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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
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Symetra |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Investments in unconsolidated affiliates on conversion of warrants | 2.65 wtm_InvestmentsInUnconsolidatedAffiliatesOnConversionOfWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 9.49 wtm_InvestmentsInUnconsolidatedAffiliatesOnConversionOfWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember

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- Details
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- Definition Represents additional shares of an unconsolidated affiliate to be obtained on conversion of warrants. + References No definition available. + Details
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