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N-2 - $ / shares | 9 Months Ended |  |  |  |  |
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Jun. 30, 2025 | Mar. 31, 2025 | Sep. 30, 2024 | Jun. 30, 2024 | Mar. 31, 2024 | Sep. 30, 2023
Cover [Abstract] |  |  |  |  |  |
Entity Central Index Key | 0001143513 |  |  |  |  |
Amendment Flag | false |  |  |  |  |
Securities Act File Number | 814-00237 |  |  |  |  |
Document Type | 10-Q |  |  |  |  |
Entity Registrant Name | GLADSTONE CAPITAL CORP |  |  |  |  |
Entity Address, Address Line One | 1521 WESTBRANCH DRIVE |  |  |  |  |
Entity Address, Address Line Two | SUITE 100 |  |  |  |  |
Entity Address, City or Town | McLean |  |  |  |  |
Entity Address, State or Province | VA |  |  |  |  |
Entity Address, Postal Zip Code | 22102 |  |  |  |  |
City Area Code | 703 |  |  |  |  |
Local Phone Number | 287-5800 |  |  |  |  |
Entity Emerging Growth Company | false |  |  |  |  |
Financial Highlights [Abstract] |  |  |  |  |  |
Senior Securities, Note [Text Block] | As of June 30, 2025, our asset coverage on our “senior securities representing indebtedness” was 267.9% and our asset coverage on our “senior securities that are stock” was 253.7%. |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |
Investment Objectives and Practices [Text Block] | We were established for the purpose of investing in debt and equity securities of established private businesses operating in the U.S. Our investment objectives are to: (1) achieve and grow current income by investing in debt securities of established lower middle market companies in the U.S. that we believe will provide stable earnings and cash flow to pay expenses, make principal and interest payments on our outstanding indebtedness and make distributions to stockholders that grow over time; and (2) provide our stockholders with long-term capital appreciation in the value of our assets by investing in equity securities, in connection with our debt investments, that we believe can grow over time to permit us to sell our equity investments for capital gains. To achieve our investment objectives, our primary investment strategy is to invest in several categories of debt and equity securities, with each investment generally ranging from $8 million to $40 million, although investment size may vary, depending upon our total assets or available capital at the time of investment. We expect that our investment portfolio over time will consist of approximately 90.0% debt investments and 10.0% equity investments, at cost.We focus on investing in lower middle market companies (which we generally define as companies with annual earnings before interest, taxes, depreciation and amortization of $3 million to $25 million) in the U.S. that meet certain criteria, including the following: the sustainability of the business’ free cash flow and its ability to grow it over time, adequate assets for loan collateral, experienced management teams with a significant ownership interest in the borrower, reasonable capitalization of the borrower, including an ample equity contribution or cushion based on prevailing enterprise valuation multiples and, to a lesser extent, the potential to realize appreciation and gain liquidity in our equity position, if any. We lend to borrowers that need funds for growth capital or to finance acquisitions or recapitalize or refinance their existing debt facilities. We seek to avoid investing in high-risk, early-stage enterprises. Our targeted portfolio companies are generally considered too small for the larger capital marketplace. |  |  |  |  |
Risk [Text Block] | RISK FACTORS. Our business is subject to certain risks and events that, if they occur, could adversely affect our financial condition and results of operations and the trading price of our securities. In addition to the risks in the section captioned “Item 1A. Risk Factors” in Part I of our Annual Report on Form 10-K for the fiscal year ended September 30, 2024, as filed with the SEC on November 13, 2024, we are subject to the following risk: Tariffs may adversely affect us or our portfolio companies. Existing or new tariffs imposed on foreign goods imported by the United States or on U.S. goods imported by foreign countries could subject us or our portfolio companies to additional risks. Among other effects, tariffs could increase the cost of production for certain of our portfolio companies or reduce demand for their products, which could affect their results of operations. We cannot predict whether, or to what extent, any tariff or other trade protections may affect us or our portfolio companies. The risks described in this section and our Annual Report are not the only risks we face. Additional risks and uncertainties not currently known to us or that we currently deem to be immaterial also may materially and adversely affect our business, financial condition and/or operating results. |  |  |  |  |
