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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-09-14</periodOfReport>

    <issuer>
        <issuerCik>0001305014</issuerCik>
        <issuerName>ASHLAND INC.</issuerName>
        <issuerTradingSymbol>ASH</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001166070</rptOwnerCik>
            <rptOwnerName>HALE ROGER W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>218 MOCKINGBIRD GARDENS DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>LOUISVILLE</rptOwnerCity>
            <rptOwnerState>KY</rptOwnerState>
            <rptOwnerZipCode>40207</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Restricted Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1083</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Common Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-09-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>29</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>76.40</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>29</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9991</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F5"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-09-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F6"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>66</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>76.40</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F7"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F7"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>66</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>22460</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Represents shares of Restricted Common Stock acquired pursuant to Ashland's stock incentive plans as approved by the shareholders and exempt pursuant to Rule 16b-3.  83 of these shares are held in the dividend reinvestment plan administered by Ashland's transfer agent.</footnote>
        <footnote id="F2">1-for-1</footnote>
        <footnote id="F3">Common Stock Units acquired pursuant to Ashland's Deferred Compensation Plan for Non-Employee Directors (the &quot;Plan&quot;) as of 9-14-2012 and exempt under Rule 16b-3. (One (1) Common Stock Unit in the Plan is the equivalent of one (1) share of Ashland Common Stock.)</footnote>
        <footnote id="F4">Subject to any deferral election on timing of distribution by the reporting person under the Plan, the Common Stock Units are payable in Common Stock upon the reporting person's separation from service as a director.</footnote>
        <footnote id="F5">Each Restricted Stock Unit represents a right to receive one (1) share of Ashland Common Stock.</footnote>
        <footnote id="F6">Restricted Stock Units acquired pursuant to Ashland's Deferred Compensation Plan for Non-Employee Directors (the &quot;Plan&quot;) as of 9-14-12 and exempt from Rule 16b-3. (One (1) Restricted Stock Unit in the Deferred Compensation Plan for Non-Employee Directors is the equivalent of one (1) share of Ashland Common Stock.)</footnote>
        <footnote id="F7">Subject to any deferral election on timing of distribution by the reporting person under the Plan, the reporting person may, prior to vesting, elect to receive the Restricted Stock Units in Common Stock or cash upon separation from service as a director.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ David A. Rines, Attorney-in-Fact</signatureName>
        <signatureDate>2012-09-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
