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

    <schemaVersion>X0508</schemaVersion>

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    <periodOfReport>2025-03-07</periodOfReport>

    <notSubjectToSection16>false</notSubjectToSection16>

    <issuer>
        <issuerCik>0001108426</issuerCik>
        <issuerName>TXNM ENERGY INC</issuerName>
        <issuerTradingSymbol>TXNM</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001637037</rptOwnerCik>
            <rptOwnerName>TARRY JOSEPH DON</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>CORPORATE HEADQUARTERS MS1275</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>ALBUQUERQUE</rptOwnerCity>
            <rptOwnerState>NM</rptOwnerState>
            <rptOwnerZipCode>87158-1275</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>true</isDirector>
            <isOfficer>true</isOfficer>
            <isTenPercentOwner>false</isTenPercentOwner>
            <isOther>false</isOther>
            <officerTitle>President and COO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>false</aff10b5One>

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                <equitySwapInvolved>false</equitySwapInvolved>
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            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
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                </transactionShares>
                <transactionPricePerShare>
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                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                <transactionPricePerShare>
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                <transactionPricePerShare>
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            <transactionTimeliness></transactionTimeliness>
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        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Rights</value>
            </securityTitle>
            <conversionOrExercisePrice>
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            <transactionDate>
                <value>2025-03-07</value>
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            <transactionTimeliness></transactionTimeliness>
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                <transactionShares>
                    <value>2422.00</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.00</value>
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                <transactionAcquiredDisposedCode>
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            </transactionAmounts>
            <exerciseDate>
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            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2422.00</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
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                <sharesOwnedFollowingTransaction>
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                </sharesOwnedFollowingTransaction>
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                <directOrIndirectOwnership>
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                <value>Restricted Stock Rights</value>
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                <value>2025-03-07</value>
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                <equitySwapInvolved>false</equitySwapInvolved>
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                <transactionShares>
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                </transactionShares>
                <transactionPricePerShare>
                    <value>0.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
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            <expirationDate>
                <footnoteId id="F4"/>
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                <underlyingSecurityTitle>
                    <value>Common Stock</value>
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                    <value>795.00</value>
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            </underlyingSecurity>
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                <sharesOwnedFollowingTransaction>
                    <value>29247.00</value>
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        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Represents the portions of previous awards of restricted stock rights that vested effective as of March 7, 2025.</footnote>
        <footnote id="F2">Represents shares withheld by TXNM Energy, Inc. to satisfy the tax withholding obligations arising in connection with the settlement of equity awards. The company utilizes a modified &quot;share withholding&quot; approach in connection with settling equity awards, in which it (i) withholds (in cash) an amount to satisfy tax withholding obligations and remits such amount to the relevant tax authorities, and (ii) directs a designated broker to purchase on the open market the number of shares of the company's common stock that can be acquired with the after-tax value of equity awards at the prevailing market price. Only these &quot;net shares&quot; are delivered to the recipient of the equity awards.</footnote>
        <footnote id="F3">Each restricted stock right represents a contingent right to receive one share of TXNM Energy, Inc. common stock.</footnote>
        <footnote id="F4">The restricted stock units vest in three equal annual installments. Vested shares will be delivered to the reporting person on the applicable vesting dates (or, if the company is in a blackout period under its insider trading policy on any vesting date, at a later date after such blackout period ends).</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Angela L. Pino, POA for Joseph Don Tarry</signatureName>
        <signatureDate>2025-03-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
