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Transactions with Related Persons (Details) (USD $) | 12 Months Ended | 0 Months Ended |
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Jul. 16, 2012 | Mar. 15, 2013
Related Party Transaction [Line Items] |  |  |  |  |  |
Purchases | $ 5,602,100,000 us-gaap_PaymentsToAcquireInvestments | $ 4,635,500,000 us-gaap_PaymentsToAcquireInvestments |  |  |  |
Sales of common equity securities | 660,900,000 wtm_ProceedsFromSaleOfTradingCommonEquitySecurities | 521,100,000 wtm_ProceedsFromSaleOfTradingCommonEquitySecurities | 192,400,000 wtm_ProceedsFromSaleOfTradingCommonEquitySecurities |  |  |
Beneficial Owner |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Per share price, class of warrant (in dollars per share) |  |  |  |  |  | $ 563.57 us-gaap_SharePrice / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_BeneficialOwnerMember
Shares purchased |  | 140,000 us-gaap_TreasuryStockSharesAcquired / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_BeneficialOwnerMember |  |  |  |
Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Asset Management Fees | 6,500,000 us-gaap_AssetManagementFees1 / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember | 6,500,000 us-gaap_AssetManagementFees1 / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember | 6,400,000 us-gaap_AssetManagementFees1 / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |
Performance shares granted at the beginning of each performance cycle |  | 4,500,000 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantedAtBeginningOfPerformanceCycleToRelatedParty / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |  |
Percentage of fees in excess of $500,000 revenue chargeable | 6.00%wtm_PercentageOfFeesInExcessOfSpecifiedAmountOfRevenueChargeablePursuantToRevenueSharingAgreementWithRelatedParty / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |  |  |
Amount of revenue over which 6% of fees chargeable |  | 500,000 wtm_AmountOfRevenueOverWhichSpecifiedPercentageOfFeesChargeablePursuantToRevenueSharingAgreementWithRelatedParty / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |  |
Invested in limited partnership investment interests | 82,400,000 wtm_LimitedPartnershipInvestmentsUnderManagementCarryingAmount / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |  |  |
Management Fees, Base Revenue | 1,100,000 us-gaap_ManagementFeeExpense / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember | 1,300,000 us-gaap_ManagementFeeExpense / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  | 1,100,000 us-gaap_ManagementFeeExpense / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |
Incentive fees incurred | 700,000 wtm_RelatedPartyTransactionIncentiveFeesFromTransactionsWithRelatedParty / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember | 3,200,000 wtm_RelatedPartyTransactionIncentiveFeesFromTransactionsWithRelatedParty / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  | 1,300,000 wtm_RelatedPartyTransactionIncentiveFeesFromTransactionsWithRelatedParty / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |
OneBeacon | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Assets under management | 431,200,000 us-gaap_AssetsUnderManagementCarryingAmount / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |  |  |
ERISA Assets | 202,900,000 wtm_AssetsunderManagementRelatedtoDefinedBenefitandDefinedContributionPlans / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |  |  |
Invested in limited partnership investment interests | 15,800,000 wtm_LimitedPartnershipInvestmentsUnderManagementCarryingAmount / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |  |  |
White Mountains Advisors LLC | Symetra |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Assets under management | 30,900,000,000 us-gaap_AssetsUnderManagementCarryingAmount / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_SymetraMember | 27,800,000,000 us-gaap_AssetsUnderManagementCarryingAmount / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_SymetraMember |  |  |  |
Asset Management Fees | 18,400,000 us-gaap_AssetManagementFees1 / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_SymetraMember | 16,500,000 us-gaap_AssetManagementFees1 / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_SymetraMember | 15,900,000 us-gaap_AssetManagementFees1 / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_SymetraMember |  |  |
White Mountains Advisors LLC | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Assets under management | 195,700,000 us-gaap_AssetsUnderManagementCarryingAmount / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |  |  |
Oakum Bay Capital |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Ownerhsip percentage |  |  |  |  | 7.50%wtm_OwnerhsipPercentageOfCommonEquity / dei_LegalEntityAxis = wtm_OakumBayCapitalMember |
Sales of common equity securities |  | 500,000 wtm_ProceedsFromSaleOfTradingCommonEquitySecurities / dei_LegalEntityAxis = wtm_OakumBayCapitalMember |  |  |  |
Number of years to receive portion of revenue |  | 10 years |  |  |  |
Oakum Bay Capital | Director |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Related party ownerhship percentage |  | 67.50%wtm_RelatedPartyOwnerhshipPercentage / dei_LegalEntityAxis = wtm_OakumBayCapitalMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_DirectorMember |  |  |  |
Maximum | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Target number of performance shares granted (as a percent) | 200.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardTargetedPerformanceGoalPercentageofPayoutforGrowthTargetPercentage / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |  |  |
