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Employee Share-Based Incentive Compensation Plans (WTM Non-Qualified and Qualified Options) (Details)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Performance Shares      
Share-based compensation arrangement by share-based payment award      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number 125,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		    
Qualified Plan [Member]      
Share-based compensation arrangement by share-based payment award      
Percentage of common shares outstanding 3.00%wtm_PercentageOfCommonSharesOutstandingOwnedByPlans
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 3.00%wtm_PercentageOfCommonSharesOutstandingOwnedByPlans
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Eligible contribution percentage 2.00%wtm_DeferredCompensationArrangementVariableContributionByEligibleParticipantsAsPercentageOfSalary
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 4.00%wtm_DeferredCompensationArrangementVariableContributionByEligibleParticipantsAsPercentageOfSalary
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 3.00%wtm_DeferredCompensationArrangementVariableContributionByEligibleParticipantsAsPercentageOfSalary
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember