XML 44 R73.htm IDEA: XBRL DOCUMENT
v2.4.0.6

Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Investments, Debt and Equity Securities [Abstract] |  |  |
Accumulated Other Comprehensive Income (Loss) Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates before Tax | $ 62.8 | $ (63.6) | $ 72.7
Income taxes | (5.1) | 5.1 | 0.8
Net change in unrealized gains (losses) on investments in unconsolidated affiliates, after tax | 57.7 | (58.5) | 73.5
Other Comprehensive Income, Foreign Currency Transaction and Translation Adjustment, Unrealized Gain (Loss) on Investments, Net of Tax Period Increase (Decrease) | 95.5 | (41.7) | 107.9
Total investments gains (losses) through accumulated other comprehensive income | 153.2 | (100.2) | 181.4
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 91.4 | 56.0 | 56.1
Total investment gains (losses) recorded during the period, after-tax | $ 244.6 | $ (44.2) | $ 237.5

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

X
- Definition Represents the change in the current period of the accumulated appreciation or loss, net of tax, in equity in unrealized gains or losses from investments in unconsolidated affiliates included in shareholders equity. + References No definition available. + Details
Name: | wtm_AccumulatedOtherComprehensiveIncomeLossEquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Represents the change in the current period of the accumulated appreciation or loss, before tax, in equity in unrealized gains or losses from investments in unconsolidated affiliates included in shareholders equity. + References No definition available. + Details
Name: | wtm_AccumulatedOtherComprehensiveIncomeLossEquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliatesBeforeTax
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_AccumulatedOtherComprehensiveIncomeLossEquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliatesTaxEffect
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 Adjustment that results from the process of translating unrealized gain (losses) on investments into the reporting currency of the reporting entity, net of tax. + References No definition available. + Details
Name: | wtm_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentUnrealizedGainLossOnInvestmentsNetOfTaxPeriodIncreaseDecrease
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration