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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-12-31</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001439296</rptOwnerCik>
            <rptOwnerName>Quinlan Raymond J.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>300 CONTINENTAL DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEWARK</rptOwnerCity>
            <rptOwnerState>DE</rptOwnerState>
            <rptOwnerZipCode>19713</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <officerTitle>Chairman &amp; CEO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2015-12-31</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>17024</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>6.52</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>759560</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">As previously reported, on January 21, 2014, Mr. Quinlan was granted 53,630 restricted stock units (&quot;RSUs&quot;) representing rights to receive shares of common stock of SLM Corporation (the &quot;Company&quot;) to vest in one-third increments on December 31, 2014, 2015 and 2016. Mr. Quinlan subsequently received an additional 873 RSUs relating to the original RSU grant and vesting on the same schedule. These additional RSUs were attributable to dividends paid by the Company prior to the separation of Navient Corporation (&quot;Navient&quot;). As previously reported, subsequently on April 30, 2014, the January 21, 2014 award was adjusted in the spin-off of Navient to reflect 149,549 RSUs outstanding. On December 31, 2015, 50,141 shares vested in connection with these RSUs, of which 17,024 shares were withheld by the Company to satisfy Mr. Quinlan's tax withholding obligations.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Nicolas Jafarieh (POA) for Raymond J. Quinlan</signatureName>
        <signatureDate>2016-01-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
