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Debt and Standby Letter of Credit Facilities (Narrative) (Details) (USD $) | 12 Months Ended | 1 Months Ended | 3 Months Ended | 5 Months Ended | 7 Months Ended | 1 Months Ended | 3 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2007 | Dec. 31, 2013 | Dec. 31, 2013 | Aug. 13, 2013 | Nov. 30, 2012 | Dec. 31, 2012 | Aug. 14, 2013 | Aug. 12, 2011 | May 31, 2003 | Oct. 10, 2014 | Nov. 25, 2014
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of debt, net of debt issuance costs | $ 68,600,000 us-gaap_ProceedsFromIssuanceOfDebt | $ 0 us-gaap_ProceedsFromIssuanceOfDebt | $ 271,900,000 us-gaap_ProceedsFromIssuanceOfDebt |  |  |  |  |  |  |  |  |  |  |
Aggregate purchase price of senior notes purchased and retired | (65,000,000)us-gaap_EarlyRepaymentOfSeniorDebt | 0 us-gaap_EarlyRepaymentOfSeniorDebt | 275,900,000 us-gaap_EarlyRepaymentOfSeniorDebt |  |  |  |  |  |  |  |  |  |  |
Interest expense on debt | 41,900,000 us-gaap_InterestExpenseDebt | 42,500,000 us-gaap_InterestExpenseDebt | 44,800,000 us-gaap_InterestExpenseDebt |  |  |  |  |  |  |  |  |  |  |
Draw down of revolving line of credit | 0 us-gaap_ProceedsFromLinesOfCredit | 200,000,000 us-gaap_ProceedsFromLinesOfCredit | 150,000,000 us-gaap_ProceedsFromLinesOfCredit |  |  |  |  |  |  |  |  |  |  |
Repayments of debt, principal | 66,500,000 us-gaap_RepaymentsOfDebt | 275,000,000 us-gaap_RepaymentsOfDebt | 75,000,000 us-gaap_RepaymentsOfDebt |  |  |  |  |  |  |  |  |  |  |
Interest Paid | 42,000,000 us-gaap_InterestPaid | 42,600,000 us-gaap_InterestPaid | 44,600,000 us-gaap_InterestPaid |  |  |  |  |  |  |  |  |  |  |
Debt | 746,600,000 us-gaap_LongTermDebt | 676,400,000 us-gaap_LongTermDebt |  |  | 676,400,000 us-gaap_LongTermDebt | 676,400,000 us-gaap_LongTermDebt |  |  |  |  |  |  |  |
SIG Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument at face value |  |  |  | 400,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |
Percentage of par value at which debt was issued |  |  |  | 99.715%wtm_DebtInstrumentIssuePriceAsPercentageOfParValue / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |
Issuance of debt, net of debt issuance costs |  |  |  | 392,000,000 us-gaap_ProceedsFromIssuanceOfDebt / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  | 6.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |
Debt issue costs |  |  |  | 3,600,000 us-gaap_DeferredFinanceCostsGross / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |
Underwriting discount |  |  |  | 2,600,000 wtm_DeferredUnderwritingDiscountGross / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |
Effective yield (as a percent) |  |  |  | 6.50%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |
Loss from interest rate hedge recorded in other comprehensive income |  |  |  | (2,400,000)us-gaap_DerivativeInstrumentsLossRecognizedInOtherComprehensiveIncomeEffectivePortion / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |
Unamortized loss remaining in accumulated other comprehensive income | 1,000,000 us-gaap_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectNetOfTax / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |  |  |  |
Interest expense on debt | 26,200,000 us-gaap_InterestExpenseDebt / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember | 26,200,000 us-gaap_InterestExpenseDebt / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember | 26,200,000 us-gaap_InterestExpenseDebt / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |  |
Expenses related to issuance, including underwriting fees |  |  |  | 3,600,000 us-gaap_DeferredFinanceCostsGross / us-gaap_DebtInstrumentAxis = wtm_SiriusGroupSeniorNotesMember |  |  |  |  |  |  |  |  |  |
WTM Bank Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Effective yield (as a percent) | 3.65%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_WtmBankFacilityMember | 3.65%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_WtmBankFacilityMember |  |  | 3.65%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_WtmBankFacilityMember | 3.65%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_WtmBankFacilityMember |  |  |  |  |  |  |  |
Interest expense on debt | 300,000 us-gaap_InterestExpenseDebt / us-gaap_DebtInstrumentAxis = wtm_WtmBankFacilityMember |  |  |  |  | 300,000 us-gaap_InterestExpenseDebt / us-gaap_DebtInstrumentAxis = wtm_WtmBankFacilityMember |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  |  |  |  | 425,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = wtm_WtmBankFacilityMember |  |  |  |
