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Income Taxes (ASC 740-10) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2014
Income Tax Contingency [Line Items] |  |  |  |
Percentage of likelihood for realization of benefit upon ultimate settlement |  | 50.00%wtm_PercentageOfLikelihoodForRealizationOfBenefitUponUltimateSettlement |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns |  |  |  |
Balance at beginning | $ 50.0 us-gaap_UnrecognizedTaxBenefits | $ 46.6 us-gaap_UnrecognizedTaxBenefits | $ 65.5 us-gaap_UnrecognizedTaxBenefits |
Changes in prior year tax positions | (4.8)us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | (4.8)us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | 1.9 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions |
Tax positions taken during the current year | 13.7 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions |  |  |
Lapse in statute of limitations | (1.1)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations |  | 0 us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations |
Settlements with tax authorities | (0.9)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities | (0.4)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities | (0.6)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities |
Tax positions taken during the current year |  | 8.6 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions | (20.2)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions |
Balance at year end | 56.9 us-gaap_UnrecognizedTaxBenefits | 50.0 us-gaap_UnrecognizedTaxBenefits | 46.6 us-gaap_UnrecognizedTaxBenefits |
Unrecognized tax benefit recognition including interest and penalties, if recognized, would effect income tax expense | 27.1 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |  |  | 5.0 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense | (2.6)us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense | 1.3 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense | 1.2 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense |
Permanent Differences [Member] |  |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns |  |  |  |
Balance at beginning | 18.2 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember | 9.7 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember | 9.6 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember |
Changes in prior year tax positions | (2.0)us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember | 0.6 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember | 0.5 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember |
Tax positions taken during the current year | 5.6 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember |  |  |
Lapse in statute of limitations | (0.8)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember |  | 0 us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember |
Settlements with tax authorities | (0.6)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember | (0.4)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember |
Tax positions taken during the current year |  | 7.9 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember |
Balance at year end | 20.4 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember | 18.2 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember | 9.7 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_PermanentDifferencesMember |
Temporary Differences [Member] |  |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns |  |  |  |
Balance at beginning | 22.5 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember | 28.9 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember | 49.1 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember |
Changes in prior year tax positions | (0.8)us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember | (7.1)us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember |
Tax positions taken during the current year | 8.1 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember |  |  |
Lapse in statute of limitations | 0 us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember |  | 0 us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember |
Settlements with tax authorities | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember |
Tax positions taken during the current year |  | 0.7 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember | (20.2)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember |
Balance at year end | 29.8 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember | 22.5 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember | 28.9 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_TemporaryDifferencesMember |
Interest and Penalties Net of Tax Benefit [Member] |  |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns |  |  |  |
Balance at beginning | 9.3 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember | 8.0 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember | 6.8 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember |
Changes in prior year tax positions | (2.0)us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember | 1.7 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember | 1.4 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember |
Tax positions taken during the current year | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember |  |  |
Lapse in statute of limitations | (0.3)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember |  | 0 us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember |
Settlements with tax authorities | (0.3)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember | (0.4)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember | (0.2)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember |
Tax positions taken during the current year |  | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember |
Balance at year end | $ 6.7 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember | $ 9.3 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember | $ 8.0 us-gaap_UnrecognizedTaxBenefits / us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis = wtm_InterestAndPenaltiesNetOfTaxBenefitMember |

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