level 3) | First lien loans

Investments at fair value

1,890,406

1,586,622

Significant Unobservable Inputs (Level 3) | First lien loans | Income Approach | Discount Rate

Investments at fair value

$ 1,786,594

1,448,656

Significant Unobservable Inputs (Level 3) | First lien loans | Income Approach | Third Party Broker Quote

Investments at fair value

$ 6,923

Significant Unobservable Inputs (Level 3) | First lien loans | Income Approach | Minimum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.040

0.052

Significant Unobservable Inputs (Level 3) | First lien loans | Income Approach | Minimum | Third Party Broker Quote

Significant Unobservable Inputs

Significant Level 3 Inputs

0.400

Significant Unobservable Inputs (Level 3) | First lien loans | Income Approach | Maximum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

1.261

0.647

Significant Unobservable Inputs (Level 3) | First lien loans | Income Approach | Maximum | Third Party Broker Quote

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

Significant Unobservable Inputs (Level 3) | First lien loans | Income Approach | Weighted Average | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.117

0.128

Significant Unobservable Inputs (Level 3) | First lien loans | Income Approach | Weighted Average | Third Party Broker Quote

Significant Unobservable Inputs

Significant Level 3 Inputs

0.798

Significant Unobservable Inputs (Level 3) | First lien loans | Market Approach

Investments at fair value

$ 85,731

$ 125,500

Significant Unobservable Inputs (Level 3) | First lien loans | Market Approach | Minimum | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

0.952

0.898

Significant Unobservable Inputs (Level 3) | First lien loans | Market Approach | Maximum | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

1.000

Significant Unobservable Inputs (Level 3) | First lien loans | Market Approach | Weighted Average | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

0.990

0.979

Significant Unobservable Inputs (Level 3) | First lien loans | Enterprise Value Waterfall Approach

Investments at fair value

$ 18,081

$ 5,543

Significant Unobservable Inputs (Level 3) | First lien loans | Enterprise Value Waterfall Approach | Minimum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.126

0.210

Significant Unobservable Inputs (Level 3) | First lien loans | Enterprise Value Waterfall Approach | Minimum | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

8.4

3.0

Significant Unobservable Inputs (Level 3) | First lien loans | Enterprise Value Waterfall Approach | Maximum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.191

0.492

Significant Unobservable Inputs (Level 3) | First lien loans | Enterprise Value Waterfall Approach | Maximum | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

9.0

9.0

Significant Unobservable Inputs (Level 3) | First lien loans | Enterprise Value Waterfall Approach | Weighted Average | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.150

0.210

Significant Unobservable Inputs (Level 3) | First lien loans | Enterprise Value Waterfall Approach | Weighted Average | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

8.6

3.0

Significant Unobservable Inputs (Level 3) | Second lien loans

Investments at fair value

$ 24,931

$ 18,066

Significant Unobservable Inputs (Level 3) | Second lien loans | Income Approach

Investments at fair value

$ 24,931

Significant Unobservable Inputs (Level 3) | Second lien loans | Income Approach | Discount Rate

Investments at fair value

$ 18,066

Significant Unobservable Inputs (Level 3) | Second lien loans | Income Approach | Minimum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.079

0.095

Significant Unobservable Inputs (Level 3) | Second lien loans | Income Approach | Maximum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.170

0.412

Significant Unobservable Inputs (Level 3) | Second lien loans | Income Approach | Weighted Average | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.111

0.133

Significant Unobservable Inputs (Level 3) | Second lien loans | Enterprise Value Waterfall Approach

Investments at fair value

$ 0

Significant Unobservable Inputs (Level 3) | Second lien loans | Enterprise Value Waterfall Approach | Minimum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.210

Significant Unobservable Inputs (Level 3) | Second lien loans | Enterprise Value Waterfall Approach | Minimum | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

3.0

Significant Unobservable Inputs (Level 3) | Second lien loans | Enterprise Value Waterfall Approach | Maximum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.210

Significant Unobservable Inputs (Level 3) | Second lien loans | Enterprise Value Waterfall Approach | Maximum | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

3.0

Significant Unobservable Inputs (Level 3) | Second lien loans | Enterprise Value Waterfall Approach | Weighted Average | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.000

Significant Unobservable Inputs (Level 3) | Second lien loans | Enterprise Value Waterfall Approach | Weighted Average | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

