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Income Taxes (Effective Rate Reconciliation) (Details) (USD $) | 3 Months Ended | 12 Months Ended | 0 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, 2012 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Jan. 02, 2013
Income Tax Contingency [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. statutory rate (as a percent) |  |  |  |  |  |  |  |  | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate |  |  |  |
Tax (expense) benefit at the U.S. statutory rate |  |  |  |  |  |  |  |  |  | $ (105,600,000)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate | $ (120,700,000)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate | $ (92,000,000)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate |
Differences in taxes resulting from: |  |  |  |  |  |  |  |  |  |  |  |  |
Non-U.S. earnings, net of foreign taxes |  |  |  |  |  |  |  |  |  | 77,700,000 us-gaap_IncomeTaxReconciliationRepatriationOfForeignEarnings | 82,900,000 us-gaap_IncomeTaxReconciliationRepatriationOfForeignEarnings | 43,000,000 us-gaap_IncomeTaxReconciliationRepatriationOfForeignEarnings |
Change in valuation allowance |  |  |  |  |  |  |  |  |  | (18,100,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance | (33,600,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance | (14,100,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance |
Effective Income Tax Rate Reconciliation, Deduction, Other, Amount |  |  |  |  |  |  |  |  |  | (5,600,000)us-gaap_IncomeTaxReconciliationDeductionsOther | (1,700,000)us-gaap_IncomeTaxReconciliationDeductionsOther | (2,900,000)us-gaap_IncomeTaxReconciliationDeductionsOther |
Tax exempt interest and dividends |  |  |  |  |  |  |  |  |  | 4,400,000 us-gaap_IncomeTaxReconciliationTaxExemptIncome | 3,200,000 us-gaap_IncomeTaxReconciliationTaxExemptIncome | 3,100,000 us-gaap_IncomeTaxReconciliationTaxExemptIncome |
Tax reserve adjustments |  |  |  |  |  |  |  |  |  | (2,200,000)us-gaap_IncomeTaxReconciliationTaxContingencies | (10,200,000)us-gaap_IncomeTaxReconciliationTaxContingencies | (1,300,000)us-gaap_IncomeTaxReconciliationTaxContingencies |
Purchase of subsidiaries |  |  |  |  |  |  |  |  |  | (300,000)wtm_EffectiveIncomeTaxRateReconciliationPurchaseofSubsidiary | 5,300,000 wtm_EffectiveIncomeTaxRateReconciliationPurchaseofSubsidiary | 5,100,000 wtm_EffectiveIncomeTaxRateReconciliationPurchaseofSubsidiary |
Other, net |  |  |  |  |  |  |  |  |  | (3,600,000)us-gaap_IncomeTaxReconciliationOtherAdjustments | (1,800,000)us-gaap_IncomeTaxReconciliationOtherAdjustments | 2,200,000 us-gaap_IncomeTaxReconciliationOtherAdjustments |
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 |
Pre-tax loss | 46,200,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 34,400,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 109,000,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 112,100,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 134,200,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 55,300,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 3,600,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 151,800,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments |  | 301,700,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 344,900,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 262,800,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments |
Tax Payments and Receipts |  |  |  |  |  |  |  |  |  |  |  |  |
Net income tax payments to (receipts from) national governments, total |  |  |  |  |  |  |  |  |  | 9,400,000 us-gaap_IncomeTaxesPaidNet | 13,900,000 us-gaap_IncomeTaxesPaidNet | 17,500,000 us-gaap_IncomeTaxesPaidNet |
Non-US |  |  |  |  |  |  |  |  |  |  |  |  |
Differences in taxes resulting from: |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax loss |  |  |  |  |  |  |  |  |  | 338,500,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_StatementGeographicalAxis = wtm_NonUSMember | 294,300,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_StatementGeographicalAxis = wtm_NonUSMember | 250,000,000 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_StatementGeographicalAxis = wtm_NonUSMember |
SWEDEN |  |  |  |  |  |  |  |  |  |  |  |  |
Differences in taxes resulting from: |  |  |  |  |  |  |  |  |  |  |  |  |
Tax rate change enacted |  |  |  |  |  |  |  |  |  | 0 us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate / us-gaap_StatementGeographicalAxis = country_SE | 0 us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate / us-gaap_StatementGeographicalAxis = country_SE | 65,400,000 us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate / us-gaap_StatementGeographicalAxis = country_SE |
Luxembourg |  |  |  |  |  |  |  |  |  |  |  |  |
Differences in taxes resulting from: |  |  |  |  |  |  |  |  |  |  |  |  |
Change in valuation allowance |  |  |  |  |  |  |  |  | 41,300,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = country_LU | (1,200,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = country_LU | (1,400,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_StatementGeographicalAxis = country_LU |  |
Tax rate change enacted |  |  |  |  |  |  |  |  |  | 0 us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate / us-gaap_StatementGeographicalAxis = country_LU | 0 us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate / us-gaap_StatementGeographicalAxis = country_LU | 7,200,000 us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate / us-gaap_StatementGeographicalAxis = country_LU |
Sirius Group | SWEDEN |  |  |  |  |  |  |  |  |  |  |  |  |
Differences in taxes resulting from: |  |  |  |  |  |  |  |  |  |  |  |  |
Effective income tax |  |  |  |  |  |  |  |  |  |  |  | 26.30%us-gaap_EffectiveIncomeTaxRateContinuingOperations / us-gaap_IncomeTaxAuthorityAxis = country_SE / dei_LegalEntityAxis = wtm_SiriusGroupMember | 22.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations / us-gaap_IncomeTaxAuthorityAxis = country_SE / dei_LegalEntityAxis = wtm_SiriusGroupMember
Increase (Decrease) in Deferred Income Taxes |  |  |  |  |  |  |  |  |  |  |  |  | $ 65,400,000 us-gaap_IncreaseDecreaseInDeferredIncomeTaxes / us-gaap_IncomeTaxAuthorityAxis = country_SE / dei_LegalEntityAxis = wtm_SiriusGroupMember

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- Definition The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income, net of any cash received during the current period as refunds for the overpayment of taxes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586 + Details
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