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Income Taxes (Deferred Tax Inventory) (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Income Tax Contingency [Line Items] |  |  |
Valuation allowance | $ 18,100,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance | $ 33,600,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance | $ 14,100,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Deferred income tax assets related to: |  |  |
Non-U.S. net operating loss carryforwards | 427,400,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsForeign | 520,600,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsForeign |
U.S. federal net operating and capital loss carryforwards | 213,400,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic | 148,600,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic |
Loss reserve discount | 79,300,000 us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves | 74,900,000 us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves |
Unearned premiums | 46,000,000 us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve | 38,900,000 us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve |
Incentive compensation | 8,900,000 us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits | 10,300,000 us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits |
Tax credit carryforwards | 25,800,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther | 23,100,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther |
Accrued interest | 5,300,000 us-gaap_DeferredTaxAssetsDeferredIncome | 1,100,000 us-gaap_DeferredTaxAssetsDeferredIncome |
Fixed assets | 3,600,000 us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment | 4,900,000 us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment |
Other items | 8,400,000 us-gaap_DeferredTaxAssetsOther | 6,900,000 us-gaap_DeferredTaxAssetsOther |
Total gross deferred income tax assets | 872,700,000 us-gaap_DeferredTaxAssetsGross | 907,400,000 us-gaap_DeferredTaxAssetsGross |
Less: valuation allowances | (274,600,000)us-gaap_DeferredTaxAssetsValuationAllowance | (289,800,000)us-gaap_DeferredTaxAssetsValuationAllowance |
Total net deferred income tax assets | 598,100,000 us-gaap_DeferredTaxAssetsNet | 617,600,000 us-gaap_DeferredTaxAssetsNet |
Deferred income tax liabilities related to: |  |  |
Deferred Tax Liabilities, Gross, Noncurrent | 0 us-gaap_DeferredTaxLiabilitiesGrossNoncurrent | 3,400,000 us-gaap_DeferredTaxLiabilitiesGrossNoncurrent |
Net unrealized investment gains | 45,300,000 us-gaap_DeferredTaxLiabilitiesUnrealizedGainsOnTradingSecurities | 44,600,000 us-gaap_DeferredTaxLiabilitiesUnrealizedGainsOnTradingSecurities |
Deferred acquisition costs | 40,500,000 us-gaap_DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost | 33,000,000 us-gaap_DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost |
Investment basis difference | 22,900,000 us-gaap_DeferredTaxLiabilitiesInvestments | 3,000,000 us-gaap_DeferredTaxLiabilitiesInvestments |
Other items | 3,100,000 us-gaap_DeferredTaxLiabilitiesOther | 7,900,000 us-gaap_DeferredTaxLiabilitiesOther |
Total deferred income tax liabilities | 424,800,000 us-gaap_DeferredTaxLiabilities | 461,700,000 us-gaap_DeferredTaxLiabilities |
Net deferred tax asset | 173,300,000 us-gaap_DeferredTaxAssetsLiabilitiesNet | 155,900,000 us-gaap_DeferredTaxAssetsLiabilitiesNet |
OneBeacon Runoff |  |  |
Deferred income tax assets related to: |  |  |
Runoff Transaction | 12,600,000 us-gaap_DeferredTaxAssetParentsBasisInDiscontinuedOperation / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember | 24,200,000 us-gaap_DeferredTaxAssetParentsBasisInDiscontinuedOperation / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember |
Safety Reserve [Member] |  |  |
Deferred income tax liabilities related to: |  |  |
Deferred Tax Liabilities, Gross, Noncurrent | 295,700,000 us-gaap_DeferredTaxLiabilitiesGrossNoncurrent / us-gaap_FairValueByLiabilityClassAxis = wtm_SafetyReserveMember | 357,200,000 us-gaap_DeferredTaxLiabilitiesGrossNoncurrent / us-gaap_FairValueByLiabilityClassAxis = wtm_SafetyReserveMember |
Purchase Accounting [Member] |  |  |
Deferred income tax liabilities related to: |  |  |
Deferred Tax Liabilities, Gross, Noncurrent | 7,000,000 us-gaap_DeferredTaxLiabilitiesGrossNoncurrent / us-gaap_FairValueByLiabilityClassAxis = wtm_PurchaseAccountingMember | 7,200,000 us-gaap_DeferredTaxLiabilitiesGrossNoncurrent / us-gaap_FairValueByLiabilityClassAxis = wtm_PurchaseAccountingMember |
Members Surplus Contributions [Member] |  |  |
Deferred income tax liabilities related to: |  |  |
Deferred Tax Liabilities, Gross, Noncurrent | 10,300,000 us-gaap_DeferredTaxLiabilitiesGrossNoncurrent / us-gaap_FairValueByLiabilityClassAxis = wtm_MembersSurplusContributionsMember | 5,400,000 us-gaap_DeferredTaxLiabilitiesGrossNoncurrent / us-gaap_FairValueByLiabilityClassAxis = wtm_MembersSurplusContributionsMember |
Incentive Compensation [Member] |  |  |
Deferred income tax assets related to: |  |  |
Incentive compensation | 42,000,000 us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits / us-gaap_FairValueByAssetClassAxis = wtm_IncentiveCompensationMember | 53,900,000 us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits / us-gaap_FairValueByAssetClassAxis = wtm_IncentiveCompensationMember |
UNITED STATES |  |  |
Deferred income tax assets related to: |  |  |
Tax credit carryforwards | 11,100,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther / us-gaap_IncomeTaxAuthorityAxis = country_US |  |
Deferred income tax liabilities related to: |  |  |
Net deferred tax asset | 214,000,000 us-gaap_DeferredTaxAssetsLiabilitiesNet / us-gaap_IncomeTaxAuthorityAxis = country_US |  |
Luxembourg |  |  |
Deferred income tax liabilities related to: |  |  |
Net deferred tax asset | 240,500,000 us-gaap_DeferredTaxAssetsLiabilitiesNet / us-gaap_IncomeTaxAuthorityAxis = country_LU |  |
UNITED KINGDOM |  |  |
Deferred income tax liabilities related to: |  |  |
Net deferred tax asset | 1,600,000 us-gaap_DeferredTaxAssetsLiabilitiesNet / us-gaap_IncomeTaxAuthorityAxis = country_GB |  |
SWEDEN |  |  |
Deferred income tax liabilities related to: |  |  |
Total deferred income tax liabilities | $ 282,800,000 us-gaap_DeferredTaxLiabilities / us-gaap_IncomeTaxAuthorityAxis = country_SE |  |

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- Definition Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=21917399&loc=d3e32123-109318 + Details
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