<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-01-27</periodOfReport>

    <notSubjectToSection16>false</notSubjectToSection16>

    <issuer>
        <issuerCik>0000876437</issuerCik>
        <issuerName>MGIC INVESTMENT CORP</issuerName>
        <issuerTradingSymbol>MTG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001793595</rptOwnerCik>
            <rptOwnerName>Thompson Steven M.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>250 EAST KILBOURN AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MILWAUKEE</rptOwnerCity>
            <rptOwnerState>WI</rptOwnerState>
            <rptOwnerZipCode>53202</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>false</isDirector>
            <isOfficer>true</isOfficer>
            <isTenPercentOwner>false</isTenPercentOwner>
            <isOther>false</isOther>
            <officerTitle>EVP&amp;CRO-Mort Gty Ins Corp</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>CommonStock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-01-27</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>false</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <footnoteId id="F1"/>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>108000.00</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>149121.00</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">These represent restricted stock units (&quot;RSUs&quot;) that will vest on February 28, 2023 to the extent, if any, the performance goal to which they are subject is met.  If it is not possible to determine whether the performance goal has been met on such vesting date, the vesting date generally shall be extended until such determination can be made.  Any of these RSUs that have not vested on April 15, 2023 shall be forfeited.</footnote>
        <footnote id="F2">These RSUs were awarded to the reporting person pursuant to the Issuer's 2015 Omnibus Incentive Plan and no price was paid by the reporting person for them.</footnote>
    </footnotes>

    <remarks>This Form 4 is being signed by the reporting person's attorney-in-fact pursuant to a previously filed power of attorney.</remarks>

    <ownerSignature>
        <signatureName>Martha F. Tsuchihashi, Attorney-in-Fact</signatureName>
        <signatureDate>2020-01-29</signatureDate>
    </ownerSignature>
</ownershipDocument>
