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Employee Share-Based Incentive Compensation Plans (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014
Performance Shares |
Share-based compensation arrangement by share-based payment award |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 3 years
One Beacon Employee Stock Ownership Plan |
Share-based compensation arrangement by share-based payment award |
Annual base contribution provided to participants expressed as percentage of salary | 3.00%wtm_EmployeeStockOwnershipPlanEsopAnnualBaseContributionProvidedToParticipantsExpressedAsPercentageOfSalary / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember
Social security wage base amount maximum | 117,000 wtm_EmployeeStockOwnershipPlanEsopSocialSecurityWageBaseAmountMaximum / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember
Variable contribution provided to participants (as percentage) | 6.00%wtm_EmployeeStockOwnershipPlanEsopVariableContributionProvidedToParticipantsNotEligibleToCertainBenefitsExpressedAsPercentageOfSalary / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember
OneBeacon | IRS |
Share-based compensation arrangement by share-based payment award |
Defined Benefit Plan Compensation LImits | 260,000 wtm_DefinedBenefitPlanCompensationLImits / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember / dei_LegalEntityAxis = wtm_OnebeaconMember

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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Details
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- Definition Defined Benefit Plan Compensation LImits + References No definition available. + Details
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Balance Type: | credit
Period Type: | instant

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- Definition Represents the employer's annual contribution (as a percentage of salary up to the Social Security wage base) to the ESOP plan. + References No definition available. + Details
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- Definition Represent the amount of contributed by the employee maximum amount applicable to social security wage base. + References No definition available. + Details
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- Definition Represents the employer's maximum annual variable contribution (as a percentage of salary up to the Social Security wage base) to the ESOP plan for employees who are not otherwise eligible for certain incentive compensation benefits. + References No definition available. + Details
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- Details
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- Details
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- Details
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