<?xml version="1.0" encoding="utf-8"?>
<FilingSummary xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Version>2.2.0.25</Version>
  <ReportType />
  <MyReports>
    <Report>
      <IsDefault>true</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>0010 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <Role>http://www.montpelierre.bm/role/BalanceSheet</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R1.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>0015 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ShortName>CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <Role>http://www.montpelierre.bm/role/BalanceSheetParenthetical</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R2.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>0020 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME</LongName>
      <ShortName>CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME</ShortName>
      <Role>http://www.montpelierre.bm/role/StatementOfIncomeAndComprehensiveIncome</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R3.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>0030 - Statement - CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY</LongName>
      <ShortName>CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY</ShortName>
      <Role>http://www.montpelierre.bm/role/StatementOfStockholdersEquity</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R4.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>0040 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <Role>http://www.montpelierre.bm/role/CashFlows</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R5.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1010 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R6.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1020 - Disclosure - Loss and LAE Reserves</LongName>
      <ShortName>Loss and LAE Reserves</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureLossAndLaeReserves</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R7.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1030 - Disclosure - Reinsurance With Third-Parties</LongName>
      <ShortName>Reinsurance With Third-Parties</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureReinsuranceWithThirdParties</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R8.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1040 - Disclosure - Investments</LongName>
      <ShortName>Investments</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureInvestments</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R9.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1050 - Disclosure - Debt and Other Financing Arrangements</LongName>
      <ShortName>Debt and Other Financing Arrangements</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureDebtAndOtherFinancingArrangements</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R10.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1060 - Disclosure - Derivative Instruments</LongName>
      <ShortName>Derivative Instruments</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureDerivativeInstruments</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R11.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1070 - Disclosure - Related Party Transactions</LongName>
      <ShortName>Related Party Transactions</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureRelatedPartyTransactions</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R12.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1080 - Disclosure - Common Shareholders' Equity</LongName>
      <ShortName>Common Shareholders' Equity</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureCommonShareholdersEquity</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R13.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1090 - Disclosure - Segment Reporting</LongName>
      <ShortName>Segment Reporting</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureSegmentReporting</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R14.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1100 - Disclosure - Earnings (Loss) Per Share</LongName>
      <ShortName>Earnings (Loss) Per Share</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureEarningsLossPerShare</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R15.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1110 - Disclosure - Commitments and Contingent Liabilities</LongName>
      <ShortName>Commitments and Contingent Liabilities</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureCommitmentsAndContingentLiabilities</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R16.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1120 - Disclosure - Regulatory Requirements</LongName>
      <ShortName>Regulatory Requirements</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureRegulatoryRequirements</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R17.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1130 - Disclosure - Share-Based Compensation</LongName>
      <ShortName>Share-Based Compensation</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureShareBasedCompensation</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R18.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1140 - Disclosure - Income Taxes</LongName>
      <ShortName>Income Taxes</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureIncomeTaxes</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R19.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1150 - Disclosure - Fair Value of Financial Instruments</LongName>
      <ShortName>Fair Value of Financial Instruments</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureFairValueOfFinancialInstruments</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R20.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>1160 - Disclosure - Subsequent Event</LongName>
      <ShortName>Subsequent Event</ShortName>
      <Role>http://www.montpelierre.bm/role/DisclosureSubsequentEvent</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R21.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Sheet</ReportType>
      <LongName>9999 - Document - Document and Entity Information</LongName>
      <ShortName>Document and Entity Information</ShortName>
      <Role>http://www.montpelierre.bm/role/DocumentAndEntityInformation</Role>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName>R22.xml</XmlFileName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ReportType>Book</ReportType>
      <LongName>All Reports</LongName>
      <ShortName>All Reports</ShortName>
      <Role />
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <XmlFileName />
    </Report>
  </MyReports>
  <EntityCount>1</EntityCount>
  <ContextCount>37</ContextCount>
  <SegmentCount>5</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <UnitCount>3</UnitCount>
  <ElementCount>125</ElementCount>
  <FootnotesReported>false</FootnotesReported>
  <TuplesReported>false</TuplesReported>
  <ContextSummary>
    <ContextSummary>
      <ContextName>D2011Q1_CommonStockMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010Q1</ContextName>
      <ElementCount>2</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2009_AccumulatedOtherComprehensiveIncomeMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2009_CommonStockMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010_CommonStockMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010_AdditionalPaidInCapitalMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010_TreasuryStockMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2011Q1_RetainedEarningsMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2011Q1_AccumulatedOtherComprehensiveIncomeMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2011Q1_AdditionalPaidInCapitalMember</ContextName>
      <ElementCount>4</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2011Q1_TreasuryStockMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010Q1_AdditionalPaidInCapitalMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2011Q1_AdditionalPaidInCapitalMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2011Q1_TreasuryStockMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010_AccumulatedOtherComprehensiveIncomeMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2011Q1_RetainedEarningsMember</ContextName>
      <ElementCount>2</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2011Q1SO</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2009_AdditionalPaidInCapitalMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2009</ContextName>
      <ElementCount>2</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2010Q1_AdditionalPaidInCapitalMember</ContextName>
      <ElementCount>4</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2009_RetainedEarningsMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2010Q1_AccumulatedOtherComprehensiveIncomeMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2011Q1</ContextName>
      <ElementCount>85</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2010Q1_RetainedEarningsMember</ContextName>
      <ElementCount>2</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2010Q1</ContextName>
      <ElementCount>59</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010</ContextName>
      <ElementCount>36</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2011Q1_CommonStockMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2011Q1_AccumulatedOtherComprehensiveIncomeMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2011Q1</ContextName>
      <ElementCount>36</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010Q1_TreasuryStockMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010Q1_AccumulatedOtherComprehensiveIncomeMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2010</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010_RetainedEarningsMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010Q1_CommonStockMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2010Q1_RetainedEarningsMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>D2010Q1_TreasuryStockMember</ContextName>
      <ElementCount>2</ElementCount>
    </ContextSummary>
    <ContextSummary>
      <ContextName>I2009_TreasuryStockMember</ContextName>
      <ElementCount>1</ElementCount>
    </ContextSummary>
  </ContextSummary>
  <BaseTaxonomies />
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