Share Price | $ 26.97 |  |  |  |  |
NAV Per Share | $ 21.25 | $ 21.41 | $ 21.18 | $ 20.18 | $ 19.80 | $ 18.79
Latest Premium (Discount) to NAV [Percent] | 26.90% |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |
Long Term Debt [Table Text Block] | Revolving Line of Credit On May 13, 2021, we, through Business Loan, entered into a ninth amended and restated credit agreement with KeyBank as administrative agent, lead arranger, managing agent and lender, the Adviser, as servicer, and certain other lenders party thereto (the “Credit Facility”). On June 23, 2025, we, through Business Loan, entered into Amendment No. 9 to the Credit Facility. As of June 30, 2025, our Credit Facility had a total commitment amount of $320.0 million with an “accordion” feature that permits us to increase the size of the facility to $400.0 million. The Credit Facility has a revolving period end date of October 31, 2027, and a final maturity date of October 31, 2029 (at which time all principal and interest will be due and payable if the Credit Facility is not extended by the revolving period end date). The interest rate margin is 2.60% during the revolving period and 3.10% thereafter. Interest is payable monthly during the term of our Credit Facility. Available borrowings are subject to various constraints imposed under our Credit Facility, based on the aggregate loan balance pledged by Business Loan, which varies as loans are added and repaid, regardless of whether such repayments are prepayments or made as contractually required. Our Credit Facility also requires that any interest or principal payments on pledged loans be remitted directly by the borrower into a lockbox account with KeyBank and with The Bank of New York Mellon Trust Company, N.A. as custodian. KeyBank, which also serves as the trustee of the account, generally remits the collected funds to us once a month. Our Credit Facility also requires that any interest or principal payments on pledged loans be remitted directly by the borrower into a lockbox account with KeyBank. KeyBank is also the trustee of the account and generally remits the collected funds to us once each month. Amounts collected in the lockbox account with KeyBank are presented as Due from administrative agent on the accompanying Consolidated Statement of Assets and Liabilities as of June 30, 2025 and September 30, 2024. Our Credit Facility contains covenants that require Business Loan to maintain its status as a separate legal entity, prohibit certain significant corporate transactions (such as mergers, consolidations, liquidations or dissolutions), and restrict material changes to our credit and collection policies without the lenders’ consent. Our Credit Facility also generally limits distributions to our stockholders on a fiscal year basis to the sum of our net investment income, net capital gains and amounts elected to have been paid during the prior year in accordance with Section 855(a) of the Code. Business Loan is also subject to certain limitations on the type of loan investments it can apply as collateral towards the borrowing base to receive additional borrowing availability under our Credit Facility, including restrictions on geographic concentrations, sector concentrations, loan size, payment frequency and status, average life, portfolio company leverage and lien property. Our Credit Facility further requires Business Loan to comply with other financial and operational covenants, which obligate Business Loan to, among other things, maintain certain financial ratios, including asset and interest coverage and a minimum number of 25 obligors required in the borrowing base. Additionally, we are required to maintain (i) a minimum net worth (defined in our Credit Facility to include any outstanding mandatorily redeemable preferred stock) of $500.0 million plus 50.0% of all equity and subordinated debt raised after May 13, 2021 less 50% of any equity and subordinated debt retired or redeemed after May 13, 2021, which equates to $500.0 million as of June 30, 2025, (ii) asset coverage with respect to “senior securities representing indebtedness” of at least 150% (or such percentage as may be set forth in Section 18 of the 1940 Act, as modified by Section 61 of the 1940 Act), and (iii) our status as a BDC under the 1940 Act and as a RIC under the Code. As of June 30, 2025, and as defined in our Credit Facility, we had a net worth of $727.0 million, asset coverage on our “senior securities representing indebtedness” of 267.9%, calculated in accordance with the requirements of Section 18 and 61 of the 1940 Act, and an active status as a BDC and RIC. In addition, we