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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2010-05-06</periodOfReport>

    <dateOfOriginalSubmission>2010-05-10</dateOfOriginalSubmission>

    <issuer>
        <issuerCik>0001094831</issuerCik>
        <issuerName>BGC Partners, Inc.</issuerName>
        <issuerTradingSymbol>BGCP</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001024896</rptOwnerCik>
            <rptOwnerName>CANTOR FITZGERALD L P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>499 PARK AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2010-05-06</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>C</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>600000</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>254617</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Cantor Fitzgerald, L.P. (&quot;CFLP&quot;) is amending Table I in the Form 4 originally filed on May 10, 2010 for the purpose of correcting the number of shares of Class A Common Stock that CFLP beneficially owned following both the reported transaction and the distribution of shares of Class A Common Stock to satisfy certain of CFLP's deferred stock distribution obligations that CFLP has to certain current and former CFLP partners.</footnote>
        <footnote id="F2">On May 6, 2010, CFLP converted 600,000 shares of Class B Common Stock into 600,000 shares of Class A Common Stock, of which 554,947 shares were distributed by CFLP on May 7, 2010 to certain current and former partners of CFLP to satisfy certain of CFLP's deferred stock distribution obligations that CFLP provided to such partners on April 1, 2008. In addition, because CFLP had previously distributed 77,791 shares of Class A Common Stock on March 23, 2010 and 71,622 shares of Class A Common Stock on April 12, 2010 to certain current and former partners of CFLP to satisfy certain of CFLP's deferred stock distribution obligations, after the distribution of the 554,947 shares of Class A Common Stock on May 7, 2010, CFLP had beneficial ownership of 254,617 shares of Class A Common Stock (rather than the 404,030 shares previously reported).  (Continue to footnote 3)</footnote>
        <footnote id="F3">After CFLP's distribution of an aggregate of 704,360 shares of Class A Common Stock on March 23, 2010, April 12, 2010 and May 7, 2010, the aggregate number of remaining shares of Class A Common Stock subject to deferred stock distribution obligations that CFLP is required to distribute to current and former CFLP partners is 22,143,400.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Howard W. Lutnick, Chairman, Chief Executive Officer and President for CANTOR FITZGERALD, L.P.</signatureName>
        <signatureDate>2010-05-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
