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Employee Share-Based Incentive Compensation Plans (OneBeacon Restricted Shares) (Details) (Restricted Stock, Long Term Incentive Plan, USD $) In Millions, except Share data, unless otherwise specified | 1 Months Ended | 0 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
May 31, 2011 | Mar. 01, 2012 Installment | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | May 25, 2011 Installment
One Beacon Insurance Company |  |  |  |  |  |
Unamortized grant date fair value |  |  |  |  |  |
Forfeited shares | 35,000 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsReductionInGrantTotal / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceCompanyMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |  |  |  |  |
One Beacon Insurance Company | Chief Executive Officer |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Issued (in shares) | 630,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceCompanyMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_ChiefExecutiveOfficerMember | 300,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceCompanyMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_ChiefExecutiveOfficerMember |  |  |  |
Unamortized grant date fair value |  |  |  |  |  |
Number of installments in which the shares vest |  | 2 wtm_NumberOfInstallmentsInWhichSharesVest / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceCompanyMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_ChiefExecutiveOfficerMember |  |  |  | 4 wtm_NumberOfInstallmentsInWhichSharesVest / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceCompanyMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_ChiefExecutiveOfficerMember
Forfeited shares | 35,000 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsReductionInGrantTotal / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceCompanyMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_ChiefExecutiveOfficerMember |  |  |  |  |
Years performance shares will be forfeited | 5 years |  |  |  |  |
OneBeacon |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Beginning of period |  |  | 915,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 927,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 630,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |
Issued (in shares) |  |  | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 300,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |
Vested (in shares) |  |  | (300,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (9,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (667)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |
Assumed forfeitures and cancellations (in shares) |  |  | (2,500)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (3,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (2,333)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |
Ending December 31, |  |  | 612,500 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 915,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 927,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |
Unamortized grant date fair value |  |  |  |  |  |
Beginning balance |  |  | $ 6.5 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | $ 9.6 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | $ 7.7 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |
Issued |  |  | 0 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsIssuedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 0 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsIssuedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 4.5 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsIssuedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |
Vested |  |  | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |  | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |
Assumed forfeitures and cancellations |  |  | 0 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsForfeitedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 0 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsForfeitedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 0 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsForfeitedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |
Expense recognized |  |  | (3.0)us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (3.1)us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (2.6)us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |
Ending balance |  |  | $ 3.5 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | $ 6.5 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | $ 9.6 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + 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 (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + 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 (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + 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 (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + 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 (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of share instruments newly issued under a share-based compensation plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The total reduction to unamortized compensation cost for performance shares forfeited during the period. + References No definition available. + Details
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- Definition The total fair value (on the date of issue) for restricted share awards issued during the period. + References No definition available. + Details
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- Definition Represents the number of installments in which the shares vest. + References No definition available. + Details
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- Definition Represents the total number of performance share grants that will be reduced. + References No definition available. + Details
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- Definition Years Performance Shares will be Forfeited Period + References No definition available. + Details
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