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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2016-02-01</periodOfReport>

    <dateOfOriginalSubmission>2016-02-02</dateOfOriginalSubmission>

    <issuer>
        <issuerCik>0001171825</issuerCik>
        <issuerName>CIT GROUP INC</issuerName>
        <issuerTradingSymbol>CIT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001661378</rptOwnerCik>
            <rptOwnerName>GALLIGAN MATTHEW</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O CIT GROUP INC. - GENERAL COUNSEL</rptOwnerStreet1>
            <rptOwnerStreet2>1 CIT DRIVE</rptOwnerStreet2>
            <rptOwnerCity>LIVINGSTON</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>07039</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>President, Real Estate Finance</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-02-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>3247</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>29.86</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12370.852</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Shares withheld to satisfy tax withholding obligations arising from 2/1/2016 settlement of restricted stock units granted on 1/30/2013, 2/20/2014 and 1/28/2015.</footnote>
        <footnote id="F2">This amendment corrects the number of shares disposed due to an administrative error in the calculation of the number of shares to be withheld to cover tax obligations on the settlement of RSUs.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/James P. Shanahan, attorney-in-fact for Mr. Galligan</signatureName>
        <signatureDate>2016-02-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
