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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-02-04</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001376712</rptOwnerCik>
            <rptOwnerName>Lutz Laurent Charles</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>EVP, GC and Secretary</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-02-04</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>18324</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>6.06</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>993128.4601</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-02-04</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1736</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>6.06</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>991392.4601</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">As previously reported, on February 4, 2014, Mr. Lutz was granted 54,556 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 February 4, 2015, 2016 and 2017. Mr. Lutz subsequently received an additional 296 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 February 4, 2014 award was adjusted in the spin-off of Navient to reflect 152,131 RSUs outstanding. On February 4, 2016, 51,006 shares vested in connection with these RSUs, of which 18,324 shares were withheld by the Company to satisfy Mr. Lutz's tax withholding obligations.</footnote>
        <footnote id="F2">On February 4, 2014, a portion of Mr. Lutz's 2013 Management Incentive Plan Award was deferred in the form of vested RSUs that carry transfer restrictions that lapse in one-third increments on February 4, 2015, February 4, 2016 and February 4, 2017. On February 4, 2016, the restrictions lapsed on 5,252 RSUs, and 31 additional shares issued to Mr. Lutz pursuant to the related divided equivalent rights, and 1,736 shares of the Company common stock were withheld to satisfy Mr. Lutz's tax obligations in connection with the lapsing of such restrictions.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Nicolas Jafarieh (POA) for Laurent C. Lutz</signatureName>
        <signatureDate>2016-02-08</signatureDate>
    </ownerSignature>
</ownershipDocument>
