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Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Stock Units) (Details) (Restricted Stock Units (RSUs) [Member], OneBeacon, One Beacon Long Term Incentive Plan, USD $)
In Millions, except Share data, unless otherwise specified
 0 Months Ended 3 Months Ended
Feb. 24, 2015
Mar. 31, 2015
Restricted Stock Units (RSUs) [Member] | OneBeacon | One Beacon Long Term Incentive Plan
    
Share-based compensation arrangement by share-based payment award    
Issued (in shares) 207,848us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ dei_LegalEntityAxis
= wtm_OnebeaconMember
/ us-gaap_PlanNameAxis
= wtm_LongTermIncentivePlanMember		  
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number   205,319us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ dei_LegalEntityAxis
= wtm_OnebeaconMember
/ us-gaap_PlanNameAxis
= wtm_LongTermIncentivePlanMember
Expense recognized   $ 0.1us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ dei_LegalEntityAxis
= wtm_OnebeaconMember
/ us-gaap_PlanNameAxis
= wtm_LongTermIncentivePlanMember