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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 | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Share-based compensation arrangement by share-based payment award |  |  |  |
Accrued incentive compensation | 184.6 wtm_IncentiveCompensationPayable | $ 218.3 wtm_IncentiveCompensationPayable |  |
Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share | 9.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardTargetedPerformanceGoalforPersonnelExpressedasPercentageofGrowthinIntrinsicBusinessValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom20142016Member |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00%wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardPayoutForMaximumGrowthTargetPercentage / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom20142016Member |  |  |
Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share | 8.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardTargetedPerformanceGoalforPersonnelExpressedasPercentageofGrowthinIntrinsicBusinessValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2013To2015Member |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00%wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardPayoutForMaximumGrowthTargetPercentage / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2013To2015Member |  |  |
Performance Cycle From 2012 to 2014 | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share | 8.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardTargetedPerformanceGoalforPersonnelExpressedasPercentageofGrowthinIntrinsicBusinessValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2012To2014Member |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00%wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardPayoutForMaximumGrowthTargetPercentage / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2012To2014Member |  |  |
White Mountains | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 126,717 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember |  |  |
Incentive compensation payable before forfeitures | 49.4 wtm_IncentiveCompensationPayableBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember |  |  |
forfeitures | (3,168)wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember |  |  |
Assumed forfeitures in period amount | (1.3)wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeituresAmount / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember |  |  |
Shares outstanding (in shares) | 123,549 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 | 119,357 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 150,064 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Accrued incentive compensation | 48.1 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 | 29.4 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 66.1 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
White Mountains | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 41,937 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom20142016Member |  |  |
Incentive compensation payable before forfeitures | 4.1 wtm_IncentiveCompensationPayableBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom20142016Member |  |  |
White Mountains | Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 46,803 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2013To2015Member |  |  |
Incentive compensation payable before forfeitures | 18.9 wtm_IncentiveCompensationPayableBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2013To2015Member |  |  |
White Mountains | Performance Cycle From 2012 to 2014 | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 37,977 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2012To2014Member |  |  |
Incentive compensation payable before forfeitures | 26.4 wtm_IncentiveCompensationPayableBeforeForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2012To2014Member |  |  |
Minimum | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 3.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardPerformanceGoalPercentageforPayout / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / us-gaap_RangeAxis = us-gaap_MinimumMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom20142016Member |  |  |
Minimum | Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 2.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardPerformanceGoalPercentageforPayout / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / us-gaap_RangeAxis = us-gaap_MinimumMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2013To2015Member |  |  |
Minimum | Performance Cycle From 2012 to 2014 | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 2.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardPerformanceGoalPercentageforPayout / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / us-gaap_RangeAxis = us-gaap_MinimumMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2012To2014Member |  |  |
Maximum | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 15.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardPerformanceGoalPercentageforPayout / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / us-gaap_RangeAxis = us-gaap_MaximumMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom20142016Member |  |  |
Maximum | Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 14.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardPerformanceGoalPercentageforPayout / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / us-gaap_RangeAxis = us-gaap_MaximumMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2013To2015Member |  |  |
Maximum | Performance Cycle From 2012 to 2014 | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 14.00%wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardPerformanceGoalPercentageforPayout / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember / us-gaap_RangeAxis = us-gaap_MaximumMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2012To2014Member |  |  |

X
- 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

X
- Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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

X
- 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

X
- Definition This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. + References No definition available. + Details
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Namespace Prefix: | wtm_
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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

X
- Definition Represents the lpercentage of target achieved, high end of range. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardPayoutForMaximumGrowthTargetPercentage
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the percentage growth in intrinsic business value per share to achieve payment. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Share-based Compensation Arrangement by Share-based Payment Award, Targeted Performance Goal for Personnel Expressed as Percentage of Growth in Intrinsic Business Value + References No definition available. + Details
Name: | wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardTargetedPerformanceGoalforPersonnelExpressedasPercentageofGrowthinIntrinsicBusinessValue
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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- Details
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- Details
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Namespace Prefix: |
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- Details
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Data Type: | na
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- Details
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