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

    <schemaVersion>X0306</schemaVersion>

    <documentType>5</documentType>

    <periodOfReport>2017-12-31</periodOfReport>

    <form3HoldingsReported>0</form3HoldingsReported>

    <form4TransactionsReported>0</form4TransactionsReported>

    <issuer>
        <issuerCik>0000007332</issuerCik>
        <issuerName>SOUTHWESTERN ENERGY CO</issuerName>
        <issuerTradingSymbol>SWN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001103813</rptOwnerCik>
            <rptOwnerName>KERLEY GREGORY D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>10000 ENERGY DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SPRING</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77389</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-04-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>689</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>8.0352</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>350810</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-07-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>3882</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>5.6266</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>354692</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-10-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F4"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>3882</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>5.6977</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>358574</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On March 21, 2017, the Issuer declared a quarterly dividend of $15.625 per share on its 6.25% Series B Mandatory Convertible Preferred Stock, payable on April 17, 2017, to holders of record on April 1, 2017.  The dividend was paid in shares of common stock based on the Volume Weighted Average Price per share over the five consecutive trading day period ending on the second trading day immediately preceding the applicable dividend payment day, multiplied by 97%.</footnote>
        <footnote id="F2">Shares of common stock acquired through the dividend paid on the 6.25% Series B Mandatory Convertible Preferred Stock that were not previously reported in the Form 4 filed on May 25, 2017.</footnote>
        <footnote id="F3">On June 20, 2017, the Issuer declared a quarterly dividend of $15.625 per share on its 6.25% Series B Mandatory Convertible Preferred Stock, payable on July 17, 2017, to holders of record on July 1, 2017. The dividend was paid in shares of common stock based on the Volume Weighted Average Price per share over the five consecutive trading day period ending on the second trading day immediately preceding the applicable dividend payment day, multiplied by 97%.</footnote>
        <footnote id="F4">On September 15, 2017, the Issuer declared a quarterly dividend of $15.625 per share on its 6.25% Series B Mandatory Convertible Preferred Stock, payable on October 16, 2017, to holders of record on October 1, 2017. The dividend was paid in shares of common stock based on the Volume Weighted Average Price per share over the five consecutive trading day period ending on the second trading day immediately preceding the applicable dividend payment day, multiplied by 97%.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Melissa D. McCarty, Attorney-in-fact for Mr. Kerley</signatureName>
        <signatureDate>2018-02-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
