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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-02-17</periodOfReport>

    <issuer>
        <issuerCik>0000702325</issuerCik>
        <issuerName>FIRST MIDWEST BANCORP INC</issuerName>
        <issuerTradingSymbol>FMBI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001634279</rptOwnerCik>
            <rptOwnerName>Putnam Angela L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>8750 WEST BRYN MAWR AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1300</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60631</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>SVP, Bank CAO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2019-02-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>660</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22.705</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10926</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On February 17, 2016 the Issuer granted the Reporting Person a Restricted Stock Award (RSA) of 2,658 restricted shares of First Midwest Bancorp, Inc. Common Stock under the First Midwest Bancorp, Inc. Omnibus Stock and Incentive Plan which were scheduled to vest in two equal annual installments on the second and third anniversary of the date of grant.  On February 17, 2019, the remaining 1,329 shares of the originally granted RSAs vested, of which 660 shares were surrendered by the Reporting Person in order to satisfy tax withholding obligations.  This form is being filed to reflect the surrender of such shares to satisfy the Reporting Person's tax withholding obligation.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Andrea L. Stangl, Attorney-in-fact for Angela L. Putnam</signatureName>
        <signatureDate>2019-02-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
