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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-11-17</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>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>BGC Holdings Exchangeable Limited Partnership Interests</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2010-11-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1509172</value>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>3.58</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A or Class B Common Stock, par value $0.01 per share</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1509172</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>47299205</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The exchange rights with respect to the BGC Holdings, L.P. (&quot;BGC Holdings&quot;) Exchangeable Limited Partnership Interests (the &quot;Interests&quot;) owned by Cantor Fitzgerald, L.P. (&quot;CFLP&quot;) are exercisable at any time for shares of Class B Common Stock, par value $0.01 per share (&quot;Class B Common Stock&quot;), of BGC Partners, Inc. (&quot;BGC Partners&quot;) (or, at CFLP's option or if there are no additional authorized but unissued shares of Class B Common Stock, shares of Class A Common Stock, par value $0.01 per share (&quot;Class A Common Stock&quot;), of BGC Partners) on a one-for-one basis (subject to adjustment). The shares of Class B Common Stock are convertible at any time on a one-for-one basis (subject to adjustment) into shares of Class A Common Stock.</footnote>
        <footnote id="F2">Exempt acquisition of Interests from BGC Holdings pursuant to Rule 16b-3 under the Securities Exchange Act of 1934, as amended, in accordance with the Agreement of Limited Partnership of BGC Holdings, as amended and restated as of March 31, 2008 (as further amended from time to time).</footnote>
        <footnote id="F3">Represents 1,509,172 Interests purchased from BGC Holdings as a result of redemptions of units of founding partners at an aggregate purchase price of $5,398,610.</footnote>
        <footnote id="F4">Does not include exchange rights with respect to 18,583,026 Interests assumed in previous reports to have been exercised for shares of Class A Common Stock to satisfy CFLP's deferred stock distribution obligations provided to certain current and former partners of CFLP on April 1, 2008.  As of the date of this report, the aggregate number of remaining shares of Class A Common Stock subject to CFLP's deferred stock distribution obligations is 19,362,830, which is assumed to be funded with 779,804 shares of Class A Common Stock acquired upon exercise of exchange rights with respect to 779,804 Interests on May 28, 2010 and the future exercise of exchange rights with respect to the 18,583,026 Interests.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Howard W. Lutnick, Chairman, Chief Executive Officer and President for Cantor Fitzgerald, L.P.</signatureName>
        <signatureDate>2010-11-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
