XML 34 R78.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract] |  |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates | $ 81.2 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio | $ (106.4)wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio | $ 62.8 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio
Income taxes | (5.9)wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliatesTaxEffect | 8.3 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliatesTaxEffect | (5.1)wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliatesTaxEffect
Net change in unrealized gains (losses) on investments in unconsolidated affiliates, after tax | 75.3 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates | (98.1)wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates | 57.7 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates
Net realized and unrealized foreign currency (losses) gains on investments through OCI | (274.3)wtm_OtherComprehensiveIncomeForeignCurrencyTransactionandTranslationAdjustmentUnrealizedGainLossonInvestmentsNetofTaxPeriodIncreaseDecrease | 11.3 wtm_OtherComprehensiveIncomeForeignCurrencyTransactionandTranslationAdjustmentUnrealizedGainLossonInvestmentsNetofTaxPeriodIncreaseDecrease | 95.5 wtm_OtherComprehensiveIncomeForeignCurrencyTransactionandTranslationAdjustmentUnrealizedGainLossonInvestmentsNetofTaxPeriodIncreaseDecrease
Total investment (losses) gains through accumulated other comprehensive income | (199.0)wtm_OtherComprehensiveIncomeChangeInUnrealizedHoldingGainLoss | (86.8)wtm_OtherComprehensiveIncomeChangeInUnrealizedHoldingGainLoss | 153.2 wtm_OtherComprehensiveIncomeChangeInUnrealizedHoldingGainLoss
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 214.2 wtm_NetRealizedAndUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax | 139.8 wtm_NetRealizedAndUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax | 91.4 wtm_NetRealizedAndUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax
Total investment gains recorded during the period, after-tax | $ 15.2 wtm_GainLossOnInvestmentsNetOfTax | $ 53.0 wtm_GainLossOnInvestmentsNetOfTax | $ 244.6 wtm_GainLossOnInvestmentsNetOfTax

X
- Details
Name: | us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The net unrealized gains (losses) from Symetra's fixed maturity portfolio. + References No definition available. + Details
Name: | wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Represents total investment gains (losses), after tax, recorded during the period. + References No definition available. + Details
Name: | wtm_GainLossOnInvestmentsNetOfTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax + References No definition available. + Details
Name: | wtm_NetRealizedAndUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Comprehensive income (loss) attributable to White Mountains' common shareholders. + References No definition available. + Details
Name: | wtm_OtherComprehensiveIncomeChangeInUnrealizedHoldingGainLoss
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Other Comprehensive Income, Foreign Currency Transaction and Translation Adjustment, Unrealized Gain (Loss) on Investments, Net of Tax Period Increase (Decrease) + References No definition available. + Details
Name: | wtm_OtherComprehensiveIncomeForeignCurrencyTransactionandTranslationAdjustmentUnrealizedGainLossonInvestmentsNetofTaxPeriodIncreaseDecrease
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Other Comprehensive Income (Loss) Equity in Unrealized Gains Losses from Investments in Unconsolidated Affiliates + References No definition available. + Details
Name: | wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Tax effect on accumulated appreciation or loss in equity in unrealized gains or losses from investments in unconsolidated affiliates. + References No definition available. + Details
Name: | wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliatesTaxEffect
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration