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Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract] |  |  |
Net realized investment gains, pre-tax | $ 233.8 wtm_TradingSecuritiesRealizedGainLossTotal | $ 104.5 wtm_TradingSecuritiesRealizedGainLossTotal | $ 68.1 wtm_TradingSecuritiesRealizedGainLossTotal
Net unrealized investment gains, pre-tax | 50.1 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities | 57.2 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities | 50.1 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities
Net realized and unrealized investment gains, pre-tax | 283.9 us-gaap_NetRealizedOrUnrealizedGainLossOnTradingSecurities | 161.7 us-gaap_NetRealizedOrUnrealizedGainLossOnTradingSecurities | 118.2 us-gaap_NetRealizedOrUnrealizedGainLossOnTradingSecurities
Income tax expense attributable to net realized and unrealized investment gains | (69.7)wtm_Taxonnetrealizedandunrealizedgainslosstradingsecurities | (21.9)wtm_Taxonnetrealizedandunrealizedgainslosstradingsecurities | (26.8)wtm_Taxonnetrealizedandunrealizedgainslosstradingsecurities
Net realized and unrealized investment gains, after tax | $ 214.2 wtm_NetRealizedandUnrealizedGainLossonTradingSecuritiesNetofTax | $ 139.8 wtm_NetRealizedandUnrealizedGainLossonTradingSecuritiesNetofTax | $ 91.4 wtm_NetRealizedandUnrealizedGainLossonTradingSecuritiesNetofTax

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- Details
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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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- Definition Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax + References No definition available. + Details
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- Definition The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. + References No definition available. + Details
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- Definition Tax on net realized and unrealized gains (loss), trading securities + References No definition available. + Details
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- Definition Trading Securities, Realized Gain (Loss), Total + References No definition available. + Details
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