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Income Taxes (Deferred Tax Inventory) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Contingency [Line Items]      
Valuation allowance $ 18,100,000us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance $ 33,600,000us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance $ 14,100,000us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Deferred income tax assets related to:      
Non-U.S. net operating loss carryforwards 427,400,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsForeign 520,600,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsForeign  
U.S. federal net operating and capital loss carryforwards 213,400,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic 148,600,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic  
Loss reserve discount 79,300,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves 74,900,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves  
Unearned premiums 46,000,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve 38,900,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve  
Incentive compensation 8,900,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits 10,300,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits  
Tax credit carryforwards 25,800,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther 23,100,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther  
Accrued interest 5,300,000us-gaap_DeferredTaxAssetsDeferredIncome 1,100,000us-gaap_DeferredTaxAssetsDeferredIncome  
Fixed assets 3,600,000us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment 4,900,000us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment  
Other items 8,400,000us-gaap_DeferredTaxAssetsOther 6,900,000us-gaap_DeferredTaxAssetsOther  
Total gross deferred income tax assets 872,700,000us-gaap_DeferredTaxAssetsGross 907,400,000us-gaap_DeferredTaxAssetsGross  
Less: valuation allowances (274,600,000)us-gaap_DeferredTaxAssetsValuationAllowance (289,800,000)us-gaap_DeferredTaxAssetsValuationAllowance  
Total net deferred income tax assets 598,100,000us-gaap_DeferredTaxAssetsNet 617,600,000us-gaap_DeferredTaxAssetsNet  
Deferred income tax liabilities related to:      
Deferred Tax Liabilities, Gross, Noncurrent 0us-gaap_DeferredTaxLiabilitiesGrossNoncurrent 3,400,000us-gaap_DeferredTaxLiabilitiesGrossNoncurrent  
Net unrealized investment gains 45,300,000us-gaap_DeferredTaxLiabilitiesUnrealizedGainsOnTradingSecurities 44,600,000us-gaap_DeferredTaxLiabilitiesUnrealizedGainsOnTradingSecurities  
Deferred acquisition costs 40,500,000us-gaap_DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost 33,000,000us-gaap_DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost  
Investment basis difference 22,900,000us-gaap_DeferredTaxLiabilitiesInvestments 3,000,000us-gaap_DeferredTaxLiabilitiesInvestments  
Other items 3,100,000us-gaap_DeferredTaxLiabilitiesOther 7,900,000us-gaap_DeferredTaxLiabilitiesOther  
Total deferred income tax liabilities 424,800,000us-gaap_DeferredTaxLiabilities 461,700,000us-gaap_DeferredTaxLiabilities  
Net deferred tax asset 173,300,000us-gaap_DeferredTaxAssetsLiabilitiesNet 155,900,000us-gaap_DeferredTaxAssetsLiabilitiesNet  
OneBeacon Runoff      
Deferred income tax assets related to:      
Runoff Transaction 12,600,000us-gaap_DeferredTaxAssetParentsBasisInDiscontinuedOperation
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= wtm_OnebeaconRunoffMember		 24,200,000us-gaap_DeferredTaxAssetParentsBasisInDiscontinuedOperation
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= wtm_OnebeaconRunoffMember		  
Safety Reserve [Member]      
Deferred income tax liabilities related to:      
Deferred Tax Liabilities, Gross, Noncurrent 295,700,000us-gaap_DeferredTaxLiabilitiesGrossNoncurrent
/ us-gaap_FairValueByLiabilityClassAxis
= wtm_SafetyReserveMember		 357,200,000us-gaap_DeferredTaxLiabilitiesGrossNoncurrent
/ us-gaap_FairValueByLiabilityClassAxis
= wtm_SafetyReserveMember		  
Purchase Accounting [Member]      
Deferred income tax liabilities related to:      
Deferred Tax Liabilities, Gross, Noncurrent 7,000,000us-gaap_DeferredTaxLiabilitiesGrossNoncurrent
/ us-gaap_FairValueByLiabilityClassAxis
= wtm_PurchaseAccountingMember		 7,200,000us-gaap_DeferredTaxLiabilitiesGrossNoncurrent
/ us-gaap_FairValueByLiabilityClassAxis
= wtm_PurchaseAccountingMember		  
Members Surplus Contributions [Member]      
Deferred income tax liabilities related to:      
Deferred Tax Liabilities, Gross, Noncurrent 10,300,000us-gaap_DeferredTaxLiabilitiesGrossNoncurrent
/ us-gaap_FairValueByLiabilityClassAxis
= wtm_MembersSurplusContributionsMember		 5,400,000us-gaap_DeferredTaxLiabilitiesGrossNoncurrent
/ us-gaap_FairValueByLiabilityClassAxis
= wtm_MembersSurplusContributionsMember		  
Incentive Compensation [Member]      
Deferred income tax assets related to:      
Incentive compensation 42,000,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits
/ us-gaap_FairValueByAssetClassAxis
= wtm_IncentiveCompensationMember		 53,900,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits
/ us-gaap_FairValueByAssetClassAxis
= wtm_IncentiveCompensationMember		  
UNITED STATES      
Deferred income tax assets related to:      
Tax credit carryforwards 11,100,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther
/ us-gaap_IncomeTaxAuthorityAxis
= country_US		    
Deferred income tax liabilities related to:      
Net deferred tax asset 214,000,000us-gaap_DeferredTaxAssetsLiabilitiesNet
/ us-gaap_IncomeTaxAuthorityAxis
= country_US		    
Luxembourg      
Deferred income tax liabilities related to:      
Net deferred tax asset 240,500,000us-gaap_DeferredTaxAssetsLiabilitiesNet
/ us-gaap_IncomeTaxAuthorityAxis
= country_LU		    
UNITED KINGDOM      
Deferred income tax liabilities related to:      
Net deferred tax asset 1,600,000us-gaap_DeferredTaxAssetsLiabilitiesNet
/ us-gaap_IncomeTaxAuthorityAxis
= country_GB		    
SWEDEN      
Deferred income tax liabilities related to:      
Total deferred income tax liabilities $ 282,800,000us-gaap_DeferredTaxLiabilities
/ us-gaap_IncomeTaxAuthorityAxis
= country_SE