Draw down of revolving line of credit | 65,000,000 us-gaap_ProceedsFromLinesOfCredit / us-gaap_DebtInstrumentAxis = wtm_WtmBankFacilityMember | 50,000,000 us-gaap_ProceedsFromLinesOfCredit / us-gaap_DebtInstrumentAxis = wtm_WtmBankFacilityMember |  |  |  |  |  |  |  |  |  |  |  |
Repayments of lines of credit |  |  |  |  | 50,000,000 us-gaap_RepaymentsOfLinesOfCredit / us-gaap_DebtInstrumentAxis = wtm_WtmBankFacilityMember |  |  |  |  |  |  |  |  |
Prior W T M Bank Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Effective yield (as a percent) |  | 2.83%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember | 3.53%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember |  | 2.83%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember | 2.83%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember |  |  | 3.53%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember |  |  |  |  |
Interest expense on debt |  |  | 1,700,000 us-gaap_InterestExpenseDebt / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember |  |  |  | 2,900,000 us-gaap_InterestExpenseDebt / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  |  |  |  |  | 375,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember |  |  |
Draw down of revolving line of credit |  | 150,000,000 us-gaap_ProceedsFromLinesOfCredit / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember | 150,000,000 us-gaap_ProceedsFromLinesOfCredit / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember |  |  |  |  |  |  |  |  |  |  |
Repayments of debt, principal |  |  | 75,000,000 us-gaap_RepaymentsOfDebt / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember |  |  |  |  |  |  |  |  |  |  |
Repayments of lines of credit |  | 225,000,000 us-gaap_RepaymentsOfLinesOfCredit / us-gaap_DebtInstrumentAxis = wtm_PriorWTMBankFacilityMember |  |  |  |  |  |  |  |  |  |  |  |
One Beacon US Holdings Inc. | 2012 OBH Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument at face value |  |  |  |  |  |  |  | 275,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |
Percentage of par value at which debt was issued |  |  |  |  |  |  |  | 99.90%wtm_DebtInstrumentIssuePriceAsPercentageOfParValue / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |
Issuance of debt, net of debt issuance costs |  |  |  |  |  |  |  | 272,900,000 us-gaap_ProceedsFromIssuanceOfDebt / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |
Interest rate (as a percent) |  |  |  |  |  |  |  | 4.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |
Debt issue costs |  |  |  |  |  |  |  | 2,800,000 us-gaap_DeferredFinanceCostsGross / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |
Underwriting discount |  |  |  |  |  |  |  | 1,800,000 wtm_DeferredUnderwritingDiscountGross / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |
Effective yield (as a percent) |  |  |  |  |  |  |  | 4.70%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |
Interest Expense | 13,000,000 us-gaap_InterestExpense / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |  |  |  |  |  |  |  |
Expenses related to issuance, including underwriting fees |  |  |  |  |  |  |  | 2,800,000 us-gaap_DeferredFinanceCostsGross / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |
One Beacon US Holdings Inc. | 2003 OBH Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest Expense |  | 13,000,000 us-gaap_InterestExpense / us-gaap_DebtInstrumentAxis = wtm_A2003ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember | 16,900,000 us-gaap_InterestExpense / us-gaap_DebtInstrumentAxis = wtm_A2003ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |  |  |  |  |  |
Guarantee fee basis points per annum on outstanding principal amount of debt (as a percent) |  |  |  |  |  |  |  |  |  |  |  | 2.50%wtm_GuarantyFeeAsPercentageOutstandingPrincipalAmountOfDebt / us-gaap_DebtInstrumentAxis = wtm_A2003ObhSeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |
One Beacon US Holdings Inc. | Senior Notes | 2003 OBH Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  |  |  |  |  | 5.875%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = wtm_A2003ObhSeniorNotesMember / us-gaap_ExtinguishmentOfDebtAxis = us-gaap_SeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |
Debt Instrument, Repurchase Amount |  | 269,800,000 us-gaap_DebtInstrumentRepurchaseAmount / us-gaap_DebtInstrumentAxis = wtm_A2003ObhSeniorNotesMember / us-gaap_ExtinguishmentOfDebtAxis = us-gaap_SeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  | 269,800,000 us-gaap_DebtInstrumentRepurchaseAmount / us-gaap_DebtInstrumentAxis = wtm_A2003ObhSeniorNotesMember / us-gaap_ExtinguishmentOfDebtAxis = us-gaap_SeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember | 269,800,000 us-gaap_DebtInstrumentRepurchaseAmount / us-gaap_DebtInstrumentAxis = wtm_A2003ObhSeniorNotesMember / us-gaap_ExtinguishmentOfDebtAxis = us-gaap_SeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |  |  |  |
