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Investment Securities (Investment Holdings) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013
Investment securities |  |
Fixed maturity investments, at fair value | $ 4,784.3 us-gaap_TradingSecuritiesDebt | $ 5,030.5 us-gaap_TradingSecuritiesDebt
Including Short Term Fixed |  |
Investment securities |  |
Duration of Fixed Maturities | 2 years |
US Government and agency obligations |  |
Investment securities |  |
Trading Securities, Cost | 184.7 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 365.5 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Net foreign currency gains (losses) | 3.6 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | (2.5)wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Fixed maturity investments, at fair value | 188.1 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 362.5 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Trading Securities, Unrealized Holding Gain | 0.1 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 0.5 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Trading Securities, Unrealized Holding Loss | (0.3)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | (1.0)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Debt securities issued by corporations |  |
Investment securities |  |
Trading Securities, Cost | 2,221.3 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 2,330.7 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Net foreign currency gains (losses) | 49.8 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | (14.3)wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Fixed maturity investments, at fair value | 2,311.2 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 2,347.2 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Trading Securities, Unrealized Holding Gain | 45.2 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 44.0 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Trading Securities, Unrealized Holding Loss | (5.1)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | (13.2)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Municipal obligations |  |
Investment securities |  |
Trading Securities, Cost | 82.0 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 18.3 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Net foreign currency gains (losses) | 0 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 0 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Fixed maturity investments, at fair value | 83.2 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 17.9 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Trading Securities, Unrealized Holding Gain | 1.4 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 0 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Trading Securities, Unrealized Holding Loss | (0.2)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | (0.4)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Mortgage-backed and asset-backed securities |  |
Investment securities |  |
Trading Securities, Cost | 1,811.1 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 2,027.3 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Net foreign currency gains (losses) | 25.7 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | (5.3)wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Fixed maturity investments, at fair value | 1,840.9 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 2,014.5 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Trading Securities, Unrealized Holding Gain | 7.6 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 2.4 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Trading Securities, Unrealized Holding Loss | (3.5)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | (9.9)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Foreign government, agency and provincial obligations |  |
Investment securities |  |
Trading Securities, Cost | 274.6 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 444.2 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Net foreign currency gains (losses) | (2.7)wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | (4.8)wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Fixed maturity investments, at fair value | 275.1 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 439.9 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Trading Securities, Unrealized Holding Gain | 4.2 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 3.7 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Trading Securities, Unrealized Holding Loss | (1.0)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | (3.2)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Preferred stocks |  |
Investment securities |  |
Trading Securities, Cost | 79.6 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | 79.9 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember
Net foreign currency gains (losses) | 0.1 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | (0.2)wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember
Fixed maturity investments, at fair value | 85.8 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | 84.8 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember
Trading Securities, Unrealized Holding Gain | 6.1 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | 5.1 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember
Trading Securities, Unrealized Holding Loss | 0 us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | 0 us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember
Fixed Income Investments [Member] |  |
Investment securities |  |
Trading Securities, Cost | 4,653.3 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeInvestmentsMember | 5,265.9 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeInvestmentsMember
Net foreign currency gains (losses) | 76.5 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeInvestmentsMember | (27.1)wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeInvestmentsMember
Total fixed maturity investments | 4,784.3 wtm_TradingSecuritiesDebtIncludingAssetsHeldForSale / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeInvestmentsMember | 5,266.8 wtm_TradingSecuritiesDebtIncludingAssetsHeldForSale / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeInvestmentsMember
Trading Securities, Unrealized Holding Gain | 64.6 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeInvestmentsMember | 55.7 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeInvestmentsMember
Trading Securities, Unrealized Holding Loss | (10.1)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeInvestmentsMember | (27.7)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeInvestmentsMember
Excluding Short Term Investments |  |
Investment securities |  |
Duration of Fixed Maturities | 2 years 3 months 18 days |
OneBeacon Runoff |  |
Investment securities |  |
Fixed maturity investments reclassified to assets held for sale (1) |  | $ (236.3)wtm_TradingSecuritiesDebtReclassifiedToAssetsHeldForSale / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconRunoffMember

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