XML 97 R77.htm IDEA: XBRL DOCUMENT
v3.3.1.900

Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details) - USD ($) $ in Millions | 10 Months Ended | 12 Months Ended
--- | --- | ---
Nov. 05, 2015 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Investments in and Advances to Affiliates [Line Items] |  |  |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  | $ (39.2) | $ 81.2 | $ (106.4)
Income tax benefit (expense) |  | 2.9 | (5.9) | 8.3
Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax |  | (36.3) | 75.3 | (98.1)
Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax |  | (34.9) | 75.3 | (98.1)
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax |  | 190.2 | 60.5 | 121.1
Total investment gains recorded during the period, after-tax |  | 155.3 | 135.8 | 23.0
Symetra |  |  |  |
Investments in and Advances to Affiliates [Line Items] |  |  |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates | $ (39.2) |  | 81.2 | (106.4)
Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax |  | (34.9) | 75.3 | (98.1)
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra |  | $ 1.4 | $ 0.0 | $ 0.0

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_InvestmentsInAndAdvancesToAffiliatesLineItems
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 Represents recognized after tax amortization through equity in net unrealized losses from investments in unconsolidated affiliates. + References No definition available. + Details
Name: | wtm_InvestmentsInUnconsolidatedAffiliatesAmortizationThroughEquityInNetUnrealizedLossesAfterTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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 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

X
- Definition Other Comprehensive Income, Reversal of accumulated other comprehensive income, Unrealized Gain (Loss) on Investments, Net of Tax Period Increase (Decrease) + References No definition available. + Details
Name: | wtm_OtherComprehensiveIncomeReversalofaccumulatedothercomprehensiveincomeUnrealizedGainLossonInvestmentsNetofTaxPeriodIncreaseDecrease
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis=wtm_SymetraMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |