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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-02-04</periodOfReport>

    <issuer>
        <issuerCik>0001032033</issuerCik>
        <issuerName>SLM CORP</issuerName>
        <issuerTradingSymbol>SLM</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001633230</rptOwnerCik>
            <rptOwnerName>Boyles Jonathan</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>300 CONTINENTAL DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEWARK</rptOwnerCity>
            <rptOwnerState>DE</rptOwnerState>
            <rptOwnerZipCode>19713</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>SVP &amp; Controller</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-02-04</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2120</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>12.04</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>188782.3867</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On February 4, 2014, Mr. Boyles was awarded 6,846 Restricted Stock Units (&quot;RSUs&quot;) representing rights to receive shares of common stock of SLM Corporation (the &quot;Company&quot;) that were subsequently adjusted in the spin-off of Navient to reflect 19,203 RSUs outstanding and an additional 112 RSUs attributable to dividends. On February 4, 2017, 6,402 shares of such RSUs vested, of which 2,120 shares of the Company common stock were withheld to satisfy Mr. Boyles' s tax withholding obligations.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Nicolas Jafarieh (POA) for Jonathan R. Boyles</signatureName>
        <signatureDate>2017-02-07</signatureDate>
    </ownerSignature>
</ownershipDocument>
