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N-2 - USD ($) $ / shares in Units, $ in Millions |  |  |  | 9 Months Ended |  |  |
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Mar. 19, 2025 | Oct. 08, 2021 | Dec. 23, 2020 | Sep. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024 | Dec. 31, 2023
Cover [Abstract] |  |  |  |  |  |  |
Entity Central Index Key |  |  |  | 0001513363 |  |  |
Amendment Flag |  |  |  | false |  |  |
Securities Act File Number |  |  |  | 814-00861 |  |  |
Document Type |  |  |  | 10-Q |  |  |
Entity Registrant Name |  |  |  | Fidus Investment Corporation |  |  |
Entity Address, Address Line One |  |  |  | 1603 Orrington Avenue, Suite 1005 |  |  |
Entity Address, City or Town |  |  |  | Evanston |  |  |
Entity Address, State or Province |  |  |  | IL |  |  |
Entity Address, Postal Zip Code |  |  |  | 60201 |  |  |
City Area Code |  |  |  | 847 |  |  |
Local Phone Number |  |  |  | 859-3940 |  |  |
Entity Emerging Growth Company |  |  |  | false |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |
Investment Objectives and Practices [Text Block] |  |  |  | Our investment objective is to provide attractive risk-adjusted returns by generating both current income from our debt investments and capital appreciation from our equity related investments. Our investment strategy includes partnering with business owners, management teams and financial sponsors by providing customized financing for ownership transactions, recapitalizations, strategic acquisitions, business expansion and other growth initiatives. We seek to maintain a diversified portfolio of investments in order to help mitigate the potential effects of adverse economic events related to particular companies, regions or industries. Investments We seek to create a diversified investment portfolio that primarily includes debt investments and, to a lesser extent, equity securities. Our investments typically range between $5.0 million to $35.0 million per portfolio company, although this investment size may vary proportionately with the size of our capital base. Our investment objective is to provide attractive risk-adjusted returns by generating both current income from our debt investments and capital appreciation from our equity related investments. We may invest in the equity securities of our portfolio companies, such as preferred stock, common stock, warrants and other equity interests, either directly or in conjunction with our debt investments. First Lien Debt. We structure some of our investments as senior secured or first lien debt investments. First lien debt investments are secured by a first priority lien on existing and future assets of the borrower and may take the form of term loans or revolving lines of credit. First lien debt is typically senior on a lien basis to other liabilities in the issuer’s capital structure and has the benefit of a first-priority security interest in assets of the issuer. The security interest ranks above the security interest of any second lien lenders in those assets. Our first lien debt may include stand-alone first lien loans, “last out” first lien loans, or “unitranche” loans. Stand-alone first lien loans are traditional first lien loans. All lenders in the facility have equal rights to the collateral that is subject to the first-priority security interest. “Last out” first lien loans have a secondary priority behind super-senior “first out” first lien loans in the collateral securing the loans in certain circumstances. The arrangements for a “last out” first lien loan are set forth in an “agreement among lenders,” which provides lenders with “first out” and “last out” payment streams based on a single lien on the collateral. Since the “first out” lenders generally have priority over the “last out” lenders for receiving payment under certain specified events of default, or upon the occurrence of other triggering events under intercreditor agreements or agreements among lenders, the “last out” lenders bear a greater risk and, in exchange, receive a higher effective interest rate, through arrangements among the lenders, than the “first out” lenders or lenders in stand-alone first lien loans. Agreements among lenders also typically provide greater voting rights to the “last out” lenders than the intercreditor agreements to which second lien lenders often are subject. Many of our debt investments also include excess cash flow sweep features, whereby principal repayment may be required before maturity if the portfolio company achieves certain defined operating targets. Additionally, our debt investments typically have principal prepayment penalties in the early years of the debt investment. The majority of our debt investments provide for a variable interest rate, generally with a prime or SOFR floor. Second Lien Debt. Some of our debt investments take the form of second lien debt, which includes senior subordinated notes. Second lien debt investments obtain security interests in the assets of the portfolio company as collateral in support of the repayment of such loans. Second lien debt typically is senior on a lien basis to other liabilities in the issuer’s capital structure and has the benefit of a security interest over assets of the issuer, though ranking junior to first lien debt secured by those assets. First lien lenders and second lien lenders typically have separate liens on the collateral, and an intercreditor agreement provides the first lien lenders with priority over the second lien lenders’ liens on the collateral. These loans typically provide for no contractual loan amortization, with all amortization deferred until loan maturity, and may include payment-in-kind (“PIK”) interest, which increases the principal balance over the term and, coupled with the deferred principal payment provision, increases credit risk exposure over the life of the loan. Subordinated Debt. These investments are typically structured as