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Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss), Total | $ 52.8 wtm_TradingSecuritiesRealizedGainLossTotal | $ 21.7 wtm_TradingSecuritiesRealizedGainLossTotal
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 50.6 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities | 42.1 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities
Net Realized and Unrealized Gain (Loss) on Trading Securities | 103.4 us-gaap_NetRealizedOrUnrealizedGainLossOnTradingSecurities | 63.8 us-gaap_NetRealizedOrUnrealizedGainLossOnTradingSecurities
Income tax expense attributable to net realized and unrealized investment gains | (26.3)wtm_Taxonnetrealizedandunrealizedgainslosstradingsecurities | (15.7)wtm_Taxonnetrealizedandunrealizedgainslosstradingsecurities
Realized and unrealized gain | 77.1 wtm_NetRealizedandUnrealizedGainLossonTradingSecuritiesNetofTax | 48.1 wtm_NetRealizedandUnrealizedGainLossonTradingSecuritiesNetofTax
Fixed maturity investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss), Total | 31.0 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 0.9 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 52.3 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 31.6 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Short-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss), Total | 3.5 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 0 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Common equity securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss), Total | 25.5 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 18.8 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (3.4)wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 4.5 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Convertible fixed maturity investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss), Total | (4.2)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 2.4 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 0.8 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 0.7 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Other long-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss), Total | (3.0)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | (0.3)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 0.9 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 5.3 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Forward Contracts |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss), Total | 0 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | (0.1)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | $ 0 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | $ 0 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember

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- Definition The sum of the realized and unrealized net gain (loss) on trading securities during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 + Details
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