Aggregate purchase price of senior notes purchased and retired |  |  |  |  |  |  |  |  | 275,900,000 us-gaap_EarlyRepaymentOfSeniorDebt / us-gaap_DebtInstrumentAxis = wtm_A2003ObhSeniorNotesMember / us-gaap_ExtinguishmentOfDebtAxis = us-gaap_SeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |
Loss on retirement of debt |  |  |  |  |  |  |  |  | (6,300,000)us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = wtm_A2003ObhSeniorNotesMember / us-gaap_ExtinguishmentOfDebtAxis = us-gaap_SeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |
Write off of deferred issuance costs |  |  |  |  |  |  |  |  | 200,000 us-gaap_WriteOffOfDeferredDebtIssuanceCost / us-gaap_DebtInstrumentAxis = wtm_A2003ObhSeniorNotesMember / us-gaap_ExtinguishmentOfDebtAxis = us-gaap_SeniorNotesMember / dei_LegalEntityAxis = wtm_OneBeaconHoldingsIncMember |  |  |  |  |
Tranzact [Member] | Tranzact Bank Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest expense on debt | 700,000 us-gaap_InterestExpenseDebt / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Fair Value of Amount Outstanding | 67,400,000 us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember | 0 us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |  |  | 0 us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember | 0 us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  |  |  |  | 70,000,000 us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |
Line of Credit Facility, Amount Outstanding | 68,700,000 us-gaap_LineOfCreditFacilityAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember | 0 us-gaap_LineOfCreditFacilityAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |  |  | 0 us-gaap_LineOfCreditFacilityAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember | 0 us-gaap_LineOfCreditFacilityAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  |  |  |  |  |
Line of Credit Facility, Remaining Borrowing Capacity | 4,500,000 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate | 4.15575%us-gaap_LongTermDebtPercentageBearingVariableInterestRate / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  |  |  |  |  |  |  |  |  |  |
Sirius International | Letter of Credit [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Fair Value of Amount Outstanding |  |  |  |  |  |  |  |  |  |  |  |  |  | 200,000,000 us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding / us-gaap_DebtInstrumentAxis = us-gaap_LetterOfCreditMember / dei_LegalEntityAxis = wtm_SiriusInternationalMember
number of Credit Lines |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 wtm_NumberofCreditLines / us-gaap_DebtInstrumentAxis = us-gaap_LetterOfCreditMember / dei_LegalEntityAxis = wtm_SiriusInternationalMember
Minimum | Tranzact [Member] | Tranzact Bank Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
LIBOR rate | 2.50%wtm_LIBORrate / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember / us-gaap_RangeAxis = us-gaap_MinimumMember |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum | Tranzact [Member] | Tranzact Bank Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
LIBOR rate | 4.50%wtm_LIBORrate / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |  |  |  |  |  |  |  |  |
Nordea Bank Finland [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  |  |  |  |  |  |  |  | 125,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = wtm_NordeaBankFinlandMember
Lloyds Bank plc [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  |  |  |  |  |  |  |  | 75,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = wtm_LloydsBankplcMember
Unsecured Debt [Member] | Nordea Bank Finland [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = wtm_NordeaBankFinlandMember / us-gaap_LongtermDebtTypeAxis = us-gaap_UnsecuredDebtMember
Unsecured Debt [Member] | Lloyds Bank plc [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 25,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = wtm_LloydsBankplcMember / us-gaap_LongtermDebtTypeAxis = us-gaap_UnsecuredDebtMember

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- Definition Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e681-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669646-108580 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=7668309&loc=d3e80784-113994 + Details
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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400 + Details
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