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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-08-16</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>2010-07-27</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>194050</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </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>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2010-08-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>200000</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>200000</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </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>2010-08-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F4"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>200000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>BGC Holdings Exchangeable Limited Partnership Interests</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2010-08-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>200000</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock, par value $0.01 per share</value>
                    <footnoteId id="F3"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>200000</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>45250457</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Represents a gift of 194,050 shares of BGC Partners, Inc. (&quot;BGC Partners&quot;) Class A Common Stock, par value $0.01 per share (the &quot;Class A Common Stock&quot;), by Cantor Fitzgerald, L.P. (&quot;CFLP&quot;) to a charitable organization.</footnote>
        <footnote id="F2">On August 16, 2010, CFLP exercised exchange rights with respect to 200,000 BGC Holdings, L.P. (&quot;BGC Holdings&quot;) Exchangeable Limited Partnership Interests for 200,000 shares of Class A Common Stock.</footnote>
        <footnote id="F3">The exchange rights with respect to the BGC Holdings Exchangeable Limited Partnership Interests (the &quot;Interests&quot;) owned by CFLP are exercisable at any time for shares of Class B Common Stock, par value $0.01 per share (the &quot;Class B Common Stock&quot;), of BGC Partners (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) 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="F4">Represents a gift of 200,000 shares of Class A Common Stock by CFLP to a charitable organization.</footnote>
        <footnote id="F5">Does not include 3,446,055 shares of Class A Common Stock acquired upon exercise of exchange rights with respect to 3,446,055 Interests (3,500,000 shares acquired by CFLP on May 28, 2010 upon exercise of exchange rights with respect to 3,500,000 Interests less 53,945 shares distributed by CFLP on August 12, 2010 to a partner of CFLP to satisfy certain of CFLP's deferred stock distribution obligations) because such 3,446,055 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.</footnote>
        <footnote id="F6">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.  In Table II, Column 9 of a Form 4 filed by CFLP on June 2, 2010, CFLP reported that it held an aggregate of 44,763,321 Interests. The number of Interests held by CFLP reported in this Form 4 decreased by 200,000 as a result of the exercise of exchange rights with respect to 200,000 Interests reported in this Form 4.  In addition, however, because CFLP previously distributed an aggregate of 687,136 shares of Class A Common Stock unrelated to the exercise of exchange rights with respect to Interests to satisfy certain of CFLP's deferred stock distribution obligations (consisting of  (Continue to footnote 7)</footnote>
        <footnote id="F7">(i) 71,622 shares of Class A Common Stock on April 12, 2010, (ii) 554,947 shares of Class A Common Stock acquired by CFLP upon the conversion of an equal number of shares of Class B Common Stock owned by CFLP on May 6, 2010 and (iii) 60,567 shares of Class A Common Stock on June 4, 2010), in this report the number of Interests held by CFLP increased by a net of 487,136 Interests to 45,250,457 Interests.  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 22,029,081, which is assumed to be funded with the 3,446,055 shares of Class A Common Stock acquired upon exercise of exchange rights with respect to 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>2010-08-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
