<?xml version="1.0"?>

<ownershipDocument>

    <documentType>4</documentType>

    <periodOfReport>2003-07-07</periodOfReport>


    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000751652</issuerCik>
        <issuerName>QUESTAR CORP</issuerName>
        <issuerTradingSymbol>STR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001179019</rptOwnerCik>
            <rptOwnerName>PARKS S E</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Sr. VP, Treasurer, and CFO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeSecurity>
        <securityTitle>
            <value>Phantom Stock Units</value>
        </securityTitle>
        <conversionOrExercisePrice>
            <value>33.64</value>
        </conversionOrExercisePrice>
        <transactionDate>
            <value>2003-07-07</value>
        </transactionDate>
        <transactionCoding>
            <transactionFormType>4</transactionFormType>
            <transactionCode>A</transactionCode>
            <equitySwapInvolved>0</equitySwapInvolved>
        </transactionCoding>
        <transactionTimeliness>
            <value></value>
        </transactionTimeliness>
        <transactionAmounts>
            <transactionShares>
                <value>6.4209</value>
            </transactionShares>
            <transactionValue>
                <value>33.64</value>
            </transactionValue>
            <transactionAcquiredDisposedCode>
                <value>A</value>
            </transactionAcquiredDisposedCode>
        </transactionAmounts>
        <exerciseDate>
            <value>1988-08-08</value>
            <footnoteId id="F1"/>
        </exerciseDate>
        <expirationDate>
            <value>1988-08-08</value>
            <footnoteId id="F2"/>
        </expirationDate>
        <underlyingSecurity>
            <underlyingSecurityTitle>
                <value>Phantom Stock Units</value>
            </underlyingSecurityTitle>
            <underlyingSecurityShares>
                <value>6.4209</value>
            </underlyingSecurityShares>
        </underlyingSecurity>
        <postTransactionAmounts>
            <sharesOwnedFollowingTransaction>
                <value>3065.503</value>
                <footnoteId id="F3"/>
            </sharesOwnedFollowingTransaction>
        </postTransactionAmounts>
        <ownershipNature>
            <directOrIndirectOwnership>
                <value>D</value>
            </directOrIndirectOwnership>
        </ownershipNature>
    </derivativeSecurity>

    <footnotes>
        <footnote id="F1">Phantom Stock Units will be converted to cash beginning at retirement; retirement date is unknown.</footnote>
        <footnote id="F2">Phantom Stock Units will be converted to cash beginning at retirement; retirement date is unknown.</footnote>
        <footnote id="F3">Total includes 1,614.3552 Phantom Stock Units in an excess benefit plan sponsored by Questar in addition to units held through an account in a deferred compensation plan.  This total does not include 76,949 shares held directly, 21,496.87 equivalent shares held indirectly through Questar's Employee Investment Plan as of June 17, 2003, or 152,449 vested stock options.  (Detailed information concerning my options has been previously disclosed).</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Connie C. Holbrook Attorney in Fact for S.E. Parks</signatureName>
        <signatureDate>2003-07-08</signatureDate>
    </ownerSignature>

</ownershipDocument>

