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v2.4.1.9

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 | $ 0.6 us-gaap_DefinedBenefitPlanServiceCost | $ 0.8 us-gaap_DefinedBenefitPlanServiceCost | $ 0.7 us-gaap_DefinedBenefitPlanServiceCost
Interest cost | 4.7 us-gaap_DefinedBenefitPlanInterestCost | 4.2 us-gaap_DefinedBenefitPlanInterestCost | 4.7 us-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 | 0.3 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses | 0.9 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses | 0.8 us-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 | 0 us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 | 0 us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 | 0.6 us-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 | $ 0.5 us-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 |

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- 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) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition The interest rate used to adjust for the time value of money for the 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 (k)(1) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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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. + 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 (q) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512136 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512171 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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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. + 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)(2) -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 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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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. + 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) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 15 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=7577095&loc=d3e8001-114927 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(7) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition 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. + 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)(1) -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 Service Cost (Component of Net Periodic Pension Cost) -URI http://asc.fasb.org/extlink&oid=6525008 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6482852&loc=d3e14931-158439 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6482852&loc=d3e14937-158439 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6482852&loc=d3e14944-158439 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 80 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6484115&loc=d3e19393-158473 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 55 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6588021&loc=d3e7104-158389 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6481800&loc=d3e11318-158416 + Details
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- Definition The total amount of periodic benefit cost before settlements, curtailments and special termination benefits for defined benefit plans for the period. + References No definition available. + Details
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