0.0

Significant Unobservable Inputs (Level 3) | Subordinated debt

Investments at fair value

$ 1,157

$ 1,218

Significant Unobservable Inputs (Level 3) | Subordinated debt | Income Approach | Discount Rate

Investments at fair value

$ 600

538

Significant Unobservable Inputs (Level 3) | Subordinated debt | Income Approach | Third Party Broker Quote

Investments at fair value

$ 58

Significant Unobservable Inputs (Level 3) | Subordinated debt | Income Approach | Minimum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.109

0.144

Significant Unobservable Inputs (Level 3) | Subordinated debt | Income Approach | Minimum | Third Party Broker Quote

Significant Unobservable Inputs

Significant Level 3 Inputs

0.250

Significant Unobservable Inputs (Level 3) | Subordinated debt | Income Approach | Maximum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.109

0.144

Significant Unobservable Inputs (Level 3) | Subordinated debt | Income Approach | Maximum | Third Party Broker Quote

Significant Unobservable Inputs

Significant Level 3 Inputs

0.250

Significant Unobservable Inputs (Level 3) | Subordinated debt | Income Approach | Weighted Average | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.109

0.144

Significant Unobservable Inputs (Level 3) | Subordinated debt | Income Approach | Weighted Average | Third Party Broker Quote

Significant Unobservable Inputs

Significant Level 3 Inputs

0.250

Significant Unobservable Inputs (Level 3) | Subordinated debt | Market Approach

Investments at fair value

$ 65

Significant Unobservable Inputs (Level 3) | Subordinated debt | Market Approach | Minimum | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

Significant Unobservable Inputs (Level 3) | Subordinated debt | Market Approach | Maximum | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

Significant Unobservable Inputs (Level 3) | Subordinated debt | Market Approach | Weighted Average | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

Significant Unobservable Inputs (Level 3) | Subordinated debt | Enterprise Value Waterfall Approach

Investments at fair value

$ 557

$ 557

Significant Unobservable Inputs (Level 3) | Subordinated debt | Enterprise Value Waterfall Approach | Minimum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.143

0.133

Significant Unobservable Inputs (Level 3) | Subordinated debt | Enterprise Value Waterfall Approach | Minimum | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

6.0

6.1

Significant Unobservable Inputs (Level 3) | Subordinated debt | Enterprise Value Waterfall Approach | Maximum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.174

0.176

Significant Unobservable Inputs (Level 3) | Subordinated debt | Enterprise Value Waterfall Approach | Maximum | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

8.0

7.6

Significant Unobservable Inputs (Level 3) | Subordinated debt | Enterprise Value Waterfall Approach | Weighted Average | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.153

0.147

Significant Unobservable Inputs (Level 3) | Subordinated debt | Enterprise Value Waterfall Approach | Weighted Average | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

6.6

6.6

Significant Unobservable Inputs (Level 3) | Preferred equity

Investments at fair value

$ 74,638

$ 102,918

Significant Unobservable Inputs (Level 3) | Preferred equity | Market Approach

Investments at fair value

$ 1,000

$ 2,900

Significant Unobservable Inputs (Level 3) | Preferred equity | Market Approach | Minimum | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

1.000

Significant Unobservable Inputs (Level 3) | Preferred equity | Market Approach | Maximum | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

1.000

Significant Unobservable Inputs (Level 3) | Preferred equity | Market Approach | Weighted Average | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

1.000

Significant Unobservable Inputs (Level 3) | Preferred equity | Enterprise Value Waterfall Approach

Investments at fair value

$ 73,638

$ 100,018

Significant Unobservable Inputs (Level 3) | Preferred equity | Enterprise Value Waterfall Approach | Minimum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.115

0.111

Significant Unobservable Inputs (Level 3) | Preferred equity | Enterprise Value Waterfall Approach | Minimum | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

4.0

3.0

Significant Unobservable Inputs (Level 3) | Preferred equity | Enterprise Value Waterfall Approach | Maximum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.476

0.492

Significant Unobservable Inputs (Level 3) | Preferred equity | Enterprise Value Waterfall Approach | Maximum | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

13.8

16.9

Significant Unobservable Inputs (Level 3) | Preferred equity | Enterprise Value Waterfall Approach | Weighted Average | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.178

0.159

Significant Unobservable Inputs (Level 3) | Preferred equity | Enterprise Value Waterfall Approach | Weighted Average | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

7.2

8.8

Significant Unobservable Inputs (Level 3) | Common equity & warrants

Investments at fair value

$ 102,276

$ 76,475

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Market Approach

Investments at fair value

$ 750

$ 1,597

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Market Approach | Minimum | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

1.000

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Market Approach | Maximum | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

1.000

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Market Approach | Weighted Average | Cost

Significant Unobservable Inputs

Significant Level 3 Inputs

1.000

1.000

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Enterprise Value Waterfall Approach

Investments at fair value

$ 101,526

$ 74,878

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Enterprise Value Waterfall Approach | Minimum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.105

0.119

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Enterprise Value Waterfall Approach | Minimum | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

3.5

3.0

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Enterprise Value Waterfall Approach | Maximum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.308

0.233

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Enterprise Value Waterfall Approach | Maximum | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

16.4

16.0

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Enterprise Value Waterfall Approach | Weighted Average | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.140

0.144

Significant Unobservable Inputs (Level 3) | Common equity & warrants | Enterprise Value Waterfall Approach | Weighted Average | EBITDA Multiple

Significant Unobservable Inputs

Significant Level 3 Inputs

7.9

8.4

Significant Unobservable Inputs (Level 3) | Earnout

Investments at fair value

$ 4,038

Significant Unobservable Inputs (Level 3) | Earnout | Income Approach

Investments at fair value

$ 4,038

Significant Unobservable Inputs (Level 3) | Earnout | Income Approach | Minimum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.334

Significant Unobservable Inputs (Level 3) | Earnout | Income Approach | Maximum | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.334

Significant Unobservable Inputs (Level 3) | Earnout | Income Approach | Weighted Average | Discount Rate

Significant Unobservable Inputs

Significant Level 3 Inputs

0.334

[1]	All debt investments are income-producing, unless otherwise noted. Equity investments are non-income producing, unless otherwise noted. (2)All of the Company’s investments and the investments of Capital Southwest SPV LLC ("SPV"), SBIC I (as defined below) and SBIC II (as defined below) are pledged as collateral for the Company’s senior secured revolving credit facility, the SPV's financing credit facility or in support of the SBA-guaranteed debentures to be issued by Capital Southwest SBIC I, LP ("SBIC I") and Capital Southwest SBIC II, LP ("SBIC II"), the Company's wholly-owned subsidiaries that operate as small business investment companies, respectively.
[2]	Non-Control/Non-Affiliate investments are generally defined by the 1940 Act as investments that are neither control investments nor affiliate investments. Investments are classified as non-control/non-affiliate investments, unless otherwise noted. At March 31, 2026, the Company held $1,682.4 million of non-control/non-affiliate investments, which represented approximately 80.2% of the Company’s investment assets. The fair value of these investments as a percent of net assets is 166.5%.
[3]	The Company's investment portfolio is comprised entirely of debt and equity securities, along with other financial instruments, 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 valuation committee comprised of certain officers of the Company (the "Valuation Committee") as the valuation designee of the Board of Directors (the "Valuation Designee") pursuant to Rule 2a-5 under the Investment Company Act of 1940, as amended (the “1940 Act”), using significant unobservable Level 3 inputs. Refer to Note 4 - Fair Value Measurements for further discussion.
[4]	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.
[5]	Unless otherwise noted, all portfolio company headquarters are based in the United States.
[6]	All debt investments are income-producing, unless otherwise noted. Equity investments are non-income producing, unless otherwise noted.
[7]	Non-Control/Non-Affiliate investments are generally defined by the 1940 Act as investments that are neither control investments nor affiliate investments. Investments are classified as non-control/non-affiliate investments, unless otherwise noted. At March 31, 2025, the Company held $1,436.3 million of non-control/non-affiliate investments, which represented approximately 80.5% of the Company’s investment assets. The fair value of these investments as a percent of net assets is 162.6%.
[8]	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 valuation committee comprised of certain officers of the Company (the "Valuation Committee") as the valuation designee of the Board of Directors (the "Valuation Designee") pursuant to Rule 2a-5 under the Investment Company Act of 1940, as amended (the “1940 Act”), using significant unobservable Level 3 inputs. Refer to Note 4 - Fair Value Measurements for further discussion.
[9]	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.
[10]	Unless otherwise noted, all portfolio company headquarters are based in the United States.