<?xml version="1.0" encoding="utf-8"?>
<xbrl
  xmlns="http://www.xbrl.org/2003/instance"
  xmlns:cef="http://xbrl.sec.gov/cef/2023"
  xmlns:dei="http://xbrl.sec.gov/dei/2023"
  xmlns:iso4217="http://www.xbrl.org/2003/iso4217"
  xmlns:link="http://www.xbrl.org/2003/linkbase"
  xmlns:xlink="http://www.w3.org/1999/xlink">
    <link:schemaRef xlink:href="ck0001513363-20240229.xsd" xlink:type="simple"/>
    <context id="P02_29_2024To02_29_2024">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001513363</identifier>
        </entity>
        <period>
            <startDate>2024-02-29</startDate>
            <endDate>2024-02-29</endDate>
        </period>
    </context>
    <context id="P02_28_2024To02_28_2024">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001513363</identifier>
        </entity>
        <period>
            <startDate>2024-02-28</startDate>
            <endDate>2024-02-28</endDate>
        </period>
    </context>
    <unit id="Unit_USD">
        <measure>iso4217:USD</measure>
    </unit>
    <dei:AmendmentFlag contextRef="P02_29_2024To02_29_2024" id="ixv-298">false</dei:AmendmentFlag>
    <dei:EntityCentralIndexKey contextRef="P02_29_2024To02_29_2024" id="ixv-299">0001513363</dei:EntityCentralIndexKey>
    <dei:DocumentType contextRef="P02_29_2024To02_29_2024" id="ixv-317">8-K</dei:DocumentType>
    <dei:DocumentPeriodEndDate contextRef="P02_29_2024To02_29_2024" id="ixv-318">2024-02-29</dei:DocumentPeriodEndDate>
    <dei:EntityRegistrantName contextRef="P02_29_2024To02_29_2024" id="ixv-319">Fidus Investment Corporation</dei:EntityRegistrantName>
    <dei:EntityIncorporationStateCountryCode contextRef="P02_29_2024To02_29_2024" id="ixv-320">MD</dei:EntityIncorporationStateCountryCode>
    <dei:EntityFileNumber contextRef="P02_29_2024To02_29_2024" id="ixv-321">814-00861</dei:EntityFileNumber>
    <dei:EntityTaxIdentificationNumber contextRef="P02_29_2024To02_29_2024" id="ixv-322">27-5017321</dei:EntityTaxIdentificationNumber>
    <dei:EntityAddressAddressLine1 contextRef="P02_29_2024To02_29_2024" id="ixv-323">1603 Orrington Avenue</dei:EntityAddressAddressLine1>
    <dei:EntityAddressAddressLine2 contextRef="P02_29_2024To02_29_2024" id="ixv-324">Suite 1005</dei:EntityAddressAddressLine2>
    <dei:EntityAddressCityOrTown contextRef="P02_29_2024To02_29_2024" id="ixv-325">Evanston</dei:EntityAddressCityOrTown>
    <dei:EntityAddressStateOrProvince contextRef="P02_29_2024To02_29_2024" id="ixv-326">IL</dei:EntityAddressStateOrProvince>
    <dei:EntityAddressPostalZipCode contextRef="P02_29_2024To02_29_2024" id="ixv-327">60201</dei:EntityAddressPostalZipCode>
    <dei:CityAreaCode contextRef="P02_29_2024To02_29_2024" id="ixv-328">847</dei:CityAreaCode>
    <dei:LocalPhoneNumber contextRef="P02_29_2024To02_29_2024" id="ixv-329">859-3940</dei:LocalPhoneNumber>
    <dei:WrittenCommunications contextRef="P02_29_2024To02_29_2024" id="ixv-330">false</dei:WrittenCommunications>
    <dei:SolicitingMaterial contextRef="P02_29_2024To02_29_2024" id="ixv-331">false</dei:SolicitingMaterial>
    <dei:PreCommencementTenderOffer contextRef="P02_29_2024To02_29_2024" id="ixv-332">false</dei:PreCommencementTenderOffer>
    <dei:PreCommencementIssuerTenderOffer contextRef="P02_29_2024To02_29_2024" id="ixv-333">false</dei:PreCommencementIssuerTenderOffer>
    <dei:Security12bTitle contextRef="P02_29_2024To02_29_2024" id="ixv-334">Common Stock, par value $0.001 per share</dei:Security12bTitle>
    <dei:TradingSymbol contextRef="P02_29_2024To02_29_2024" id="ixv-335">FDUS</dei:TradingSymbol>
    <dei:SecurityExchangeName contextRef="P02_29_2024To02_29_2024" id="ixv-336">NASDAQ</dei:SecurityExchangeName>
    <dei:EntityEmergingGrowthCompany contextRef="P02_29_2024To02_29_2024" id="ixv-337">false</dei:EntityEmergingGrowthCompany>
