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Retirement and Postretirement Plans (Periodic Benefit Cost) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Components of net periodic benefit cost (income)
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
Expected return on plan assets	(8.5)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets	(7.1)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets	(6.9)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets
Amortization of unrecognized loss	0.3us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses	0.9us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses	0.8us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses
Net periodic pension income before settlements, curtailments and special termination benefits	(2.9)wtm_DefinedBenefitPlanBeforeSettlementsCurtailmentsAndSpecialTerminationBenefits	(1.2)wtm_DefinedBenefitPlanBeforeSettlementsCurtailmentsAndSpecialTerminationBenefits	(0.7)wtm_DefinedBenefitPlanBeforeSettlementsCurtailmentsAndSpecialTerminationBenefits
Settlement loss	0us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1	0us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1	0.6us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1
Special termination benefits expense	(0.3)us-gaap_DefinedBenefitPlanCostOfProvidingSpecialOrContractualTerminationBenefitRecognizedDuringPeriod	(0.3)us-gaap_DefinedBenefitPlanCostOfProvidingSpecialOrContractualTerminationBenefitRecognizedDuringPeriod	(0.6)us-gaap_DefinedBenefitPlanCostOfProvidingSpecialOrContractualTerminationBenefitRecognizedDuringPeriod
Total net periodic (income) benefit cost	$ (2.6)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost	$ (0.9)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost	$ 0.5us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount Yield (as a percent)	3.91%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember	4.66%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Weighted average discount rate assumptions used to determine benefit obligations
Discount rate (as a percent)	4.66%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Pension Costs [Member]
Weighted average discount rate assumptions used to determine benefit obligations
Discount rate (as a percent)		3.64%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / us-gaap_RegulatoryLiabilityAxis = us-gaap_PensionCostsMember
Long-Duration Contracts, Assumptions by Product and Guarantee, Estimated Average Investment Yield	6.00%us-gaap_LongDurationContractsAssumptionsByProductAndGuaranteeEstimatedAverageInvestmentYield / us-gaap_RegulatoryLiabilityAxis = us-gaap_PensionCostsMember	5.75%us-gaap_LongDurationContractsAssumptionsByProductAndGuaranteeEstimatedAverageInvestmentYield / us-gaap_RegulatoryLiabilityAxis = us-gaap_PensionCostsMember

X

- Definition

The amount of gains or losses recognized in net periodic benefit cost.

+ 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 (h)(4)

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- Definition

The interest rate used to adjust for the time value of money for the plan.

+ References

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- Definition

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

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- Definition

An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

+ References

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- Definition

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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- Definition

The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

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- Definition

Amount of net gain (loss) recognized in net periodic benefit cost as a result of an irrevocable action that relieves the employer (or the plan) of primary responsibility for a benefit obligation and eliminates significant risks related to the obligation and the assets used to effect the settlement. Examples of transactions that constitute a settlement include, but are not limited to, lump-sum cash payments to plan participants in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contracts to cover vested benefits.

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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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Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

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Estimated average rate of return on investments which is included in the calculation of the amount of the specified guaranteed benefit obligation as of the balance sheet date.

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The total amount of periodic benefit cost before settlements, curtailments and special termination benefits for defined benefit plans for the period.

+ References

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