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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-05-01</periodOfReport>

    <issuer>
        <issuerCik>0000084246</issuerCik>
        <issuerName>RLI CORP</issuerName>
        <issuerTradingSymbol>RLI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001354119</rptOwnerCik>
            <rptOwnerName>Kennedy Daniel O</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>9025 N. LINDBERGH DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>PEORIA</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>61615</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>Vice President General Counsel</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>36801.364</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>8080.6105</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Esop</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1145.7037</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Executive Deferred Compensation</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>43.21</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2014-05-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>13500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>43.21</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2015-05-01</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2022-05-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>13500</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>13500</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>13.45</value>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
                <footnoteId id="F7"/>
                <footnoteId id="F8"/>
                <footnoteId id="F9"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2010-05-07</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-05-07</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4120</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4120</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>18.17</value>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
                <footnoteId id="F7"/>
                <footnoteId id="F8"/>
                <footnoteId id="F9"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2011-05-06</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2018-05-06</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>7040</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7040</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>22.865</value>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
                <footnoteId id="F7"/>
                <footnoteId id="F9"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2012-05-05</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2019-05-05</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>12000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>30.35</value>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
                <footnoteId id="F9"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2013-05-03</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2020-05-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>20000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>20000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>34.42</value>
                <footnoteId id="F5"/>
                <footnoteId id="F9"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2014-05-02</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2021-05-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>20000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>20000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Ownership reflects dividend reinvestment.</footnote>
        <footnote id="F2">Balance reflects annual company contributions and dividend reinvestment.</footnote>
        <footnote id="F3">Ownership reflects dividend reinvestment</footnote>
        <footnote id="F4">Pursuant to option schedule wherein 20% of the aggregate number of shares granted may be exercised commencing one year from grant date and each year thereafter in 20% increments.</footnote>
        <footnote id="F5">Stock option grant price adjusted to reflect $3.00 extraordinary dividend declared 11/14/13.</footnote>
        <footnote id="F6">Stock option grant price adjusted to reflect $5 extraordinary dividend declared 11/14/12.</footnote>
        <footnote id="F7">Stock Option grant price adjusted to reflect $5 RLI extraordinary dividend declared 11/17/11.</footnote>
        <footnote id="F8">Stock Option grant price adjusted to reflect $7 RLI extraordinary dividend paid 12/29/10.</footnote>
        <footnote id="F9">Stock option grant price and number of stock options adjusted to reflect 2-for-1 stock split on 01/15/14.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Daniel O. Kennedy</signatureName>
        <signatureDate>2014-05-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