had 33 obligors in our Credit Facility’s borrowing base as of June 30, 2025. As of June 30, 2025, we were in compliance with all of our Credit Facility covenants. Refer to Note 5—Borrowings of the notes to our Consolidated Financial Statements included elsewhere in this Quarterly Report for additional information regarding our Credit Facility. Notes Payable In August 2023, we completed an offering of $57.0 million aggregate principal amount of 7.75% Notes due 2028 (the “2028 Notes”) for net proceeds of approximately $55.1 million after deducting underwriting discounts, commissions and offering expenses borne by us. The 2028 Notes are traded under the ticker symbol “GLADZ” on the Nasdaq Global Select Market. The 2028 Notes will mature on September 1, 2028 and may be redeemed in whole or in part at any time or from time to time at our option on or after September 1, 2025. The 2028 Notes bear interest at a rate of 7.75% per year. Interest is payable quarterly on March 1, June 1, September 1, and December 1 of each year (which equates to approximately $4.4 million per year). In November 2021, we completed a private placement of $50.0 million aggregate principal amount of 3.75% Notes due 2027 (the “2027 Notes”) for net proceeds of approximately $48.5 million after deducting initial purchasers’ costs, commissions and offering expenses borne by us. The 2027 Notes will mature on May 1, 2027 and may be redeemed in whole or in part at any time or from time to time at the Company’s option prior to maturity at par plus a “make-whole” premium, if applicable. The 2027 Notes bear interest at a rate of 3.75% per year. Interest is payable semi-annually on May 1 and November 1 of each year (which equates to approximately $1.9 million per year). In April 2022, pursuant to the registration rights agreement we entered into in connection with the 2027 Notes, we conducted an exchange offer through which we offered to exchange all of our then outstanding 2027 Notes (the “Restricted Notes”) that were issued on November 4, 2021, for an equal aggregate principal amount of our new 3.75% Notes due 2027 (the “Exchange Notes”) that had been registered with the SEC under the Securities Act of 1933, as amended. The terms of the Exchange Notes are identical to those of the Restricted Notes, except that the transfer restrictions and registration rights relating to the Restricted Notes do not apply to the Exchange Notes, and the Exchange Notes do not provide for the payment of additional interest in the event of a registration default. In December 2020, we completed an offering of $100.0 million aggregate principal amount of 5.125% Notes due 2026 (the “2026 Notes”) for net proceeds of approximately $97.7 million after deducting underwriting discounts, commissions and offering expenses borne by us. In March 2021, we completed an offering of an additional $50.0 million aggregate principal amount of the 2026 Notes for net proceeds of approximately $50.6 million after adding premiums and deducting underwriting costs, commissions and offering expenses borne by us. The 2026 Notes will mature on January 31, 2026 and may be redeemed in whole or in part at any time or from time to time at the Company’s option prior to maturity at par plus a “make-whole” premium, if applicable. The 2026 Notes bear interest at a rate of 5.125% per year. Interest is payable semi-annually on January 31 and July 31 of each year (which equates to approximately $7.7 million per year). The indenture relating to the 2028 Notes, the 2027 Notes, and the 2026 Notes contains certain covenants, including (i) an inability to incur additional debt or issue additional debt or preferred securities unless the Company’s asset coverage meets the threshold specified in the 1940 Act after such borrowing, (ii) an inability to declare any dividend or distribution (except a dividend payable in our stock) on a class of our capital stock or to purchase shares of our capital stock unless the Company’s asset coverage meets the threshold specified in the 1940 Act at the time of (and giving effect to) such declaration or purchase, and (iii) if, at any time, we are not subject to the reporting requirements of the Exchange Act, we will provide the holders of the 2028 Notes, the 2027 Notes, and the 2026 Notes, as applicable, and the trustee with audited annual consolidated financial statements and unaudited interim consolidated financial statements. The 2028 Notes, 2027 Notes, and 2026 Notes are recorded at the principal amount, plus applicable premiums, less discounts and offering costs, on our Consolidated Statements of Assets and Liabilities. |  |  |  |  |
7.75% Notes due 2028 [Member] |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |
