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

    <schemaVersion>X0304</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-05-31</periodOfReport>

    <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, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2011-05-31</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>42527</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2011-07-27</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F5"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>150000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents a gift of 42,527 shares of BGC Partners, Inc. (&quot;BGC&quot;) Class A Common Stock, par value $0.01 per share (&quot;Class A Common Stock&quot;), by Cantor Fitzgerald, L.P. (&quot;CFLP&quot;) to a charitable organization.</footnote>
        <footnote id="F2">Does not include an aggregate of 9,780,646 shares of Class A Common Stock acquired by CFLP upon exercise of exchange rights with respect to 9,780,646 BGC Holdings, L.P. Exchangeable Limited Partnership Interests (&quot;Interests&quot;), because such aggregate 9,780,646 shares of Class A Common Stock were assumed in previous reports to have been distributed in connection with CFLP's deferred stock distribution obligations provided to certain current and former partners of CFLP on April 1, 2008. (continued in footnote 3)</footnote>
        <footnote id="F3">Following the reported transaction, the aggregate number of remaining shares of Class A Common Stock subject to CFLP's deferred stock distribution obligations was 19,363,672, which were assumed in prior filings to be funded with (i) 780,646 shares of Class A Common Stock acquired by CFLP upon exercise of exchange rights with respect to 780,646 Interests on May 28, 2010, (ii) the 9,000,000 shares of Class A Common Stock acquired by CFLP upon exercise of exchange rights with respect to 9,000,000 Interests on May 5, 2011, and (iii) the future exercise of exchange rights with respect to 9,583,026 Interests for 9,583,026 shares of Class A Common Stock.  However, since the 42,527 shares of Class A Common Stock gifted on May 31, 2011 and an additional 150,000 shares gifted on July 27, 2011 (see footnote 5) were derived from the exchange of Interests, (continued in footnote 4)</footnote>
        <footnote id="F4">CFLP will need to exchange 192,527 (42,527 plus 150,000) more Interests for 192,527 shares of Class A Common Stock than previously reported to satisfy all of its 19,363,672 remaining deferred stock distribution obligations.</footnote>
        <footnote id="F5">Represents a gift of 150,000 shares of Class A Common Stock by CFLP to a charitable organization.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Howard W. Lutnick, Chairman, Chief Executive Officer and President for Cantor Fitzgerald, L.P.</signatureName>
        <signatureDate>2012-02-13</signatureDate>
    </ownerSignature>
</ownershipDocument>
