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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-05-20</periodOfReport>

    <issuer>
        <issuerCik>0001342126</issuerCik>
        <issuerName>Morgans Hotel Group Co.</issuerName>
        <issuerTradingSymbol>MHGC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001421021</rptOwnerCik>
            <rptOwnerName>Malone Michael D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O MORGANS HOTEL GROUP CO.</rptOwnerStreet1>
            <rptOwnerStreet2>475 TENTH AVENUE</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10018</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>LTIP Units</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2008-05-20</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3623</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3623</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3623</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Represents units of membership interest (&quot;Membership Units&quot;) in Morgans Group LLC, the operating company and a subsidiary of Morgans Hotel Group Co. (the &quot;Company&quot;), which are structured as profits interests (&quot;LTIP Units&quot;). Conditioned upon minimum allocations to the capital accounts of the LTIP Units for federal income tax purposes, each LTIP Unit may be converted, at the election of the holder, into one Membership Unit. Each of these Membership Units is redeemable at the election of the holder for: (1) one share of Common Stock, or (2) at the option of the Company, cash equal to the then fair market value of one share of Common Stock. The right of a holder to convert LTIP Units into Membership Units and to have the Membership Units redeemed does not have an expiration date. LTIP Units granted to the reporting person are subject to the terms and conditions of the Company's Amended and Restated 2007 Omnibus Incentive Plan. The LTIP Units vested immediately on the date of the grant.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Richard Szymanski, by power of attorney</signatureName>
        <signatureDate>2008-05-22</signatureDate>
    </ownerSignature>
</ownershipDocument>
