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Discontinued Operations Discontinued Operations (Surplus Note Table) (Details) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 23, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
(Loss) gain on sale of discontinued operations, net of tax | $ (1.6)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax | $ 46.6 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax | $ (91.0)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax |
OneBeacon Runoff |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
Surplus notes |  |  |  | 101.0 us-gaap_SurplusNotes / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember
OneBeacon Runoff | Other long-term investments | Fair Value [Member] |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
Surplus notes |  |  |  | 64.9 us-gaap_SurplusNotes / us-gaap_FairValueByAssetClassAxis = wtm_FairValueMember / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
OneBeacon Runoff | Surplus Note |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
Percentage of Total Adjusted Capital | 200.00%wtm_PercentageofTotalAdjustedCapital / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / invest_InvestmentAxis = wtm_SurplusNoteMember |  |  |
OneBeacon Runoff | Surplus Note | Other long-term investments |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
Interest rate (as a percent) |  |  |  | 6.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / invest_InvestmentAxis = wtm_SurplusNoteMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Discontinued Operations |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
(Loss) gain on sale of discontinued operations, net of tax | (1.6)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember | 46.6 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember | (91.0)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |
Discontinued Operations | OneBeacon Runoff |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
RBC Score | 250.00%wtm_RBCScore / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Current Market Rate | (6.6)wtm_CurrentMarketRate / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Regulatory Approval Payment | (12.6)wtm_RegulatoryApprovalPayment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Liquidity Adjustments | (16.7)wtm_LiquidityAdjustments / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
(Loss) gain on sale of discontinued operations, net of tax | (35.9)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  | 91.5 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |
Surplus notes | 101.0 us-gaap_SurplusNotes / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Discontinued Operations | OneBeacon Runoff | Seller Priority Surplus Note [Member] |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
Current Market Rate | 1.6 wtm_CurrentMarketRate / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Regulatory Approval Payment | (4.6)wtm_RegulatoryApprovalPayment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Liquidity Adjustments | (11.0)wtm_LiquidityAdjustments / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
(Loss) gain on sale of discontinued operations, net of tax | (14.0)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Surplus notes | 57.9 us-gaap_SurplusNotes / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Discontinued Operations | OneBeacon Runoff | Pari Passu Surplus Note [Member] |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
Current Market Rate | (8.2)wtm_CurrentMarketRate / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_PariPassuSurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Regulatory Approval Payment | (8.0)wtm_RegulatoryApprovalPayment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_PariPassuSurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Liquidity Adjustments | (5.7)wtm_LiquidityAdjustments / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_PariPassuSurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
(Loss) gain on sale of discontinued operations, net of tax | (21.9)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_PariPassuSurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Surplus notes | 43.1 us-gaap_SurplusNotes / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_PariPassuSurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Discontinued Operations | OneBeacon Runoff | Fair Value [Member] |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
Surplus notes | 65.1 us-gaap_SurplusNotes / us-gaap_FairValueByAssetClassAxis = wtm_FairValueMember / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Discontinued Operations | OneBeacon Runoff | Fair Value [Member] | Seller Priority Surplus Note [Member] |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
Surplus notes | 43.9 us-gaap_SurplusNotes / us-gaap_FairValueByAssetClassAxis = wtm_FairValueMember / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Discontinued Operations | OneBeacon Runoff | Fair Value [Member] | Pari Passu Surplus Note [Member] |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
Surplus notes | 21.2 us-gaap_SurplusNotes / us-gaap_FairValueByAssetClassAxis = wtm_FairValueMember / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_PariPassuSurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Discontinued Operations | OneBeacon Runoff | Surplus Note |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |  |  |
(Loss) gain on sale of discontinued operations, net of tax | $ (23.5)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / invest_InvestmentAxis = wtm_SurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |

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