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SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS |  |  |
Rate at which reserves are discounted (as a percent) | 2.50%us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersInterestRateAtWhichDiscountComputed | 3.50%us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersInterestRateAtWhichDiscountComputed | 3.50%us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersInterestRateAtWhichDiscountComputed
OneBeacon |  |  |
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS |  |  |
Deferred acquisition costs | $ 103.2 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | $ 103.7 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | $ 123.9 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Reserves for Unpaid Claims and Claims Adjustment Expenses | 1,342.2 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 1,054.3 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 1,000.0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Discount, if any, deducted in Column C | 1.0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDiscountDeductedFromReserves / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 3.0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDiscountDeductedFromReserves / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 4.6 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDiscountDeductedFromReserves / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Unearned Premiums | 588.3 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 544.9 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 573.8 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Earned Premiums | 1,177.1 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 1,120.4 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 1,132.0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Net investment income | 41.7 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 41.1 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 53.6 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Claims and ClaimsAdjustment ExpensesIncurred Related to, Current Year | 725.3 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 622.1 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 657.4 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Claims and ClaimsAdjustment ExpensesIncurred Related to, Prior Year | 89.8 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | (7.4)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Amortization of deferred policy acquisition | 203.3 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 208.9 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 249.4 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Paid Claims and Claims Adjustment Expenses | 608.6 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 540.7 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 565.1 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Premiums written | 1,216.9 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 1,088.6 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember | 1,179.2 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OnebeaconMember
Sirius Group |  |  |
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS |  |  |
Deferred acquisition costs | 69.9 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 69.3 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 71.4 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Reserves for Unpaid Claims and Claims Adjustment Expenses | 1,809.8 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 2,025.0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 2,168.9 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Discount, if any, deducted in Column C | 0.7 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDiscountDeductedFromReserves / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 1.7 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDiscountDeductedFromReserves / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 2.4 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDiscountDeductedFromReserves / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Unearned Premiums | 338.6 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 343.3 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 350.2 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Earned Premiums | 873.9 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 866.4 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 931.6 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Net investment income | 41.1 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 30.3 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 65.0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Claims and ClaimsAdjustment ExpensesIncurred Related to, Current Year | 443.2 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 466.8 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 578.4 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Claims and ClaimsAdjustment ExpensesIncurred Related to, Prior Year | (98.0)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | (48.4)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | (34.5)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Amortization of deferred policy acquisition | 193.6 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 166.5 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 180.8 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Paid Claims and Claims Adjustment Expenses | 544.9 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 628.1 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 741.2 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Premiums written | 882.5 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 876.6 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember | 947.7 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_SiriusGroupSegmentMember
Other Operations |  |  |
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS |  |  |
Deferred acquisition costs | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember
Reserves for Unpaid Claims and Claims Adjustment Expenses | 7.8 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember
Discount, if any, deducted in Column C | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDiscountDeductedFromReserves / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDiscountDeductedFromReserves / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDiscountDeductedFromReserves / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember
Unearned Premiums | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember
Earned Premiums | 0.8 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember
Net investment income | 0.2 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember
Claims and ClaimsAdjustment ExpensesIncurred Related to, Current Year | 6.7 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember
Claims and ClaimsAdjustment ExpensesIncurred Related to, Prior Year | 2.2 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember
Amortization of deferred policy acquisition | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember
Paid Claims and Claims Adjustment Expenses | 14.8 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember
Premiums written | $ 5.9 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | $ 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember | $ 0 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten / us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis = wtm_OtherOperationsMember

X
- Definition Amount of amortization expense on deferred policy acquisition cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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X
- Definition Amount of insurance benefit, claims, loss and settlement expenses paid to the policyholders during the current accounting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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X
- Definition Amount of deferred policy acquisition cost related to property-casualty insurance policy written. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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Balance Type: | debit
Period Type: | instant

X
- Definition The amount of discount that may be deducted from the reserve for unpaid claims and claims adjustment expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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X
- Definition Amount of property-casualty insurance premium revenue earned on insurance policies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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Balance Type: | credit
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X
- Definition The rate at which the discount was computed for each business segment reported. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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- Definition Amount of net investment income earned by property-casualty insurance underwriter. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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- Definition Payments of claims and claims adjustment expenses during the period of a property-casualty insurance entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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