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Investments in Unconsolidated Affiliates (Investment in Unconsolidated Affiliates Table) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Investments in unconsolidated affiliates        
Total investments in unconsolidated affiliates $ 450.8wtm_InvestmentsInUnconsolidatedAffiliates $ 414.4wtm_InvestmentsInUnconsolidatedAffiliates    
Symetra        
Investments in unconsolidated affiliates        
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates 68.4wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_SymetraMember		 37.6wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_SymetraMember		    
Symetra | Common Stock        
Investments in unconsolidated affiliates        
Symetra Common Shares excluding unrealized gains (losses) 378.9wtm_SharesExcludingUnrealizedGainsLosses
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_SymetraMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember		 373.8wtm_SharesExcludingUnrealizedGainsLosses
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_SymetraMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember		    
Total investments in unconsolidated affiliates 447.3wtm_InvestmentsInUnconsolidatedAffiliates
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_SymetraMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember		 411.4wtm_InvestmentsInUnconsolidatedAffiliates
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_SymetraMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember		 369.0wtm_InvestmentsInUnconsolidatedAffiliates
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_SymetraMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember		 317.3wtm_InvestmentsInUnconsolidatedAffiliates
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_SymetraMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember
Hamer, LLC        
Investments in unconsolidated affiliates        
Total investments in unconsolidated affiliates $ 3.5wtm_InvestmentsInUnconsolidatedAffiliates
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_HamerLlcMember		 $ 3.0wtm_InvestmentsInUnconsolidatedAffiliates
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= wtm_HamerLlcMember