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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
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Investments in unconsolidated affiliates |  |  |  |
Total investments in unconsolidated affiliates | $ 450.8 wtm_InvestmentsInUnconsolidatedAffiliates | $ 414.4 wtm_InvestmentsInUnconsolidatedAffiliates |  |
Symetra |  |  |  |
Investments in unconsolidated affiliates |  |  |  |
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates | 68.4 wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 37.6 wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |
Symetra | Common Stock |  |  |  |
Investments in unconsolidated affiliates |  |  |  |
Symetra Common Shares excluding unrealized gains (losses) | 378.9 wtm_SharesExcludingUnrealizedGainsLosses / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 373.8 wtm_SharesExcludingUnrealizedGainsLosses / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |
Total investments in unconsolidated affiliates | 447.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 411.4 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 369.0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember
Hamer, LLC |  |  |  |
Investments in unconsolidated affiliates |  |  |  |
Total investments in unconsolidated affiliates | $ 3.5 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember | $ 3.0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember |  |

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