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Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments $ 5,957.2us-gaap_InvestmentsFairValueDisclosure $ 5,991.4us-gaap_InvestmentsFairValueDisclosure $ 6,736.7us-gaap_InvestmentsFairValueDisclosure $ 6,766.5us-gaap_InvestmentsFairValueDisclosure
Non-Agency Residential Mortgage [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 85.4us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= wtm_NonAgencyResidentialMortgageMember		      
Residential Mortgage Backed Securities        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 194.3us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 131.2us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		    
Residential Mortgage Backed Securities | Super Senior        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 49.3us-gaap_InvestmentsFairValueDisclosure
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SuperSeniorMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Residential Mortgage Backed Securities | Senior        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 145.0us-gaap_InvestmentsFairValueDisclosure
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SeniorMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Residential Mortgage Backed Securities | Subordinate        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0us-gaap_InvestmentsFairValueDisclosure
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SubordinateMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Prime | Residential Mortgage Backed Securities        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 186.4us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPrimeFinancingReceivableMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Prime | Residential Mortgage Backed Securities | Super Senior        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 48.3us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPrimeFinancingReceivableMember
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SuperSeniorMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Prime | Residential Mortgage Backed Securities | Senior        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 138.1us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPrimeFinancingReceivableMember
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SeniorMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Prime | Residential Mortgage Backed Securities | Subordinate        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPrimeFinancingReceivableMember
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SubordinateMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Non-prime | Residential Mortgage Backed Securities        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 6.9us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= wtm_ResidentialNonprimeFinancingReceivableMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Non-prime | Residential Mortgage Backed Securities | Super Senior        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= wtm_ResidentialNonprimeFinancingReceivableMember
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SuperSeniorMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Non-prime | Residential Mortgage Backed Securities | Senior        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 6.9us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= wtm_ResidentialNonprimeFinancingReceivableMember
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SeniorMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Non-prime | Residential Mortgage Backed Securities | Subordinate        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= wtm_ResidentialNonprimeFinancingReceivableMember
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SubordinateMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Sub-prime | Residential Mortgage Backed Securities        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 1.0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialSubprimeFinancingReceivableMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Sub-prime | Residential Mortgage Backed Securities | Super Senior        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 1.0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialSubprimeFinancingReceivableMember
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SuperSeniorMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Sub-prime | Residential Mortgage Backed Securities | Senior        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialSubprimeFinancingReceivableMember
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SeniorMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Sub-prime | Residential Mortgage Backed Securities | Subordinate        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments $ 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialSubprimeFinancingReceivableMember
/ wtm_InvestmentByTrancheLevelsAxis
= wtm_SubordinateMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember