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Retirement and Postretirement Plans (Benefit Obligations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 102.9us-gaap_DefinedBenefitPlanBenefitObligation	$ 119.5us-gaap_DefinedBenefitPlanBenefitObligation
Service cost	0.6us-gaap_DefinedBenefitPlanServiceCost	0.8us-gaap_DefinedBenefitPlanServiceCost	0.7us-gaap_DefinedBenefitPlanServiceCost
Interest cost	4.7us-gaap_DefinedBenefitPlanInterestCost	4.2us-gaap_DefinedBenefitPlanInterestCost	4.7us-gaap_DefinedBenefitPlanInterestCost
Special termination benefits expense	0.3us-gaap_DefinedBenefitPlanCostOfProvidingSpecialOrContractualTerminationBenefitRecognizedDuringPeriod	0.3us-gaap_DefinedBenefitPlanCostOfProvidingSpecialOrContractualTerminationBenefitRecognizedDuringPeriod	0.6us-gaap_DefinedBenefitPlanCostOfProvidingSpecialOrContractualTerminationBenefitRecognizedDuringPeriod
Assumption changes	17.5us-gaap_DefinedBenefitPlanPlanAmendments	(13.0)us-gaap_DefinedBenefitPlanPlanAmendments
Actuarial loss (gain)	1.1us-gaap_DefinedBenefitPlanActuarialGainLoss	(0.5)us-gaap_DefinedBenefitPlanActuarialGainLoss
Benefits and expenses paid with plan assets	(5.2)wtm_DefinedBenefitPlanBenefitsAndExpensesPaidNetOfParticipantContributions	(6.2)wtm_DefinedBenefitPlanBenefitsAndExpensesPaidNetOfParticipantContributions
Benefits paid directly by OneBeacon	(2.2)us-gaap_DefinedBenefitPlanBenefitsPaid	(2.2)us-gaap_DefinedBenefitPlanBenefitsPaid
Projected benefit obligation at end of year	119.7us-gaap_DefinedBenefitPlanBenefitObligation	102.9us-gaap_DefinedBenefitPlanBenefitObligation	119.5us-gaap_DefinedBenefitPlanBenefitObligation
Change in plan assets:
Fair value of plan assets at beginning of year	142.8us-gaap_DefinedBenefitPlanFairValueOfPlanAssets	124.7us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Actual return on plan assets	8.4us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets	24.3us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets
Benefits and expenses paid with plan assets	(5.2)wtm_DefinedBenefitPlanBenefitsAndExpensesPaidNetOfParticipantContributions	(6.2)wtm_DefinedBenefitPlanBenefitsAndExpensesPaidNetOfParticipantContributions
Fair value of plan assets at end of year	146.0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets	142.8us-gaap_DefinedBenefitPlanFairValueOfPlanAssets	124.7us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Funded status at end of year	26.3us-gaap_DefinedBenefitPlanFundedStatusOfPlan	39.9us-gaap_DefinedBenefitPlanFundedStatusOfPlan
Amounts recognized in the financial statements
Net assets of the Qualified Plan recorded in other assets	53.4us-gaap_DefinedBenefitPlanAssetsForPlanBenefitsCurrentAndNoncurrent	64.1us-gaap_DefinedBenefitPlanAssetsForPlanBenefitsCurrentAndNoncurrent
Net liabilities of the Non-qualified Plan recorded in other liabilities	(27.1)us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent	(24.2)us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent
Defined Benefit Plan, Amounts Recognized in Balance Sheet	26.3us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet	39.9us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet
Qualified Plan [Member]
Change in plan assets:
Fair value of plan assets at end of year	143.2us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember	142.1us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Amounts recognized in the financial statements
Net assets of the Qualified Plan recorded in other assets	53.4us-gaap_DefinedBenefitPlanAssetsForPlanBenefitsCurrentAndNoncurrent / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Other Postretirement Benefit Plan, Defined Benefit [Member]
Amounts recognized in the financial statements
Net liabilities of the Non-qualified Plan recorded in other liabilities	(27.1)us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Other Postretirement Benefit Plan, Defined Benefit [Member] | OneBeacon Ltd.
Change in plan assets:
Amount set aside in irrevocable rabbi trust	$ 18.2wtm_DefinedBenefitPlanAmountSetAsideUnderIrrevocableTrustForBenefitOfPlanParticipant / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember

X

- Definition

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

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- Definition

Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan.

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The aggregate net amount recognized in the balance sheet associated with the defined benefit plan(s). Will normally be the same as the Defined Benefit Plan, Funded Status of Plan, Total.

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This represents the entire assets recognized in the balance sheet that are associated with the defined benefit plans.

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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.

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The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

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If applicable, the cost of providing special or contractual termination benefits recognized during the period on the projected benefit obligation.

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Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

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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.

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The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

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The amount of increase or decrease due to a change in the terms of an existing plan or the initiation of a new plan. A plan amendment may increase or decrease benefits, including those attributed to years of service already rendered.

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The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

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Represents the amount set aside under irrevocable rabbi trust for the benefit of Plan participants.

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