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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-01-03</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001690454</rptOwnerCik>
            <rptOwnerName>BROSNAN MICHAEL L.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O CIT GROUP INC.</rptOwnerStreet1>
            <rptOwnerStreet2>340 MOUNT KEMBLE AVENUE, SUITE 100</rptOwnerStreet2>
            <rptOwnerCity>MORRISTOWN</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>07960</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2022-01-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>22039.571</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2022-01-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2752.114</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2752.114</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Pursuant to the Agreement and Plan of Merger, dated October 15, 2020, by and among CIT Group Inc. (&quot;CIT&quot;), First Citizens BancShares, Inc. (&quot;BancShares&quot;), First-Citizens Bank &amp; Trust Company, and FC Merger Subsidiary IX, Inc. (as amended from time to time, the &quot;Merger Agreement&quot;), a transaction exempt under Rule 16b-3, each issued and outstanding share of common stock, par value $0.01 per share, of CIT (&quot;CIT Common Stock&quot;) was converted into 0.06200 (the &quot;Exchange Ratio&quot;) shares of Class A common stock, par value $1.00 per share, of BancShares (the &quot;BancShares Class A Common Stock&quot;). On December 31, 2021 (the business day prior to the merger), the closing price of one share of CIT Common Stock was $51.34.</footnote>
        <footnote id="F2">Each CIT RSU had the economic equivalent of one share of CIT Common Stock.</footnote>
        <footnote id="F3">For Mr. Brosnan, pursuant to the Merger Agreement, a transaction exempt under Rule 16b-3, the unvested restricted stock units in respect of shares of CIT Common Stock (the &quot; CIT RSUs&quot;) (not subject to deferral) automatically vested upon the effective time and were converted into 170 shares of BancShares Class A Common Stock as multiplied by the Exchange Ratio less fractional shares paid in cash.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ James P. Shanahan, attorney-in-fact for Mr. Brosnan</signatureName>
        <signatureDate>2022-01-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
