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 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 718

 -SubTopic 10

 -Section 50

 -Paragraph 2

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 -URI http://asc.fasb.org/extlink&amp;oid=6415400&amp;loc=d3e5070-113901



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 -Number 123R

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 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



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 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



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 -Publisher FASB

 -Name Accounting Standards Codification

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 -Section 50

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 -URI http://asc.fasb.org/extlink&amp;oid=6415400&amp;loc=d3e5070-113901



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 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



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 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 718

 -SubTopic 10

 -Section 50

 -Paragraph 2

 -Subparagraph (a)(1)

 -URI http://asc.fasb.org/extlink&amp;oid=6415400&amp;loc=d3e5070-113901



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 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 718

 -SubTopic 10

 -Section 50

 -Paragraph 2

 -Subparagraph (a)(1)

 -URI http://asc.fasb.org/extlink&amp;oid=6415400&amp;loc=d3e5070-113901



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 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 718

 -SubTopic 10

 -Section 50

 -Paragraph 2

 -Subparagraph (a)(1)

 -URI http://asc.fasb.org/extlink&amp;oid=6415400&amp;loc=d3e5070-113901



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</LabelSeparator><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>true</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol>$</CurrencySymbol><contextRef><ContextID>D2013Q2_RestrictedStockUnitsFixedMember</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001165880</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-04-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments><anyType xsi:type="Segment"><IsDefaultForEntity>false</IsDefaultForEntity><Name /><IsFromEntityGroup>false</IsFromEntityGroup><ValueName>Fixed RSUs Awarded</ValueName><ValueType>us-gaap_AwardTypeAxis</ValueType><Namespace>xbrldi</Namespace><Schema>http://xbrl.org/2006/xbrldi</Schema><DimensionInfo><Id>mrh_RestrictedStockUnitsFixedMember</Id><dimensionId>us-gaap_AwardTypeAxis</dimensionId><type>explicitMember</type></DimensionInfo></anyType></Segments><Scenarios /></contextRef><UPS><UnitProperty><UnitID>Shares</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema><MeasureValue>shares</MeasureValue><MeasureNamespace>xbrli</MeasureNamespace><MeasureDescription /><MeasureType /></StandardMeasure><MultiplyMeasures><Measure><MeasureSchema /><MeasureValue /><MeasureNamespace /><MeasureDescription /><MeasureType /></Measure><Measure><MeasureSchema /><MeasureValue /><MeasureNamespace /><MeasureDescription /><MeasureType /></Measure></MultiplyMeasures><NumeratorMeasure><MeasureSchema /><MeasureValue /><MeasureNamespace /><MeasureDescription /><MeasureType /></NumeratorMeasure><DenominatorMeasure><MeasureSchema /><MeasureValue /><MeasureNamespace /><MeasureDescription /><MeasureType /></DenominatorMeasure><Scale>0</Scale></UnitProperty><UnitProperty><UnitID>USD</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace><MeasureDescription /><MeasureType /></StandardMeasure><MultiplyMeasures><Measure><MeasureSchema /><MeasureValue /><MeasureNamespace /><MeasureDescription /><MeasureType /></Measure><Measure><MeasureSchema /><MeasureValue /><MeasureNamespace /><MeasureDescription /><MeasureType /></Measure></MultiplyMeasures><NumeratorMeasure><MeasureSchema /><MeasureValue /><MeasureNamespace /><MeasureDescription /><MeasureType /></NumeratorMeasure><DenominatorMeasure><MeasureSchema /><MeasureValue /><MeasureNamespace /><MeasureDescription /><MeasureType /></DenominatorMeasure><Scale>0</Scale></UnitProperty></UPS><CurrencyCode>USD</CurrencyCode><OriginalCurrencyCode>USD</OriginalCurrencyCode></MCU><CurrencySymbol>$</CurrencySymbol><Labels><Label Key="us-gaap_AwardTypeAxis:mrh_RestrictedStockUnitsFixedMember" Id="0" Label="Fixed RSUs Awarded" /><Label Key="Calendar" Id="1" Label="4/1/2013 - 6/30/2013" /></Labels></OriginalInstanceReportColumn><ElementDataType>na</ElementDataType><SimpleDataType>na</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Fixed RSUs Awarded</Label></Row><Row FlagID="0"><Id>22</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

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 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 718

 -SubTopic 10

 -Section 50

 -Paragraph 2

 -Subparagraph (c)(2)(iii)(1)

 -URI http://asc.fasb.org/extlink&amp;oid=6415400&amp;loc=d3e5070-113901



Reference 2: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Statement of Financial Accounting Standard (FAS)

 -Number 123R

 -Paragraph A240

 -Subparagraph b(2)(c)

 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



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</LabelSeparator><Level>4</Level><ElementName>mrh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedGrantsInPeriodAtTargetROE</ElementName><ElementPrefix>mrh_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="D2013Q2_RestrictedStockUnitsVariableMember_FourYearPerformanceCycleFrom2013To2016Member" UnitID="Shares"><Id>1</Id><IsNumeric>true</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>37807</NumericAmount><RoundedNumericAmount>37807</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell><Cell FlagID="0" ContextID="" UnitID=""><Id>2</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell><Cell FlagID="0" ContextID="D2013Q2YTD_RestrictedStockUnitsVariableMember_FourYearPerformanceCycleFrom2013To2016Member" UnitID="Shares"><Id>3</Id><IsNumeric>true</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>536761</NumericAmount><RoundedNumericAmount>536761</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell><Cell FlagID="0" ContextID="" UnitID=""><Id>4</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:sharesItemType</ElementDataType><SimpleDataType>shares</SimpleDataType><ElementDefenition>Represents the number of grants expected to be made at target ROE, during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>1</UnitID><Label>RSUs to be awarded at target ROE (in shares)</Label></Row><Row FlagID="0"><Id>30</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>4</Level><ElementName>mrh_ThresholdReturnOnEquity</ElementName><ElementPrefix>mrh_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>instant</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="I2013Q2_RestrictedStockUnitsVariableMember_FourYearPerformanceCycleFrom2013To2016Member" UnitID="Pure"><Id>1</Id><IsNumeric>true</IsNumeric><IsRatio>true</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0.0276</NumericAmount><RoundedNumericAmount>0.0276</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell><Cell FlagID="0" ContextID="" UnitID=""><Id>2</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell><Cell FlagID="0" ContextID="I2013Q2_RestrictedStockUnitsVariableMember_FourYearPerformanceCycleFrom2013To2016Member" UnitID="Pure"><Id>3</Id><IsNumeric>true</IsNumeric><IsRatio>true</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0.0276</NumericAmount><RoundedNumericAmount>0.0276</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell><Cell FlagID="0" ContextID="" UnitID=""><Id>4</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>num:percentItemType</ElementDataType><SimpleDataType>pure</SimpleDataType><ElementDefenition>Represents the threshold return on equity.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Threshold ROE (as a percent)</Label></Row><Row FlagID="0"><Id>31</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

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