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Income Taxes (Valuation Allowance) (Details) (USD $) | 3 Months Ended | 12 Months Ended | 3 Months Ended
--- | --- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2012
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance | $ 274,600,000 us-gaap_DeferredTaxAssetsValuationAllowance |  |  |  | $ 289,800,000 us-gaap_DeferredTaxAssetsValuationAllowance |  |  |  | $ 274,600,000 us-gaap_DeferredTaxAssetsValuationAllowance | $ 289,800,000 us-gaap_DeferredTaxAssetsValuationAllowance |  |
Valuation allowance |  |  |  |  |  |  |  |  | 18,100,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance | 33,600,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance | 14,100,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance |
Income Tax Expense (Benefit) | (8,700,000)us-gaap_IncomeTaxExpenseBenefit | 7,800,000 us-gaap_IncomeTaxExpenseBenefit | 23,900,000 us-gaap_IncomeTaxExpenseBenefit | 30,300,000 us-gaap_IncomeTaxExpenseBenefit | 27,400,000 us-gaap_IncomeTaxExpenseBenefit | 8,200,000 us-gaap_IncomeTaxExpenseBenefit | (600,000)us-gaap_IncomeTaxExpenseBenefit | 41,600,000 us-gaap_IncomeTaxExpenseBenefit | 53,300,000 us-gaap_IncomeTaxExpenseBenefit | 76,600,000 us-gaap_IncomeTaxExpenseBenefit | (15,700,000)us-gaap_IncomeTaxExpenseBenefit |
Net operating losses in Luxembourg subsidiaries |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance | 164,800,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_NetOperatingLossesInLuxembourgSubsidiariesMember |  |  |  | 195,800,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_NetOperatingLossesInLuxembourgSubsidiariesMember |  |  |  | 164,800,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_NetOperatingLossesInLuxembourgSubsidiariesMember | 195,800,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_NetOperatingLossesInLuxembourgSubsidiariesMember |  |
U.S. losses and other federal deferred tax benefits |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance | 107,600,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_USLossesAndOtherFederalDeferredTaxBenefitsMember |  |  |  | 93,700,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_USLossesAndOtherFederalDeferredTaxBenefitsMember |  |  |  | 107,600,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_USLossesAndOtherFederalDeferredTaxBenefitsMember | 93,700,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_USLossesAndOtherFederalDeferredTaxBenefitsMember |  |
Net Operating Losses in Israel Subsidiaries [Member] |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance | 1,900,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_NetOperatingLossesinIsraelSubsidiariesMember |  |  |  |  |  |  |  | 1,900,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_NetOperatingLossesinIsraelSubsidiariesMember |  |  |
Net Operating Losses in Netherlands Subsidiaries [Member] [Member] |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance | 300,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_NetOperatingLossesinNetherlandsSubsidiariesMemberMember |  |  |  | 300,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_NetOperatingLossesinNetherlandsSubsidiariesMemberMember |  |  |  | 300,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_NetOperatingLossesinNetherlandsSubsidiariesMemberMember | 300,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_NetOperatingLossesinNetherlandsSubsidiariesMemberMember |  |
Luxembourg |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  |  |  | 1,200,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = country_LU | 1,400,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = country_LU |  | (41,300,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = country_LU
Luxembourg | Tax Benefit [Member] |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  |  |  | (14,300,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = country_LU / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_TaxBenefitMember | (16,700,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = country_LU / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_TaxBenefitMember |  |
Luxembourg | Tax Expense [Member] |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  |  |  | 15,500,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = country_LU / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_TaxExpenseMember | 18,100,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = country_LU / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_TaxExpenseMember |  |
Sirius Re Holdings Inc. |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance | 100,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = wtm_SiriusReHoldingsIncMember |  |  |  |  |  |  |  | 100,000 us-gaap_DeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = wtm_SiriusReHoldingsIncMember |  |  |
Foreign tax credit carryforwards, subject to expiration | 12,100,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign / us-gaap_StatementGeographicalAxis = wtm_SiriusReHoldingsIncMember |  |  |  |  |  |  |  | 12,100,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign / us-gaap_StatementGeographicalAxis = wtm_SiriusReHoldingsIncMember |  |  |
Sirius Re Holdings Inc. | Tax Benefit [Member] |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  |  |  | (3,800,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = wtm_SiriusReHoldingsIncMember / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_TaxBenefitMember |  |  |
Sirius Re Holdings Inc. | Tax Expense [Member] |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  |  |  |  |  |  | 3,900,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = wtm_SiriusReHoldingsIncMember / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = wtm_TaxExpenseMember
Guilford Holdings, Inc. | UNITED STATES |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  |  |  | 7,200,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / dei_LegalEntityAxis = wtm_GuilfordHoldingsIncMember / us-gaap_StatementGeographicalAxis = country_US | 16,600,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / dei_LegalEntityAxis = wtm_GuilfordHoldingsIncMember / us-gaap_StatementGeographicalAxis = country_US |  |
BAM |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  |  |  | 8,500,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / dei_LegalEntityAxis = wtm_BamMember | 21,300,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / dei_LegalEntityAxis = wtm_BamMember |  |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 4,900,000 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = wtm_BamMember | 5,400,000 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = wtm_BamMember |  |
Houston General Insurance [Member] |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  |  |  | (100,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / dei_LegalEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember | 6.0 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / dei_LegalEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |
Star & Shield Insurance Exchange [Member] |  |  |  |  |  |  |  |  |  |  |  |
Valuation Allowance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance |  |  |  |  |  |  |  |  | $ (3,200,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |

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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible foreign tax credit carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 + Details
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