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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2005-12-20</periodOfReport>

    <dateOfOriginalSubmission>2005-12-21</dateOfOriginalSubmission>

    <issuer>
        <issuerCik>0001041665</issuerCik>
        <issuerName>PMA CAPITAL CORP</issuerName>
        <issuerTradingSymbol>PMACA</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001055629</rptOwnerCik>
            <rptOwnerName>MALONE JAMES F III</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>7720 S.E. LAKE SHORE DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>HOBE SOUND</rptOwnerCity>
            <rptOwnerState>FL</rptOwnerState>
            <rptOwnerZipCode>33455</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2500</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes></footnotes>

    <remarks>This Amendment is being filed for the purpose of correcting the reporting person's direct holdings on the date of the original report, at which time 2,500 shares of Class A Common Stock were mistakenly omitted.</remarks>

    <ownerSignature>
        <signatureName>Joseph W. La Barge - Attorney-in-Fact</signatureName>
        <signatureDate>2006-05-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