unsecured, subordinated notes. Structurally, subordinated debt usually ranks subordinate in priority of payment to first lien and second lien debt and may not have the benefit of financial covenants common in first lien and second lien debt. Subordinated debt may rank junior as it relates to proceeds in certain liquidations where it does not have the benefit of a lien in specific collateral held by creditors (typically first lien and/or second lien) who have a perfected security interest in such collateral. However, subordinated debt ranks senior to common and preferred equity in an issuer’s capital structure. These loans typically have relatively higher fixed interest rates (often representing a combination of cash pay and PIK interest) and amortization of principal deferred to maturity. The PIK feature (meaning a feature allowing for the payment of interest in the form of additional principal amount of the loan instead of in cash), which effectively operates as negative amortization of loan principal, coupled with the deferred principal payment provision, increases credit risk exposure over the life of the loan. Equity Securities. Our equity investments typically consist of either a direct minority equity investment in common or preferred stock or membership/partnership interests of a portfolio company, or we may receive warrants to buy a minority equity interest in a portfolio company in connection with a debt investment. Warrants we receive with our debt investments typically require only a nominal cost to exercise, and thus, as a portfolio company appreciates in value, we may achieve additional investment return from this equity interest. Our equity investments are typically not control-oriented investments, and in many cases, we acquire equity securities as part of a group of private equity investors in which we are not the lead investor. We may structure such equity investments to include provisions protecting our rights as a minority-interest holder, as well as a “put,” or right to sell such securities back to the issuer, upon the occurrence of specified events. In many cases, we may also seek to obtain registration rights in connection with these equity interests, which may include demand and “piggyback” registration rights. Our equity investments typically are made in connection with debt investments to the same portfolio companies. |  |  |
Risk Factors [Table Text Block] |  |  |  | Item 1A. Risk Factors. Other than the risk factor described below, there have been no material changes to the risk factors previously disclosed under "Item 1A. Risk Factors” previously disclosed in our Annual Report on Form 10-K for the year ended December 31, 2024 filed with the SEC on March 6, 2025, which are incorporated herein by reference. The risk factors therein could materially affect our business, financial condition and/or operating results. Additional risks and uncertainties not currently known to us or that we currently deem to be immaterial also may materially affect our business, financial condition and/or operating results. Changes to U.S. tariff and import/export regulations may have a negative effect on the operations of our portfolio companies and, in turn, negatively impact us. The U.S. government has recently imposed, and may in the future increase, tariffs on specific countries and commodities. In response, certain foreign trading partners, and others in the future, may impose retaliatory tariffs on certain U.S. goods. The foregoing has created significant uncertainty about the future relationship between the United States and certain other countries with respect to trade policies, treaties and new and increased tariffs. These developments, or the continued uncertainty relating to U.S. trade policies, may have a material adverse effect on global economic conditions and the stability of global financial markets, and may significantly reduce global trade and, in particular, trade between the impacted nations and the United States. The uncertainty relating to U.S. trade policies has increased market volatility. Any of these factors could depress economic activity and restrict certain of our portfolio companies’ access to suppliers or customers, and increase costs, decrease margins, and reduce the competitiveness of products and services offered by our portfolio companies. The foregoing may adversely affect the revenues and profitability of such portfolio companies and, in turn, negatively affect our results of operations, which could cause the market value of our shares of common stock to decline. |  |  |
Share Price |  |  |  | $ 20.26 |  | $ 19.6 |
NAV Per Share |  |  |  | $ 19.56 | $ 19.33 | $ 19.42 | $ 19.37
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |  |
Long Term Debt [Table Text Block] |  |  |  | Notes On December 23, 2020, we closed the offering of $125.0 million in aggregate principal amount of our 4.75% notes due 2026 (the “January 2026 Notes”). The total net proceeds to us from the January 2026 Notes, based on a public offering price of 100.00% of par, after deducting underwriting discounts of $2.5 million and offering expenses of $0.4 million, were $122.1 million. The January 2026 Notes will mature on January 31, 2026 and bear interest at a rate of 4.75%. The January 2026 Notes may be redeemed in whole or in part at any time or from time to time at our option subject to a make whole provision if redeemed before October 31, 2025 (the date falling three months prior to maturity) and at par thereafter. Interest on the January 2026 Notes is payable on January 31 and July 31 of each year. We do not intend to list the January 2026 Notes on any securities exchange or automated dealer quotation system. On May 21, 2025, we redeemed $25.0 million of the $125.0 million aggregate principal amount on the January 2026 Notes, resulting in a realized loss on extinguishment of debt of approximately $0.1 million. As of September 30, 2025, the outstanding