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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-01-19</periodOfReport>

    <issuer>
        <issuerCik>0001364742</issuerCik>
        <issuerName>BlackRock Inc.</issuerName>
        <issuerTradingSymbol>BLK</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001542481</rptOwnerCik>
            <rptOwnerName>Mallow Matthew J.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>BLACKROCK, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>55 EAST 52ND STREET</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10055</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>General Counsel and Senior MD</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-01-19</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>4221</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>19839.12</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Reflects an award value of $1,225,000 converted to a number of Restricted Stock Units by dividing the approved award value by $296.120, which was the average of the high and low price per share of Common Stock on January 19, 2016. This grant was approved by the Management Development and Compensation Committee on January 13, 2016. Restricted Stock Units vest in equal installments on 1/31/17, 1/31/18 and 1/31/19.</footnote>
        <footnote id="F2">Includes 300.12 shares of Common Stock acquired by the reporting person under the BlackRock, Inc. Employee Stock Purchase Plan. Also includes (i) 1,720 and 2,149 Restricted Stock Units vesting on 1/31/16, (ii) 3,030 Restricted Stock Units vesting in equal installments on 1/31/16 and 1/31/17, (iii) 3,853 Restricted Stock Units vesting in equal installments on 1/31/16, 1/31/17 and 1/31/18, (iv) 4,221 Restricted Stock Units that will vest in equal installments on 1/31/17, 1/31/18 and 1/31/19 and (v) 1,723 Restricted Stock Units vesting on 1/31/17. Each Restricted Stock Unit is payable solely by delivery of an equal number of shares of Common Stock.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Daniel R. Waltcher as Attorney-in-Fact for Matthew J. Mallow</signatureName>
        <signatureDate>2016-01-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
