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Retirement and Postretirement Plans (Cash Flow) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Employee savings plan |  |  |
Defined Benefit Plan, Benefit Obligation | $ 119,700,000 us-gaap_DefinedBenefitPlanBenefitObligation | $ 102,900,000 us-gaap_DefinedBenefitPlanBenefitObligation | $ 119,500,000 us-gaap_DefinedBenefitPlanBenefitObligation
Funded status at end of year | 26,300,000 us-gaap_DefinedBenefitPlanFundedStatusOfPlan | 39,900,000 us-gaap_DefinedBenefitPlanFundedStatusOfPlan |
OneBeacon Ltd. |  |  |
Expected Benefit Payments |  |  |
2015 | 4,900,000 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |
2016 | 5,300,000 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |
2017 | 5,600,000 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |
2018 | 5,900,000 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |
2019 | 6,100,000 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |
2020-2024 | 34,400,000 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |
Employee savings plan |  |  |
Anticipated contribution to plan | 2,200,000 us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |
OneBeacon |  |  |
Employee savings plan |  |  |
Post-employment benefit liability | 4,800,000 us-gaap_PostemploymentBenefitsLiabilityNoncurrent / dei_LegalEntityAxis = wtm_OnebeaconMember | 5,800,000 us-gaap_PostemploymentBenefitsLiabilityNoncurrent / dei_LegalEntityAxis = wtm_OnebeaconMember |
Sirius Group |  |  |
Employee savings plan |  |  |
Defined Contribution Plan, Cost Recognized | 500,000 us-gaap_DefinedContributionPlanCostRecognized / dei_LegalEntityAxis = wtm_SiriusGroupMember | 700,000 us-gaap_DefinedContributionPlanCostRecognized / dei_LegalEntityAxis = wtm_SiriusGroupMember | 400,000 us-gaap_DefinedContributionPlanCostRecognized / dei_LegalEntityAxis = wtm_SiriusGroupMember
Government Pension Plans [Member] | Sirius Group |  |  |
Employee savings plan |  |  |
Defined Contribution Plan, Cost Recognized | 10,300,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember / dei_LegalEntityAxis = wtm_SiriusGroupMember | 9,700,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember / dei_LegalEntityAxis = wtm_SiriusGroupMember | 7,500,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember / dei_LegalEntityAxis = wtm_SiriusGroupMember
Defined Benefit Plan, Benefit Obligation | 14,100,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember / dei_LegalEntityAxis = wtm_SiriusGroupMember | 13,100,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember / dei_LegalEntityAxis = wtm_SiriusGroupMember |
Funded status at end of year | (1,800,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember / dei_LegalEntityAxis = wtm_SiriusGroupMember | (1,000,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember / dei_LegalEntityAxis = wtm_SiriusGroupMember |
Qualified Plan [Member] | Sirius Group |  |  |
Employee savings plan |  |  |
Defined Contribution Plan, Cost Recognized | 700,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = wtm_SiriusGroupMember | 600,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = wtm_SiriusGroupMember | 600,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = wtm_SiriusGroupMember
Combination One [Member] | Sirius Group |  |  |
Employee savings plan |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 2.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / dei_LegalEntityAxis = wtm_SiriusGroupMember / wtm_MatchingContributionAxis = wtm_CombinationOneMember |  |
Combination Two [Member] | Sirius Group |  |  |
Employee savings plan |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 4.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / dei_LegalEntityAxis = wtm_SiriusGroupMember / wtm_MatchingContributionAxis = wtm_CombinationTwoMember |  |
Maximum | Sirius Group |  |  |
Employee savings plan |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Match | 100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch / dei_LegalEntityAxis = wtm_SiriusGroupMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |
Employer matching contribution variable contributions | 7.00%wtm_DefinedContributionPlanEmployerMatchingContributionVariableContributions / dei_LegalEntityAxis = wtm_SiriusGroupMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |
Minimum | Sirius Group |  |  |
Employee savings plan |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Match | 50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch / dei_LegalEntityAxis = wtm_SiriusGroupMember / us-gaap_RangeAxis = us-gaap_MinimumMember |  |
One Beacon Employee Stock Ownership Plan |  |  |
Employee savings plan |  |  |
Defined Contribution Plan, Cost Recognized | 4,400,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember | 6,300,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember | 4,800,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember
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 |  |
One Beacon Employee Stock Ownership Plan | OneBeacon |  |  |
Employee savings plan |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Match | 50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember / dei_LegalEntityAxis = wtm_OnebeaconMember |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 6.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember / dei_LegalEntityAxis = wtm_OnebeaconMember |  |
Defined Contribution Plan, Cost Recognized | 2,700,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember / dei_LegalEntityAxis = wtm_OnebeaconMember | 2,300,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember / dei_LegalEntityAxis = wtm_OnebeaconMember | 2,800,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember / dei_LegalEntityAxis = wtm_OnebeaconMember
IRS | OneBeacon |  |  |
Employee savings plan |  |  |
Defined Benefit Plan Compensation LImits | 260,000 wtm_DefinedBenefitPlanCompensationLImits / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember / dei_LegalEntityAxis = wtm_OnebeaconMember |  |
IRS | Sirius Group |  |  |
Employee savings plan |  |  |
Defined Benefit Plan Compensation LImits | $ 260,000 wtm_DefinedBenefitPlanCompensationLImits / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember / dei_LegalEntityAxis = wtm_SiriusGroupMember |  |

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- Definition 1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Projected Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6522206 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Postretirement Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503904 + Details
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- Definition Amount of benefits expected to be paid in the five fiscal years after the fifth fiscal year following the latest fiscal year from a defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Amount of benefits expected to be paid in the next fiscal year following the latest fiscal year from a defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Amount of benefits expected to be paid in the fifth fiscal year following the latest fiscal year from a defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Amount of benefits expected to be paid in the fourth fiscal year following the latest fiscal year from a defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Amount of benefits expected to be paid in the third fiscal year following the latest fiscal year from a defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Amount of benefits expected to be paid in the second fiscal year following the latest fiscal year from a defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=21915240&loc=d3e1703-114919 + Details
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- Definition Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
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- Definition For a classified balance sheet, the carrying amount as of the balance sheet date of the portion of the obligations recognized for the various benefits provided to former or inactive employees, their beneficiaries, and covered dependents after employment but before retirement that is payable after one year (or beyond the operating cycle if longer). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 712 -SubTopic 10 -Section 25 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6410138&loc=d3e79691-111665 + Details
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- Definition Defined Benefit Plan Compensation LImits + References No definition available. + Details
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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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