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Investment Securities (Debt Securities Issued By Corporation) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Investment securities      
Fair value investments $ 5,991.4us-gaap_InvestmentsFairValueDisclosure $ 6,766.5us-gaap_InvestmentsFairValueDisclosure $ 6,950.7us-gaap_InvestmentsFairValueDisclosure
Debt securities issued by corporations      
Investment securities      
Fair value investments 2,311.2us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 2,347.2us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Other Standard Poors Rating [Member] | Debt securities issued by corporations      
Investment securities      
Fair value investments 10.6us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= wtm_OtherStandardPoorsRatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 3.4us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= wtm_OtherStandardPoorsRatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Standard & Poor's, BB Rating [Member] | Debt securities issued by corporations      
Investment securities      
Fair value investments 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBRatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBRatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
BBB | Debt securities issued by corporations      
Investment securities      
Fair value investments 1,105.9us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBRatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 1,075.5us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBRatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
A | Debt securities issued by corporations      
Investment securities      
Fair value investments 957.8us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsARatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 1,039.5us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsARatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
AA | Debt securities issued by corporations      
Investment securities      
Fair value investments 236.9us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAARatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 228.8us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAARatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
AAA | Debt securities issued by corporations      
Investment securities      
Fair value investments $ 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAAARatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 $ 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAAARatingMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember