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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-01-24</periodOfReport>

    <issuer>
        <issuerCik>0000086104</issuerCik>
        <issuerName>SAFECO CORP</issuerName>
        <issuerTradingSymbol>SAFC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001205174</rptOwnerCik>
            <rptOwnerName>RUDDY JAMES W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>SAFECO PLAZA</rptOwnerStreet1>
            <rptOwnerStreet2>4333 BROOKLYN AVENUE N.E.</rptOwnerStreet2>
            <rptOwnerCity>SEATTLE</rptOwnerCity>
            <rptOwnerState>WA</rptOwnerState>
            <rptOwnerZipCode>98185</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Executive VP &amp; Gen. Counsel</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2005-01-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1.125</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>29.09</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1.125</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>241.549</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">These Phantom Stock Units may be converted to shares of Safeco stock as follows:  The number of Phantom Stock Units acquired multiplied by the price of Phantom Stock Units on the transaction date divided by the closing price of Safeco stock on the transaction date.</footnote>
        <footnote id="F2">These Phantom Stock Units accrued under the Safeco Corporation Deferred Compensation Plan for Executives (the &quot;Plan&quot;) and will be settled in cash following termination of the reporting person's employment with Safeco Corporation or may be reallocated to another investment option pursuant to the terms of the Plan.</footnote>
        <footnote id="F3">Consists of previously reported Phantom Stock Units under the Plan that were converted into Phantom Stock Units reflecting a measurement fund that is parallel to and tracks the Safeco Stock Ownership Fund under Safeco's 401(k) /Proft Sharing Retirement Plan, and that are convertible into shares of Safeco stock as explained in footnote (1), and new units acquired as reported above.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Stephanie Daley-Watson,  Attorney-in-Fact for James W. Ruddy</signatureName>
        <signatureDate>2005-01-26</signatureDate>
    </ownerSignature>
</ownershipDocument>