    <cef:LongTermDebtTableTextBlock contextRef="P02_29_2024To02_29_2024" id="ixv-180">&lt;div style="margin-top:6pt; margin-bottom:0pt; text-indent:4%; font-size:10pt; font-family:Times New Roman"&gt;On November&#160;10, 2022, Fidus Investment Corporation (the &#x201c;Company&#x201d;) established an &lt;div style="white-space: nowrap; letter-spacing: 0px; top: 0px;;display:inline;"&gt;&lt;div style="white-space: nowrap; letter-spacing: 0px; top: 0px;;display:inline;"&gt;&#x201c;at-the-market&#x201d;&lt;/div&gt;&lt;/div&gt; offering (the &#x201c;ATM Program&#x201d;) through which the Company may sell, from time to time through the Sales Agents (as defined below), shares of the Company&#x2019;s common stock, par value $0.001 per share (the &#x201c;Shares&#x201d;). &lt;/div&gt;&lt;div style="margin-top:12pt; margin-bottom:0pt; text-indent:4%; font-size:10pt; font-family:Times New Roman"&gt;On February&#160;29, 2024, the Company increased the maximum amount of Shares to be sold through the ATM Program to $300.0&#160;million from $150.0&#160;million. In connection with the upsize of the ATM Program, the Company entered into amendment no. 2 (&#x201c;Amendment No.&#160;2&#x201d;) to the equity distribution agreement, dated November&#160;10, 2022 (the &#x201c;Equity Distribution Agreement&#x201d;) with Fidus Investment Advisors, LLC, Raymond James&#160;&amp;amp; Associates, Inc. (&#x201c;Raymond James&#x201d;) and B. Riley Securities, Inc. (&#x201c;B. Riley&#x201d; and together with Raymond James, the &#x201c;Sales Agents&#x201d;). Under the Equity Distribution Agreement,&#160;as amended by Amendment No. 2,&#160;the Company may, but has no obligation to, issue and sell up to $300.0&#160;million in aggregate amount of Shares in the ATM Program, from time to time through the Sales Agents, or to them, as principal for their own account. As of February&#160;28, 2024 and after giving effect to Amendment No.&#160;2, up to approximately $178.1&#160;million in aggregate amount of the Shares will remain available for sale under the ATM Program. &lt;/div&gt;</cef:LongTermDebtTableTextBlock>
    <cef:LongTermDebtTitleTextBlock contextRef="P02_29_2024To02_29_2024" id="ixv-338">ATM Program</cef:LongTermDebtTitleTextBlock>
    <cef:LongTermDebtPrincipal
      contextRef="P02_28_2024To02_28_2024"
      decimals="-5"
      id="ixv-339"
      unitRef="Unit_USD">178100000</cef:LongTermDebtPrincipal>
    <cef:LongTermDebtStructuringTextBlock contextRef="P02_29_2024To02_29_2024" id="ixv-185">&lt;div style="margin-top:12pt; margin-bottom:0pt; text-indent:4%; font-size:10pt; font-family:Times New Roman"&gt;Further details regarding the Equity Distribution Agreement, as amended by Amendment No.&#160;2, and the ATM Program are set forth in the Company&#x2019;s prospectus supplement, dated November&#160;10, 2022 (the &#x201c;ATM Prospectus Supplement&#x201d;), supplement no. 1 to the ATM Prospectus Supplement, dated March&#160;2, 2023 (&#x201c;Supplement No.&#160;1&#x201d;), supplement no. 2 to the ATM Prospectus Supplement, dated May&#160;8, 2023 (&#x201c;Supplement No.&#160;2&#x201d;), supplement no. 3 to the ATM Prospectus Supplement, dated August&#160;4, 2023 (&#x201c;Supplement No.&#160;3&#x201d;), supplement no. 4 to the ATM Prospectus Supplement, dated August&#160;11, 2023 (&#x201c;Supplement No.&#160;4&#x201d;), supplement no. 5 to the ATM Prospectus Supplement, dated November&#160;2, 2023 (&#x201c;Supplement No.&#160;5&#x201d;), and supplement no. 6 to the ATM Prospectus Supplement, dated February&#160;29, 2024 (&#x201c;Supplement No.&#160;6&#x201d; and together with the ATM Prospectus Supplement, Supplement No.&#160;1, Supplement No.&#160;2, Supplement No.&#160;3, Supplement No.&#160;4, and Supplement No.&#160;5 and including, in each case, any information incorporated by reference therein, the &#x201c;Prospectus&#x201d;), filed by the Company with the Securities and Exchange Commission. &lt;/div&gt;</cef:LongTermDebtStructuringTextBlock>
</xbrl>
