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

    <schemaVersion>X0305</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-05-04</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>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>BGC Holdings Exchangeable Limited Partnership Interests</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-05-04</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>34160</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>3.96</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A or Class B Common Stock, par value $0.01 per share</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>34160</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F3"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>35786608</value>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On May 4, 2012, Cantor Fitzgerald, L.P. (&quot;CFLP&quot;) purchased from BGC Holdings, L.P. (&quot;BGC Holdings&quot;) an aggregate of 34,160 exchangeable limited partnership interests in BGC Holdings (the &quot;Interests&quot;) at a price of $3.96 per Interest in transactions exempt 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, the &quot;BGC Holdings Agreement&quot;).</footnote>
        <footnote id="F2">The exchange rights with respect to the Interests 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="F3">Includes 9,380 Interests purchased from BGC Holdings as a result of the redemption of 9,380 non-exchangeable founding partner units, at an aggregate purchase price of $37,145, and 24,780 Interests purchased from BGC Holdings as a result of the redemption of 24,780 non-exchangeable founding partner units, at an aggregate purchase price of $98,129.</footnote>
        <footnote id="F4">Does not include exchange rights with respect to an aggregate of 12,996,325 Interests assumed in previous reports to have been exercised for shares of Class A Common Stock to satisfy CFLP's deferred share distribution obligations pursuant to rights provided to certain current and former partners of CFLP on April 1, 2008 and February 14, 2012.  As of the date of this report, an aggregate of 17,473,709 shares of Class A Common Stock remain subject to CFLP's deferred share distribution obligations pursuant to rights to receive shares provided to certain current and former partners of CFLP on April 1, 2008 (15,545,606 remaining shares) and the deferral by certain partners of CFLP of receipt of shares distributed by CFLP on February 14, 2012 in connection with payment of previous quarterly CFLP distributions (1,928,103 remaining shares).</footnote>
        <footnote id="F5">As of the date of this report, the aggregate 17,473,709 remaining shares of Class A Common Stock subject to CFLP's deferred share distribution obligations are assumed to be funded with (i) 588,119 shares of Class A Common Stock acquired by CFLP upon exercise of exchange rights with respect to 588,119 Interests on May 28, 2010, (ii) 3,889,265 shares of Class A Common Stock acquired by CFLP upon exercise of exchange rights with respect to 3,889,265 Interests on May 5, 2011, and (iii) the future exercise of exchange rights with respect to 12,996,325 Interests for 12,996,325 shares of Class A Common Stock.  As of the date of this report, CFLP held an aggregate of 48,782,933 Interests.</footnote>
        <footnote id="F6">Due to a typographical error, the number of shares in Column 9 of Table II in a Form 4 filed by CFLP on March 15, 2012 was incorrectly inserted as 35,169,615 and should have been 35,752,448.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Howard W. Lutnick, Chairman, Chief Executive Officer and President for Cantor Fitzgerald, L.P.</signatureName>
        <signatureDate>2012-05-08</signatureDate>
    </ownerSignature>
</ownershipDocument>
