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Derivatives Derivatives (Interest Rate Cap) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended 1 Months Ended
Dec. 31, 2014
Rating
Dec. 31, 2013
May 31, 2007
Interest Rate Cap      
Derivative [Line Items]      
Derivative Collateral Right to Reclaim $ 4.3wtm_DerivativeCollateralRightToReclaim
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
   
Beginning of period 11.1us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
0us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
 
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
9.9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
 
Change in fair value of derivatives, included in other revenue 7.0us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
(1.2)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
 
Ending balance 4.1us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
11.1us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
 
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements 0wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
0wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
 
S&P Credit Ratings 21wtm_SAndPCreditRatings
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
   
Barclays [Member] | A | Interest Rate Cap      
Derivative [Line Items]      
Derivative Collateral Right to Reclaim 3.0wtm_DerivativeCollateralRightToReclaim
/ us-gaap_CounterpartyNameAxis
= wtm_BarclaysMember
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsARatingMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
   
Nordea Bank Finland [Member] | Standard & Poor's, AA- Rating [Member] | Interest Rate Cap      
Derivative [Line Items]      
Derivative Collateral Right to Reclaim 1.3wtm_DerivativeCollateralRightToReclaim
/ us-gaap_CounterpartyNameAxis
= wtm_NordeaBankFinlandMember
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAAMinusRatingMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
   
Sirius Internation Group Ltd | Interest Rate Cap      
Derivative [Line Items]      
basis points 3.95%wtm_Basispoints
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
/ dei_LegalEntityAxis
= wtm_SiriusGroupLtdMember
   
years of interest rate cap 5 years 5 years  
Derivative, Cap Interest Rate 8.30%us-gaap_DerivativeCapInterestRate
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
/ dei_LegalEntityAxis
= wtm_SiriusGroupLtdMember
8.30%us-gaap_DerivativeCapInterestRate
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
/ dei_LegalEntityAxis
= wtm_SiriusGroupLtdMember
 
Derivative, Cost of Hedge $ 9.9us-gaap_DerivativeCostOfHedge
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
/ dei_LegalEntityAxis
= wtm_SiriusGroupLtdMember
   
Sirius Internation Group Ltd | Preferred stocks      
Derivative [Line Items]      
Non-cumulative perpetual preference shares issued     250,000,000us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction
/ dei_LegalEntityAxis
= wtm_SiriusGroupLtdMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_PreferredStockMember
Sirius Internation Group Ltd | Preferred stocks | Interest Rate Cap      
Derivative [Line Items]      
Preferred stock dividend rate     7.506%us-gaap_PreferredStockDividendRatePercentage
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
/ dei_LegalEntityAxis
= wtm_SiriusGroupLtdMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_PreferredStockMember
Preference shares reference rate for dividend     3
Non-cumulative perpetual preference shares issued     250,000,000us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember
/ dei_LegalEntityAxis
= wtm_SiriusGroupLtdMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_PreferredStockMember