Minimum | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Target number of performance shares granted (as a percent) | 0.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardTargetedPerformanceGoalPercentageofPayoutforGrowthTargetPercentage / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |  |  |
Preferred Equity Securities | Oakum Bay Capital |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Purchases |  |  |  |  | 2,000,000 us-gaap_PaymentsToAcquireInvestments / us-gaap_InvestmentTypeAxis = wtm_PreferredEquitySecuritiesMember / dei_LegalEntityAxis = wtm_OakumBayCapitalMember |
revenue sharing agreement | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Fees from Revenue Sharing Agreement with Related Party | 400,000 us-gaap_RevenueFromRelatedParties / us-gaap_RelatedPartyTransactionAxis = wtm_RevenuesharingagreementMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember | 900,000 us-gaap_RevenueFromRelatedParties / us-gaap_RelatedPartyTransactionAxis = wtm_RevenuesharingagreementMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember | 500,000 us-gaap_RevenueFromRelatedParties / us-gaap_RelatedPartyTransactionAxis = wtm_RevenuesharingagreementMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember |  |  |
Trimarc Capital Fund | Oakum Bay Capital |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Management fees |  | 100,000 us-gaap_PaymentForManagementFee / invest_InvestmentAxis = wtm_TrimarcCapitalFundMember / dei_LegalEntityAxis = wtm_OakumBayCapitalMember |  |  |  |
Trimarc Capital Fund | Hedge funds | Oakum Bay Capital |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Trading Securities, Other | 7,800,000 us-gaap_TradingSecuritiesOther / invest_InvestmentAxis = wtm_TrimarcCapitalFundMember / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / dei_LegalEntityAxis = wtm_OakumBayCapitalMember |  |  |  |  |
First Threshold Limit | White Mountains Advisors LLC | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Assets under management | 200,000,000 us-gaap_AssetsUnderManagementCarryingAmount / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_FirstThresholdLimitMember |  |  |  |  |
basis points | 100.00%wtm_WTM_InvestmentAdvisoryFees / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_FirstThresholdLimitMember |  |  |  |  |
Second Threshold Limit | White Mountains Advisors LLC | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Assets under management | 200,000,000 us-gaap_AssetsUnderManagementCarryingAmount / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_SecondThresholdLimitMember |  |  |  |  |
basis points | 50.00%wtm_WTM_InvestmentAdvisoryFees / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_SecondThresholdLimitMember |  |  |  |  |
Third Threshold Limit | White Mountains Advisors LLC | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Assets under management | 400,000,000 us-gaap_AssetsUnderManagementCarryingAmount / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_ThirdThresholdLimitMember |  |  |  |  |
basis points | 25.00%wtm_WTM_InvestmentAdvisoryFees / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_ThirdThresholdLimitMember |  |  |  |  |
Range Of Performance Cycle From 2015 to 2017 [Member] | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Number of performance shares granted | 6,250 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2015to2017Member |  |  |  |  |
Range Of Performance Cycle From 2014-2016 [Member] | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Number of performance shares granted | 6,250 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom20142016Member |  |  |  |  |
Performance Cycle From 2013 to 2015 | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Number of performance shares granted | 7,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2013To2015Member |  |  |  |  |
Performance Cycle From 2011 to 2013 | Prospector |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Number of performance shares granted | 7,750 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2011To2013Member |  |  |  |  |
Percentage of target number granted actually received by prospector | 145.50%wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesActuallyAwardedAsPercentageOfTargetAwardsGranted / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2011To2013Member |  |  |  |  |
Payment made with respect to the performance shares granted to the related party | $ 7,500,000 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodPaymentMade / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_ProspectorFundsMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2011To2013Member |  |  |  |  |

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- Definition Represents the percentage of performance shares that are actually awarded as a percentage of the target grant. + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award Targeted Performance Goal Percentage of Payout for Growth Target Percentage + References No definition available. + Details
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- Definition Investment Advisory Fees + References No definition available. + Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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Data Type: | na
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- Details
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- Details
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- Details
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