principal balance of the January 2026 Notes was $100.0 million. On October 8, 2021, we closed the offering of $125.0 million in aggregate principal amount of our 3.50% notes due 2026 (the “November 2026 Notes”). The total net proceeds to us from the November 2026 Notes, based on a public offering price of 99.996% of par, after deducting underwriting discounts of $2.5 million and offering expenses of $0.3 million, were $122.2 million. The November 2026 Notes will mature on November 15, 2026 and bear interest at a rate of 3.50%. The November 2026 Notes may be redeemed in whole or in part at any time or from time to time at our option subject to a make whole provision if redeemed before August 15, 2026 (the date falling three months prior to maturity) and at par thereafter. Interest on the November 2026 Notes is payable on May 15 and November 15 of each year. We do not intend to list the November 2026 Notes on any securities exchange or automated dealer quotation system. As of September 30, 2025, the outstanding principal balance of the November 2026 Notes was $125.0 million. On March 19, 2025, we closed the offering of $100.0 million in aggregate principal amount of our 6.75% notes due 2030 (the “March 2030 Notes” and together with the January 2026 Notes and the November 2026 Notes, the “Notes”). The total net proceeds to us from the March 2030 Notes, based on a public offering price of 99.29954% of par, after deducting underwriting discounts of $2.0 million and offering expenses of $0.4 million, were approximately $96.9 million. The March 2030 Notes will mature on March 19, 2030 and bear interest at a rate of 6.75%. The March 2030 Notes may be redeemed in whole or in part at any time or from time to time at our option subject to a make whole provision if redeemed before September 19, 2029 (the date falling six months prior to maturity) and at par thereafter. Interest on the March 2030 Notes is payable on March 19 and September 19 of each year, beginning September 19, 2025. We do not intend to list the March 2030 Notes on any securities exchange or automated dealer quotation system. As of September 30, 2025, the outstanding principal balance of the March 2030 Notes was $100.0 million. Each of the Notes are unsecured obligations and rank pari passu with our existing and future unsecured indebtedness; effectively subordinated to all of our existing and future secured indebtedness; and structurally subordinated to all existing and future indebtedness and other obligations of any of our subsidiaries, financing vehicles, or similar facilities we may form in the future, with respect to claims on the assets of any such subsidiaries, financing vehicles, or similar facilities, including the Revolving Credit Facility. |  |  |
Long Term Debt, Rights Limited by Other Securities [Text Block] |  |  |  | Each of the Notes are unsecured obligations and rank pari passu with our existing and future unsecured indebtedness; effectively subordinated to all of our existing and future secured indebtedness; and structurally subordinated to all existing and future indebtedness and other obligations of any of our subsidiaries, financing vehicles, or similar facilities we may form in the future, with respect to claims on the assets of any such subsidiaries, financing vehicles, or similar facilities, including the Revolving Credit Facility. |  |  |
January 2026 Notes |  |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |  |
Long Term Debt, Title [Text Block] |  |  |  | January 2026 Notes |  |  |
Long Term Debt, Principal |  |  | $ 125.0 |  |  |  |
Long Term Debt, Structuring [Text Block] |  |  |  | The January 2026 Notes will mature on January 31, 2026 and bear interest at a rate of 4.75%. The January 2026 Notes may be redeemed in whole or in part at any time or from time to time at our option subject to a make whole provision if redeemed before October 31, 2025 (the date falling three months prior to maturity) and at par thereafter. Interest on the January 2026 Notes is payable on January 31 and July 31 of each year. |  |  |
November 2026 Notes |  |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |  |
Long Term Debt, Title [Text Block] |  |  |  | November 2026 Notes |  |  |
Long Term Debt, Principal |  | $ 125.0 |  |  |  |  |
Long Term Debt, Structuring [Text Block] |  |  |  | The November 2026 Notes will mature on November 15, 2026 and bear interest at a rate of 3.50%. The November 2026 Notes may be redeemed in whole or in part at any time or from time to time at our option subject to a make whole provision if redeemed before August 15, 2026 (the date falling three months prior to maturity) and at par thereafter. Interest on the November 2026 Notes is payable on May 15 and November 15 of each year. We do not intend to list the November 2026 Notes on any securities exchange or automated dealer quotation system. As of September 30, 2025, the outstanding principal balance of the November 2026 Notes was $125.0 million. |  |  |
March 2030 Notes |  |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |  |
Long Term Debt, Title [Text Block] |  |  |  | March 2030 Notes |  |  |
Long Term Debt, Principal | $ 100.0 |  |  |  |  |  |
Long Term Debt, Structuring [Text Block] |  |  |  | The March 2030 Notes will mature on March 19, 2030 and bear interest at a rate of 6.75%. The March 2030 Notes may be redeemed in whole or in part at any time or from time to time at our option subject to a make whole provision if redeemed before September 19, 2029 (the date falling six months prior to maturity) and at par thereafter. Interest on the March 2030 Notes is payable on March 19 and September 19 of each year, beginning September 19, 2025. We do not intend to list the March 2030 Notes on any securities exchange or automated dealer quotation system. As of September 30, 2025, the outstanding principal balance of the March 2030 Notes was $100.0 million. |  |  |

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