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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-10-15</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001699426</rptOwnerCik>
            <rptOwnerName>Van Arsdell Stephen C</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>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock Under NQ Deferred Comp. Plan</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2018-10-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>587</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>25.567</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>587</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4016</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Shares of phantom stock acquired pursuant to the First Midwest Bancorp, Inc. Deferred Compensation Plan for Nonemployee Directors (the &quot;Plan&quot;) represent shares of First Midwest Bancorp, Inc. common stock on a 1-for-1 basis and are payable in cash upon distribution to the participant in accordance with the terms of the Plan.</footnote>
        <footnote id="F2">Between July 14, 2018 and October 9, 2018, the Reporting Person acquired 587 shares of First Midwest Bancorp, Inc. common stock under the Plan. The information in this report is based on a plan statement dated October 9, 2018.</footnote>
        <footnote id="F3">Between July 11, 2018 and October 9, 2018, the Reporting Person acquired 14 shares of First Midwest Bancorp, Inc. common stock pursuant to the dividend reinvestment feature under the Plan. The information in this report is based on a plan statement dated October 9, 2018.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Andrea L. Stangl, Attorney-in-fact for Stephen C. Van Arsdell</signatureName>
        <signatureDate>2018-10-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
