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Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Warrants) (Details) (Symetra)
In Millions, unless otherwise specified
Jun. 20, 2013
Dec. 31, 2012
Symetra
    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Investments in unconsolidated affiliates on conversion of warrants 2.65wtm_InvestmentsInUnconsolidatedAffiliatesOnConversionOfWarrants
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_SymetraMember		 9.49wtm_InvestmentsInUnconsolidatedAffiliatesOnConversionOfWarrants
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_SymetraMember