Long Term Debt, Structuring [Text Block] | In August 2023, we completed an offering of $57.0 million aggregate principal amount of 7.75% Notes due 2028 (the “2028 Notes”) for net proceeds of approximately $55.1 million after deducting underwriting discounts, commissions and offering expenses borne by us. The 2028 Notes are traded under the ticker symbol “GLADZ” on the Nasdaq Global Select Market. The 2028 Notes will mature on September 1, 2028 and may be redeemed in whole or in part at any time or from time to time at our option on or after September 1, 2025. The 2028 Notes bear interest at a rate of 7.75% per year. Interest is payable quarterly on March 1, June 1, September 1, and December 1 of each year (which equates to approximately $4.4 million per year). |  |  |  |  |
3.75% Notes due 2027 [Member] |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |
Long Term Debt, Structuring [Text Block] | In November 2021, we completed a private placement of $50.0 million aggregate principal amount of 3.75% Notes due 2027 (the “2027 Notes”) for net proceeds of approximately $48.5 million after deducting initial purchasers’ costs, commissions and offering expenses borne by us. The 2027 Notes will mature on May 1, 2027 and may be redeemed in whole or in part at any time or from time to time at the Company’s option prior to maturity at par plus a “make-whole” premium, if applicable. The 2027 Notes bear interest at a rate of 3.75% per year. Interest is payable semi-annually on May 1 and November 1 of each year (which equates to approximately $1.9 million per year). In April 2022, pursuant to the registration rights agreement we entered into in connection with the 2027 Notes, we conducted an exchange offer through which we offered to exchange all of our then outstanding 2027 Notes (the “Restricted Notes”) that were issued on November 4, 2021, for an equal aggregate principal amount of our new 3.75% Notes due 2027 (the “Exchange Notes”) that had been registered with the SEC under the Securities Act of 1933, as amended. The terms of the Exchange Notes are identical to those of the Restricted Notes, except that the transfer restrictions and registration rights relating to the Restricted Notes do not apply to the Exchange Notes, and the Exchange Notes do not provide for the payment of additional interest in the event of a registration default. |  |  |  |  |
5.125% Notes due 2026 [Member] |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |
Long Term Debt, Structuring [Text Block] | In December 2020, we completed an offering of $100.0 million aggregate principal amount of 5.125% Notes due 2026 (the “2026 Notes”) for net proceeds of approximately $97.7 million after deducting underwriting discounts, commissions and offering expenses borne by us. In March 2021, we completed an offering of an additional $50.0 million aggregate principal amount of the 2026 Notes for net proceeds of approximately $50.6 million after adding premiums and deducting underwriting costs, commissions and offering expenses borne by us. The 2026 Notes will mature on January 31, 2026 and may be redeemed in whole or in part at any time or from time to time at the Company’s option prior to maturity at par plus a “make-whole” premium, if applicable. The 2026 Notes bear interest at a rate of 5.125% per year. Interest is payable semi-annually on January 31 and July 31 of each year (which equates to approximately $7.7 million per year). |  |  |  |  |
7.75% Notes Due 2028, 3.75% Notes Due 2027, And 5.125% Notes Due 2026 [Member] |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |
Long Term Debt, Dividends and Covenants [Text Block] | The indenture relating to the 2028 Notes, the 2027 Notes, and the 2026 Notes contains certain covenants, including (i) an inability to incur additional debt or issue additional debt or preferred securities unless the Company’s asset coverage meets the threshold specified in the 1940 Act after such borrowing, (ii) an inability to declare any dividend or distribution (except a dividend payable in our stock) on a class of our capital stock or to purchase shares of our capital stock unless the Company’s asset coverage meets the threshold specified in the 1940 Act at the time of (and giving effect to) such declaration or purchase, and (iii) if, at any time, we are not subject to the reporting requirements of the Exchange Act, we will provide the holders of the 2028 Notes, the 2027 Notes, and the 2026 Notes, as applicable, and the trustee with audited annual consolidated financial statements and unaudited interim consolidated financial statements. The 2028 Notes, 2027 Notes, and 2026 Notes are recorded at the principal amount, plus applicable premiums, less discounts and offering costs, on our Consolidated Statements of Assets and Liabilities |  |  |  |  |

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