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Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) (USD $) In Millions, except Share data, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014 | Dec. 31, 2013
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Share-based compensation arrangement by share-based payment award |  |  |  |
Total Accrued Expense | $ 100.5 wtm_IncentiveCompensationPayable | $ 184.6 wtm_IncentiveCompensationPayable |  |
White Mountains | Performance Share | W T M Incentive Plan |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding | 119,380 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember |  |  |
Accrued Expense | 28.3 wtm_IncentiveCompensationPayableBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember |  |  |
Target Performance Shares Outstanding, Assumed forfeitures | (2,984)wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember |  |  |
Accrued Expense, Assumed forfeitures | (0.7)wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeituresAmount / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember |  |  |
Total Target Performance Shares Outstanding | 116,396 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 123,549 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 127,537 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 119,220 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Total Accrued Expense | 27.6 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 48.1 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 42.3 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 60.2 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2015 to 2017 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding | 30,640 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2015to2017Member |  |  |
Accrued Expense | 1.8 wtm_IncentiveCompensationPayableBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2015to2017Member |  |  |
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2014 to 2016 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding | 41,937 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom20142016Member |  |  |
Accrued Expense | 5.6 wtm_IncentiveCompensationPayableBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom20142016Member |  |  |
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2013 to 2015 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding | 46,803 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2013To2015Member |  |  |
Accrued Expense | $ 20.9 wtm_IncentiveCompensationPayableBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2013To2015Member |  |  |

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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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

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- Definition Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. + References No definition available. + Details
Name: | wtm_IncentiveCompensationPayable
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Carrying value as of the balance sheet date for performance shares before forfeitures. + References No definition available. + Details
Name: | wtm_IncentiveCompensationPayableBeforeForfeitures
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeitures
Namespace Prefix: | wtm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Definition This element represents the reduction to accrued expense for assumed forfeitures of performance shares. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeituresAmount
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition This element represents the number of non-vested performance shares before forfeiture assumptions. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures
Namespace Prefix: | wtm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | dei_LegalEntityAxis=wtm_WhiteMountainsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |