XML 153 R137.htm IDEA: XBRL DOCUMENT v2.4.1.9
Employee Share-Based Incentive Compensation Plans (Under Qualified Retirement Plans) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
One Beacon Employee Stock Ownership Plan      
Share-based compensation arrangement by share-based payment award      
Defined Contribution Plan, Cost Recognized $ 4.4us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= wtm_OneBeaconEmployeeStockOwnershipPlanMember		 $ 6.3us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= wtm_OneBeaconEmployeeStockOwnershipPlanMember		 $ 4.8us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= wtm_OneBeaconEmployeeStockOwnershipPlanMember
Qualified Plan [Member]      
Share-based compensation arrangement by share-based payment award      
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
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